ii:1.:1:. >A..1..A. M o()i!I1 O Ii01i' M O 1 O(. 0 N. BtY JOH'N T.YNi)AT. I,, EL.,.ItS,,. t',1 IPRtOiESSORl O NATURAL IliOTr t'1tOSOP)ttY IN'T'11 RtOYALh INSlTUTION OF' (tlIiWAT BRIlt'AIN,'tRO,it'if t XO UItfIf ANiD) L, LASf?' S A, Ir XIJff LS' EDIT2IO~N. NEW YORK:. 1). A PPTllt:'ON AN^1) COMAt1ANY, 649 (%, 61 BROADWAY. 1 8 3. 1'O H]iS FRIEiINT1 ) AND1 TEACJIIll, ~ 1, ()'1 E XI, 1 II3 U LIT SE:IN, Tills 1 OOK IS t)EDIOAT.EI).JOHN TY AI, 0"Again the cocI'etiojl of iCe will not endure a dry ttrition withoutt liquation; for if it be rubbeh d long with a clotll it melteth." SIR TH'OMA$ BRowNE, "Peltudodt;ifl ]pitcitat" IBook II., lchap. i.,'PiC(krclg's edition. 15 Vol.tt. H., 1). 211. lRJFAO'CI TO TH'II r'I TRll) IED )ITION. SO1GTAM years ago I ha(d the honor of holding a0, number of E Lxaminerships under the Council for AMilitfary l'ducation, It was also my privilege to be ]4xanlminer for the University of London. These, and the examinations connected with my public lcctures, gave iec an opportunity of makting myself acquaintcd, to some extent., with the knowledge. and needs of lE'ngland as regards thile delartment of Natural Knowledge which' it is my Voc&ation to cultivate. Th. experience thus obtained was supp)lemented )y tlhat (crivcd( firom colvcrsation vwith cminent scholars, who dleprecated and depllored the utter want of scienCific knOwledge, and the utter absence of symp)athy with scientific stludies, which mark thle grcat bulkl of our otherwise cultivated E1l1glishl public. Tl'houghl regarding original itlvestigation as the great object of my life, I {thought it no unworthy work to attemplt to stlply th le deficiencies here indicated. The idea arose, and gained consistencle by reflection, of taking, as far as time permitted, the various parts of Natural Ph'ilosoplhy, treated in viii rPREFACE TO THE THIRD ED)ITION. lmy lectlrcs, ill stccessiolln, alnd of describing and illustratingl wlitll clarnitss and simplicity such conccptions rcgarding theml as the best culture tl could command enabled me to ellntcrtain, Thlc first fruit of this idea was the work on IHeat, tile fltirli edition of which is no10 bfor3 tlC reader. Th rccception of thte work( )1rovctd tt it Imet a general wNant. Not only has its success in this country bccen far greater than was cevr hloped for, but large cditions of it Ihave been publishcd anld circulated in 1rance, lRussia, and the United States. Something more, however, thtan its ralid( diflusion among the gelncral public wars needed to convince mi c thlat thle.work twas such as I desired it to be. This assurance camei to ic, both p)i\at.ely and plublicly, from scientific Sour1ccs, and lately in at very strikingt form from Germany. The beautiful translation of the work by I e'lmholtz and \ricdemann, issued by Vicew\c of ]h'3unlswick, al(l the rccc)tion of tflit translation by the press of Germany, are to me the best guarantee and tile most gratifying cvidtence that I have not entirely missed mly (taim. Tha:t aim was to combine soundllness of matter with a style whfich should arouse interest; and sympathy ill p)olso1ns uncultured in science. I h}ad, also, lreason to believe that tthe more specially scientific student would find iln the Nworlik hbelp and furtherance, in forming' defilite conceptions of those 1molecular processes which underlie both chcmical and t)hlysical p)llclomCla, The secorld instalmncnt of the task contempl(ated was the work oin SounId recently published by Tongmans. Thle reception of the work iln this countly lhas been also far more flatter PRI E'ISC TO TIlEl TIRiD) EDITION. ix ing' tlhanl I had ventured to anticipate. It hlas, morov'Cr, been already publishcd in America. n1i France a, translation of it is b)eing preparcd I.A Gauthier-Villars, while in Germany thle tw.o eminent men already named have taken itl nder their prlotect ion. All this convillces mle that if a scientific mall take the trouble, which ill my case is imllelnSe of t!inkingl, andl writing, with life and clearness, hie is sure to gain general attention. It; can lhardly lbe doubted, if fostered and st-rengthened in thlis way, that the desire for scicntific knowledge will ultimately correct the anomalics which beset our presOnt system of education. * B esides other additions and alterations, a, conlsiderable tamllount of matter, derived mainly ifrom my own recent investig'ations, is added in a new chlapter, to thel present edition. n11 order to prevent th;e ook fiom assuming an incotnvcllient siz.e, I have olmitted most of the Sut)})llcentary Appendices. Vit;hinl the cominFg Tyearl I: hope to collect and 1ublislh, in a single volumel the original memoirs oil 1xperimental PIllysics, whfich I have communicated to the " hilosophical Tlranslactions" and "X Philosophical Nlagazinel" during the last cighteen ycars.'t'hcse memoirs wtill embrace all the su!ppllcmentary matter refcrred to, and they may b1) consultcd by those who Awish to carry their studies beyond the limits l)rescrib)ed to an elementary work, It will interest the scientific student to learn that Mayer and Clausius have rccntly pu)blishlcd, in a collected form, their celebrated researces, on the D)ynamical Theory of Hieat, an E0nglish tranlslation of the first part of the memoirs of Clausius *I tluxlcy's " Les sonis in Elcmntart y Physiology}" is a great step ill this direction, hlaving bcon cdited by Prof. Htirst. Ilt is to be hoped that thle investigations of Joulet, Illmholtz, Thomson, tand tlankine on this, the greatest scientific subjcct hitherto unfolded by thto human mindll may ultimately be renderecd equally accessible. The memoirs of Sir W'illiam Thlonlsont at once varicd and profound, would be of especial interest and importance. ROYAL SNSTI'TUTION, J&tUary, 1868. PRI-l.]AOE],'TO 1.1T1i, SECOOIND) l).[IT ONN. IlN tie first edlition of this work, t'h language emllloyed was. almost verbatim tlhat of the lecture-room. For thi., had time pCrmittcd, I should willingly, iln this edition, have substituted a graver' style, though it may be doubted wh\ther the chlange awould hlave Iadded to the clearness of the exposition. The word " Lccturc," formerly usted as a hctadi ng, has, lolcvcl, bccn abandoned, the Nwork being now divided into thirteen chapters. 1 have sought to embody in it an abstract of my own researches on radiant heat, completed since the )ublication of the first edlition. That plortion of the work which treats of the cphelnomena of vitality has also, for the most part, 1)Ccn rewritten. RIOYA1, NShTI1TUi'IONX L}ftIttwIyt 1866. 'IPR:FAOE'0 TI E11 ]T {ST'..I:Dr)IT]iON. iN tle following L4ectures T: lave endeavored to bring the.. rudimllcents of a new philosophy Nwithin the reach of a pcrson1 of ordinary intclligence and( culture. The first seven Lectures of the course deal wiith teraeeo metriC heati its generation and consumption in mIcchanical processes; the determination of tfo mechlanical equivalenlt of heat; the conception of heat as molecular motion; thte application of this conception to the solid, liquid, and gascous formls of imatter; to cxpansion and comlbustion; to specific and atentt lhcat; and to calorific conduction. The remaining five Lcctures treat of radiantt eat; the interstellar medium, and t.he propagation of motion through this medtium; the relations of radiant heat to ordinary matter in its several states of aggregation; terrcstrial, lunar, and solar r1adiation; the constitution of the sun; the possible sources of his cnergy; the relation of this cnllrgy to terrestrial forces, and to vegetable and animal life. AMy aim has been to rise to the level of these que1stions from ta basis so clemeltary, that a person possessing any ilmainaitive faculty and power of concentration, might accomnpan)y nle. PRIiFACE TO THIE FJIRST EI)ITION. xiii;Wherlver additional reimarks, or extracts, seemed likely to rcnder the reader's knovwledge of tile subjects reflrred to in any Lecture more accurate or complete, It have introducccd such extracts, or rcnmarks, as an Appndix to the Lecture. For the use of the plate at the cnd of thc volumc, I am i ndebted to the Councilo of the Royal Society; it was cngravced to iltlstrate some of my own mtlc oirs in the " Philosophical Transactions. " Fi0or soml of the wvoodcuts I am also ind(lelted to tih'e same learned body.'To thle sciCntific public, the names of the builders of this nem' philosophy are alrecady familiIar. As exlcerimnntal contriblutors, R1uford, Davy, Faraday, and Joule, stand p!rominently forward. As tlheorctic writers (p)lacing t.heCm falphabct-ically), we have Clausius, HIclmholtz, Kirchhoftl, Aayer, Rank(ine, I'homson; and in the memoirs of tLese sc minent men the student vwho desires it must scek a deeper ac(quaintance with tlle subjcct. ALM. ]Relgnautlt and S6guin also staind in honorable rclationship to the i)ynamical Thleory of IHeat, and AL. \rVrdet has recently p)ublished two lectures on it, narkled by thec learning forl~ which he is conspiculous.'.'o the Englislh reader it is superfluous to mention the wcell-known and highly-l)rized Nwork of MrI. Grovc.* I[ have called the plhilosophy of Heat a new philosophy, without, hlowcvcr, resti.ieting t.hle term to the subjet of hIeat. The flct is, i)t canlot be so restricted; for the connection of thills agent Awith th}l general cnlergies of tle universe is such that;t, if we rimaster it perfectly, we nmaster all. Evenl now we X* trbetlutaitiful experimlcnts of M. Favre ought to be referred to here: and nlso, in Connection with a subject trcated inl Chap. XIII., a most important experiment1 by M. Foucau\lt which is described in the " Philosophical 1Magoasine," vol. Pix,. 19 (February, 1865), Axiv P1PEtACE TO TII1EF FIRST EDITION. call discern, thloughl but darkly, the greatness of the issues which connect themlsclves with the l>ogrcss wte ave maden. —issues which welrc p)robably bc3yond the contemplation of those by whose industry and geniuts the fotndations of olr p)rcscnt knowlvedge wcere laid. fIn a lccture onl the " Influence of the lhistory of Science onl Intellectual E]ducation," dclivered at the ]lRoyal Institution, Dr. W5howell has shown " that every advance in intellectual cducation has been the effect of some considerable scicntific discovcry or group of discoveries." If the association here indicated be inivritabl, then, assuredly, t.he views of the connection and interaction of natural forces-........ -organic as well as inorganic —-vitl as ivell as physicabl.-.. which hlave grown, and whvlich are to grow, out of the investigation of the la^ws andl relations of hleat, will profoundly affect the intellectual discipline of the comning age. In the study of Nature two elements come into play, which belong respectively to the world of sense and to the world of thought.,VWe observe a fact and seek to refer it to its laws.......we al)prelhend the law, and seek to make it good in fact. The one is theory, the other is experiment; whlich, whcn applied to the ordinary purposes of life, becomes Practical Science. iNothlling could illustrate more forcibly the vwholesome interactionI of thetst two elemenllts, than the history of our l)r'cscet subjccL If tthe steanm-engince iad not been invented, wr should assuredly stand below the theoretic level whichl we now occuipy. The achievements of heat through t.le steam-engine hallve forced, with augmented e)mplasis, tthe question upoll thinking millnds, "'lWhat is this agloent, by llceats of whlicl e can supersede the force of'wi. uds and rivrs....of hor.ss ad of menl? hleat can pt oduce mechanlic2al force, and mchalniceal force can produce heat; some comnon qtuality must therelfore unlite this PREFACUE TO THIIE FIRST E3DITIO0N. xv aIget and the ordiniary formls of mcchanical power." This'rchttiolnship it stablished, the gciieralizing intellect could pass at once to the other energies of the universe, land it now pe'rccivcs thle principle which unites them all. Thus the triumphs of practical skill have promoted the devlopm, lnt of philosophy. Thus, by thc interaction of thought and fact, of trutlt conceivcd and truth executcdl, we ha\ e lnlade our Science whlat it is.......-the nolest growth of moldern illles, thoulgh as yet but l)Iartially appealedl to as a soutrce of individual and national might. As a meastl of intellectual education its claims are still disputed, thfough, once properlly organized, greater and more bcneficentt revolutions await its em1lployment hlere, ttall those vwhich have alrcady marked its al))licattions in the material world. Surely t1he men whose noble vocation it is to systematize the culture of England, can never allow this giant l)owcr to grow up in tlheir midst withllout endeavoring to turn it to )praetical account. Science does,not need tlheir protcction, but it desires their friendship on honorable terms; it wishes to work Nwith thetm townari thle great end of all cducat.ion-..the bettering of man's state. B3y continuilng to decline the offered hand, they invoke a contest which can have but one result. Science must; grow. Its development is as necessary and as irresistible as the motion of the tides, or the flowing of tlhe Gulf Stream.:It is ka phase of the energy of Nature, and as suceh is sure, in dtue time, to compeI) the recognitio1n, if not to winl the alliance, of those whlo now decry its influence and discourage its Iadvance. 1RoYA.L Ifs'TniTurioNX Bi'rltai, 1863. OtrAPTER I. tns utrtentis....... enration of lieat by Frictionl, Clompreasson, and Pe rcussilono. -. Exlprhlrent of lutnlford..t Water boiled by Friction..t.Conslltlintiona of [eat. n Work. lPAog 1 APPEN'DIX TO CHAP:TER 1. Nhotes on the Theranoelectrlo Pile andl ('alvanometer..... 16 l APT!R If. Tho Natuore of I teat —'le Mo terial Theory —fhe D)ynamical Theory — r':ltlmal Eftest of Air in tMotion- -..... enration of Heat by Rotation between the Poles of a tMagnet^- -.lxpetrimlets of Il.unfrdt )vy, I)ay, and Joiule — Theo Mtchanical Equivaltentt of lteat....ieat generated by Projectiles-. lteat vwhich would o gellnerated by stopping the V.Earth's Motion -Metteoric Theory of the Slun's I teat- -Flam1o i its Relatilon to the D1)ynaminal The.ory.. APPSENDI) TO CtHfAPt'R Ii. Extraets fronm Bacon and Rluoford......... CHi[APTIEt II1, Expansion: the 8olidt, Iquill, anld Caseous Ftorms of Matter —-— New Ifypotlecss reg arding the Constitution ase of es-xel of Xpanslon.. -l ieat ilmpartcd to a (;as t1under (Con.stqt Preesure.... — -lieat Iliparted to a Gtas at Constant Voluinne. —..Mayer's Calculationl of the Mechanlicl Equivalent of ietat —.Dilatation of (lases vithout lRefrigeration - Absolutte kero of Tet perature....ExaJnsotll of Liquids and Sotidt: Aunoalous )eportwsent of Water and 13islnutht. —A.En erg y of the Force of Crystalie.ation- -'hllermat l Effect of stretching Wires -— A. nomalous Deportment of India-rubbr... 5 APPENDIX. TO C0iAPTI'ER Il. Addltltonal Data concernin g E;xpaslo —los.-Extiactas froml Sir i. )Davy's firtt 8elintille Moemlolr: Fusion of oee by Frietioll, etc,.., xviii CCONTElNTS. CHAPTE!R IV.'ile l'rrvely an Ilstrun Gtt -ore's revolvinlg lls.. ~- Inftiulc e of Pressure on Fusing Point — Atquelactioll nan1d LXtamllltion of Ic by 1Pressure-. Diseetlo of leo bya Catoilflu B3eam- -.Liquid Flowrs and their Clentral -Spot- -..l.Mehlnit!A Properties of Wat er purged of Alt:- -4lt Boiling-P'oit of Liquidts: Itllueitg C;ireuiustlanees —- Con-velrsion of {eat, tlto Work In the Steam-Enlgine: the G ysrs of Icelanld.... At 93 CIfAPTER V. Application of thle Dynaltical i lTheory to the ihllenomcna of Specfle auid TI en t ieat^ — 1f)e-fi ftilonl of Eu'legy: l'otenta;l and l)ynamie Energy-.. —]'Energy of Moilcullar Forcess — --— Xperimental lutl1ust iations of Specifie and Latent leat —-Mtechaicl e Vatlces of the Acts of C(omblllntlo, (noslndelsto, anld Cogelatlon ini the Cxsw of Water — -SoliA Clarbol;ic Acld -. -fot Sphellroidal State of tiquids. — Floating of a Spheroid on its own apor —-'reeing of AWat.er and Mcelur fIn a fred-hot Crteiblo... 12 (CIfAPTlFR V. Convection of l e;ated Airu. l. s inds- -the Upper and Lower "'I'(Udt"esL" —E-fict of thle E-atl's Rotation ou the Direction of Wtind t —-- Jlluenco of Aqueois Vapor upou C'iuant —-— Europe thle C'ondenscr of the Western Atlantie. -.: allaf..tl in Ir''land- — tho Gu\llf Streua- —.Forlmation of 8 FSnow-i —uol' aion of Ice frlio Snow -. —Gflaclers...l. elloenia of Glac1ier Motlo -Retlationu... lt --.. Mou lditngof Ice by Pressure..lt.-Aneiet Glaciers.. 125 CHOAPTE'I VII, Ctonlduction a'tanusmtissilou of Motion.-.Good Condlctors and B]ad Conductors —— l.Conductv. ity of tlto Metals for t ecat: Relation between tie Condluctivity of [eat andl that of "lectr etityt — -IT nftluence of Temlperature oul thle Conductionl of Eicetriceity —...l'nluh lcnc of Molecular Constitutionl ot te Condtctiol of te[(,at —-lelatlon of Specific ilHeat to Coludutctiot.- -lPhilosophlv of Clot hes: lRumford's ExAperil cn tn s -t...lutlluco of Meehanleal Texture on Conductionl — Inlrustatiollns of Boilers - -'1te Safety Lamp.it. — Coplnductivity of Liquids cad (il es: Experimeults of Rumford and l)CesI)rCt — -— oing Etffet of 1lydrogert Gas.-.-,xperilejnts of cta"ts on the Condulelit.y of Gases.... 1 CIIAPTER VIII. Cooling a Loss of'Motioln: to what is thils Motion imparted?-.-Expperiments oni Sound bearlug on thlls Qulestion-... Experiments onl Lig ht bearing oin thils Questiou- -'thu l'lheolris of Emission ald InduJlation.. —-. —lctgt of aes ncd Nllumber of fImpulses of light-.-'11hys. teal Cause of Color- -— lwilsiblo tays of the Spcctriln —'l1ie Caltorlfl Rays beyond the Redl-..-'The Chemical Rays beyond thue fBlue-. l)cfinitl'on of Raidiant ltea.t —-Rellection of Radliant tieat from llalno l d Curved Surfac(s: Lass the lsame as tihose of.itit. — Conjugate Mirfors. 2183 APiPENDIX TO) CHIAPiERt Vill. On Singing Flames. 237 CONTENTS. xix C0IAPTI'R IX, Law of )inilmition \wilt tile Distance- tfhe Wa'ves of So0lttd lonlgitrrdirit; those of Light ttrasversal- -W- hen they o.sltate, the Mtolcules of )irireilt Bodies coimurlicato D)ifferet. t Amoult s of Motion to thoe Ether — 1,adation the Coinrr ication of Motion to the Ether; Abtsorption thle Acc:ptane of Motion from the Ether-.-Those Surfacs whichlt ilradiato well absorib wvell -A close Woollten Covelring trellitates Coolivg- -.Presce rativo Inftluece of (ohl-leaf - -The Atoms of Bodlies int,.relp)t certain Waves, and allow others to pass -T'liansmparency and -)iDatlheml ancy~..)iathermic )exoies Bald Radiator.s-..e.l)finttioll of the'Termlt "~Quality' as applied to Rladiant lteat — The Rays wlhichl passs,without: Aslsorption (do not hleat the Mediutl- -- Proportllon of lluminous and Obscure Rays in Various Flamues...........P: 219 C0IAPTEIR X. Absorption of Heat by Glascous Mantter — Apparatus employed —.arly ifllufltlies —i)iathermlancy of Air and of thoe Transparent lemlelltay (lases -— thrallnc of Oleflant Gas e1nd of other compoilund (Oases- Absorption of Radiant Iealt by Vapors —-1,adiation of Heat by' (iases.. -.Re l procity of R adiation and Absorptionl.. —-..Inftluenc of Molecular Collstitution o0n the Passageg of R'adtianlt iteat-.-l-rausmisslon of lieat throllu1h Opaqrpte rles..-... hleat-Speletrult deltached tfrom Telit-Spetrtil Iby an Opaque Prism. 274i APPENDIX TO OtiAP-'Eit N. Ctalibration of the (alvanom1eter......... 01 Action of Odorosl Substances upon Radiant eiat- -Actioin of Oz(one upon Radiant li eat —)Detrmintiuatlon of thie 1Radaltion arin Absorption of (aises and Vaplors, without any Source of Ieat external to the Gascons Blody —-). tDynamic Radiation and Absorption...-t Rladiationt throug1t1 the Eartht's Atmosphlrc-.- nlfluec of the Aqueous Vapor of trhe Atmlosphere on IRadiant lleat-. —-Conncicton of the Radiant and Absorbent Power of Aqleous Vapor witih Meteorological Phenooena....... 810 APPENDIX TO C)iAN"~ER XI, Further l)etails of tlre Aetion of Hluid Air... 8:14 CH[APTER XII. Absorption of l[ernt by Volatile Liqurids-.-Absorption of [eat by the Vaplors of those Liqulids at a ('Commoll Pr'esstire —-Absorpflion of IfHeat by tife atliro Vapors whenV tl e QuantIities of Vapor nare prolportlonat to tile Qualities of llqtuid..... Comparative View of tile Action of,iqiis anrid their Valtors upotln i adiant lcat-.-Physical'Cause of Opacity lant'T1aramstarnelry. -ln.rllulece of'T'etperatliure onl thle T''rarnsmnisslon of tadiantt lie at —.-C.langes of Position thirourghi Clhaniges of'i'tetper-atrrc —..Radiationr from ld'ainc —. flnuence of Oscillating Period on tire'lransmiltsslorl of Ralialt 11ctat-.-Explallnation of Certain Results of Mellonl alnd Knobach.. 851 S.x CONTENTS. CIJ[tAPTE11 XIII, Dis covery of ])ark Solar Rays..e..ichet l nlnd M3tIlter's E)}xpecrelicnts —]liso of Inlt-ensity with Temlperatr.tlre-. I[cat of Electrle Spectrunt.-. -lRay-iilttcs: Siftilng the Electcitec Light —'llanlslmtatlon of layvs -— Tl'lermal Illnago relered ilminlllus —Comnblustlon and Incandetscence by X)a'rk Rlays. —. F'It resene anll It Catlerescen c.-. i)ark Solar ]layis.-D.ark Lime-light Rlays -—.fanklin's E.xperimelnt on Coloris. —Its Analysis and Explana[iOl. PA.A:. 3t1 C011APTER'l,' XIV, D)ew: A Clear Sky and C(altn but ]Damp Atmosphere necessary for its Coplous Formationl 4... Deowed Substan;es colter than Undowed Ones-...-Dowed Substancies better R1adiatolr than UIndotweAl Olnes.-l)ow i tthe Condenation of tlhe Atmosphllerc Vnpor on SubstlInces whi8ch h av been chilled by Radlatton —-I-imnar RadiatioLunar Rada tConstitutilion of th Sun-tll fl'c rigt t,tiles ill the Spcctra of the MAetals-. —An Incandescent lapor absorbs the Ralys which'it cant iIStf (tmit-e - mitrelhotl's (entcralisation.io....Fraunhofecfs lines -.. Solar Cihelisti13....... - siotn of thet 8u11n- -te fcrlet el8and 1'outtlt's tl Experh'ltclts....Mayer's hMeteoric LThor — -- hcorles of Itellmtnholtz and tlhomson.l.-.'Effect of the idels on t1he'artlih's Rotationl -..niergdies of the Solar Syast.inl —i [icnheltt',h Thomson, Wato'rstton-ll Relation of the Stiun to Animal and Vegetable Lif.25. CHiAPTER{ XV. Action of Ethler-Wn aves of Short 1'eriod upon G(aseous hMatter —Clouds formed byT Actinic Decomposition- -Color lrodiicel by Small Pasrlicles -—. Polarizatlon of Light by Nebullous Mattci-.-Tlhe Blue of the Sky and tloe 1Polar.a0tlo11 of its Light.. 461 Rtemnarks onl lthe Convertlibility of Natturalt orccs.o... s603 Oi Cometartt ylhcory.. O'....0 l'olartizstion of lHeat.. 61 lContc-ldinmi Additions; Anticipattons of [tle Mcniital Thleory of I cat,. t2 Production of Fire by Savage' s.6 t *.. t14 ornting Chill ptrodtIttcint Snowl t 0 a Room01. 61 iNDEX.X. ~ ~ 611 AS.A M.I O:1):' () ]i' Y 0 N1:1:0 1ON. 1NSTRUJM;IH g-.0C ——.O} AHI~ P0 1.AT II Y gRITIO10N, CO.MPR11-,',i'O-, AND PRtVQU. SOS-']X.-RIt-lN'!'' l*UMYI......'g'',,}:1L; t~'lC IO NS....... —-ONSUSM''ION 0I' 111-'.AT II APPENDlX l..-N'-'0>T ON IEll'fEI- M0-E:EOI1:r10 l:,} ANI) AND (AtiYANM:OFI'Il-. (1) ri'v-wl aispeets of Nature plovolke in man thle spirit of I inqu'ry. As the eye is made for seeing, and the car for lcaring, so the human mind is formcd for exploring and understanding the relationship of natural phenomenla, the science of -our day being the direct issue of an intellect; tus endowed. One great characteristic of Natural Knowlvedge is its growtht; all its results are fiuitful, every new discovery becoming instantly the germ of fresh investigation. 3But no nobler example of this growyth can be adduceed than the expansion and develolmlent, during the last fivc-and-twenty ycars, of the great su bject whic is now to occupy our attention. In scientific manuals, only scanty reference was, until lately, made to the modern 1)hilosol)hy of hcat, and thus the public knowledge regarding it remained below the attainable level. Thel reserve, 0however, was natural, for the subject is an entangled one, andt, in the pursuit; of it, we must be prpl)ared to encoulter difficulties. In thle whole range of Natural 1 2 lISEAT AS A IMODE, 1OF MOTION. Science, however, there are none more wortthy of being overcome -none the subjugation of whichl insurc s a greater reward to the worker. For the various agencies of Nature are so connIctcd, tlhat, ill mastering the laws and relations of heatr, we make clear to our minds the interdependence of natural poywers generally. L-et us then commence our labors with heart and hope; let us familiariz.e ourselves withl the latcst facts, and concepltions regarding this all-pervading agent, and sccek diligently the links of law which collnect the fact's and give unity to their most diverse appl)earances. If rwe succecd helre, we shall satisfy, to an extent unlnown before, that love of order ald of beauty which, no doubt, is impllantcd in tlhe mind of every I)elson here p)-resent. From the heights at which we aim we shall have nobler gliml)SCS of the system of Nature than could possibly be obtained, if 1, while actilng as your guide in the region which we this day enter, were to confile myself to its lower levels and already trodden roads. (2) It is ly first duty to makle you acquainted with some of the instrumnents intended to be employed ill tli examiltnation of this question., Some means must be devised of mnaking the indicationls of hleat and cold visible to you, and for this pl)')Iose an ordinary thermometor would be useless. You could not see its action; and I am anxious that y6u should see,'with your own eyes, the ficets on which our su\bseqluent )philosoplhy is to be based. I wish to give you the material on wthich an inldetlndent judgment may be founded; to enable you to reason as I reason if you deem me right., to correct me if I go astray, and to censure me if you finld me dealing unfairly withl ly subtjct. To secure tlhese ends I have been obliged to abandon the use of a common ther)nrometer, and to resort to the little instrumenlit iwhich yout see before me on the table. (3) Thllis instrumenIt A n (fiF. 1) is called a thetmo-eecthiec pile.* It acts tIhus: The heat which tie pile receives gener*A brief description of the thenrro-olcetrie pile is given inl the Appendix to this Chapter. TI IARMO- EE TRI P1,E AND) FGALANOMJIT. 8 ates nl electric clurrent; and an electrio current has the power of deflecting a freely suspended magnetic needle, to which it flows parallel. Before you is placed such a needle nm " (fig. 1), surrounded by a. covered copper wire, the free ends of Fro. 1. f, wichi, W are connected c( with tt tiher lno-eceltric pile. The needlG is suspended by a. fibre, s s, of unspun silk, and proteeted by a glass sha de %w from all distturlbmnce by currents o~ air. To oei end of the needle is fixed a. piece of red, and to the othler end a piece of' b)lue paper. All of you see, these pieces of iampelr, and when the nceedle moves, its mot.ion will be clearly visi)blc to the most distiant p)ersonl in this room. Thllis instrument is cal(led a gal(JanOmeteI-'.' (4) At )preseIntthle nec(lec is quite at rest, and points to the zero-mark on the graduated disk underneath it. This shows tlhat therc is no currenlt passing. It lbreathe for an instant against the naked face A of the pile. -a single puMr of l)reati is sufiecient for 1my l)urpOsc-t.e needle starts off and passes x Ti tlhe actual arrangeent the galva.nomleter here deseribed stood onl a stool in fr'ont of the lecture table, tlhe wires vw w bleing sufticiently long to reach from the table to the stool. For a flut.her description oof tle instrument ace the Appendix to this Chapter. 4 HH I1bAT AS A MOD)E OF MOTION. througlh an are of 90. IXt would go fartlher did w\e not limit its swing by fixing, edgcways, a, tllin plate of mica at this )oint. T.his actioll of thile nccdle is )rodlucctd by tihe small Iamountlit of w\armth commlllllicafted by my breath to the face of the pile, and no ordinalry thermometer could give so largIe and rolmpt anl ildication. T.ake notice of the direct iion of the defleetion; the red end of thle lneelle moved from m towlard, you. We will loet the heat waste itself; it will do so inl a very short time, and you notice, as the pile cools, thlat the needle returns to its first position. Observe now the effect of cold on tile same face of the l)ilc, After chilling this plato of metal by ptlacing it onil ice, I wi)pe thle metal, and touch with it the face of the pile. A moment's contact sutffices to produrce a prompt and energetic deflection of the neCdlc, B13t mlark tle direction of tlhe deflection. Wherln tile pile was w-armed, the red end of the; needle moved f0rom me toward you; now time same }end moves from yol toward inle,:Ti.e implortant 1)oint here establislhed is, thlat frolm tihe direction in which the 1needle lloves we can, witth certainty, infer whether cold or lheat; has been commlunicated to tile pile; andl the energy with whtich the needle movNres-.t.the p)romptnest s w\ith -which it is driven aside firom its position of rcst — gives us somoe idea of the quantity of hlat or cold il)artced in difterent cases. IOn a fiture occasion we slhall learn how to express the relative quantities of heat communlicated to thie pile -witt numerical accuracy; for the lpresent a general lknowledge of thle action of our instruments is sufficient. (6) MTy (desire now is to connect heat with tile more famniliar forms of force, furnishing you, in the first l)lacc, with a store of facts illustrative of the generation of hleat by mechlanical processes. In the next room are somle pieces of wood, which rmy assistant; will hand to me.'.Phe temptlrature of that- room is slightly lower than the telclerat.lure of this ole, and hlence the wood which is now before me muslt he1 slightly coldler than the faee of th;e pile. Let us p)rove this. T'1he face of the pile being pl)aced against the picce of wood, the red end lA]'ATI' 01 lF:RICTION. 5 of tlil nccdle moves from you toward me, thus slhowing that tlhe contact hlas chilled tthe inst, rument;. I now carefutlly rub the ftce of the pile along tile surfitce of tile w eood —-. —" carefully," because the lile is brittle, and rough u.sage would destroy it; narklt what occurs. Th'e romplt and energetic motion of the needle towa(rd you declares that the fee of the lpile hlas been cheated by this small anioutlt of friict ion. Thl'e ncedle you observe, goes quite up to 90~ ol tlhe side oplposite to that toward which it mltoved before the friction was applied. (6) Tltlese experl'mnlts, which illustrate theII( development of heat by mechanicai means, must;be to us w\\latt a hoy's school cxrciscs are to him. In order to fix them in our mindsll and obtain due mastlry over themll, we must repl)eat tllem and vary them in many ways. fInl this task tyot lha\e now to accompany nme. This flat piece of lbrass is attached to a cork, which, whe)1 taken hold ofl, 1 )leserveC t1he brass fr'om all contact with my warm hand. ~W, hen' the brass is 1)laced agtainst the face of the pile, thle necdle moves, showing thlat; thle metal is cold. I now lrub the brass onl the surface of tllhs cold piece of wood, alld lay it once more agailnst tile pile.:It is so hot, that if allowed to remain ii contaet:t witht the instrument), the current generated would dasht tle nccdle violently against its st0ops, and probably derange its magnetism. You see the strong' d(lefction which even an instant's contact canl produce. Ilndeed, when a, boSy aat school, I h]ave often blistered Jmly hand by a brass button \which had been rubbed energetically against a form, This razor has been cooled by contact with ice; and along this hone, wit. hout; oil, I rub t he cool razor as if to sharpen it. 01ln placing the razor toagainst the facc of the pile, thle steel, Mwhtich a. momen nllt ago was cold, is declared ]hot. Similarly, I take this knife and k(nife-board, whlich are both cold, and rub the, knife along' the board. Tlhe knife, placced against tlhe )pile, declares itself to be hlot. I )ass this cold saw throutrgh this cold piece of wood, alld lace, in the first; instant e, tile surface of fthle wood alainst; which tile saw has rubbed, ill contact with tlhe pile. The needle in 0 1 t111T AS A MOD1), 01F MOTION. st-atily moves in a direction which shows thl e -wood to bo lheated. Allowinlt' the needle to return to zero, I apply thle sawt itself to tihe p)ile. It also is hot. These are t}he simplest and most commonplace examnples of the generation of leat )by friction, and they are chosen for this reason. Mecan as they applcar, they are illustrations of a principle Nlich d(etermines the 1polity of tlhe whole material univ6c.l1se. (7) ^No have now to consider 1the dovelopment of heat by compression.'This piece of deal is cooled below t1he tempcrature of the room, and gives, when placed in contact with our )ile, the deflection which indicates cold. I introduce the wool b)et\eenl tile plates of a small hydraulic press, and squeeze it forcibly.'When, after comtilession, thle wood is brought into contact with thlle ile, the galvanometer declares that heat lhas been develoled lby the act of compression. Pr1ecisely the same thing occurs wlen tltis blockl of lead is fixed between the pllates of the press and squeezed thus to flatness. (8) And now for the effect of percussioni. I place a cold lead bullet upon this cold anvil, and strike it with a cold sledgc-hammer..lte sledge dscends witht a. certain mechanical force, and its motion is sutddenly larrested by the bullet and an.vil; apparently thle force of the sledge is destroyed. But whentl we examine the lead wre find it is heated, and we shall iby-and-lby lclarn that; if we could gather u1p all tfite heat generated b)y the shockl of the sledg'e, and apply it without loss mcchanically, we should boe able, b1y means of it, to lift the hammer t to the heig)t; from wlich it fell. Another experiment is here arranl ed, whichl is al1most too delicate to bef periforICmed with the large taplaratus necessar to render lecture experitlents visible, but whvich, nevertheless, is easily executed with 1)roper instruments. This small basin contains a quant-ity of merculll which has been cooled ill the nlext room0. One( of the faee's of 1the thetrneo-electric pile is coated Nwith varnlishl, to deftend it froml the merculry, which would otherwise destroy thle pile. Thus )protected it l mayi, as you observe, be pliunged into lthe liquid metal. T'he ideftct.ion I1ATlOX' OF COMPREfSSION AND) PfRHUSSIfON. 7 of thle needle proves t that the mercury is cold. These two glasses, A and J~ (fig. 2), are swmathed thlickly round by listing, to )prvent thle warmttlh of my han(ds from reaching the mercury. I pour tlhe cold mlrcury ftrom the one glass into the other, and back. it fills with a certail mechanical forcet its motion is destroyed, )but heat is developed. The amount of heat generated by a single pouring out is extremely small; the exact amount might be easily determined, but we shall defer (luanltitative considcnrtions for the present; so we will pour the miercury from gl ass to glass tenll or fifteen tiilcs. Now mark the result whecn tile pile is plungc d r. 2 into tle liquid.'The needle mo ovesl and its motion declares that the iA llf mercur'y, which at the t )! begilnning of t1he expleriment was cooler, is now warmetr ftlan th le )ile, We here introduce into' _. the lecture-ro00om1 an effeet whichl occurs nt the base of every watcr-fall, -—. There are friellnds bcfore:'<': 9 me who }have stood amid tlhe foam of Niagara. IIad they, when there, dli)ped sutficiently sensitive thermometers into tile water at the top and bottom of the cataract, they would ave foiund thel lattcr warmler thlan the former. The sailor's traditionl, also, is theoretically correct; tile sea is rendered warmller by a. storm, lthe mechanical dash of its billows being ultimately converted into heat. (9) Whetnever friction is overcome, iheat is 1troduced, and th;le heat produced is the exact measure of the force cxpendled inl overcoming the friction. The hlcat is simply the prinlitiveo force in another form and if we wish to avoid this conversioll, 8 H VIIEAT AS A MODEl OF OTION. we mtust abolish the friction. tie put oil upon thle surface of a hone, we grease a saw, and are careful to hlubricate the axles of our railway carri-ages. What is the real meaning of these acts? Let us obtain general notions first, and( aim at strict accuracy afterward(, It is the object of a railway cngincer to urge his trailn fiom one place to another; lhe Nwishlcs to apply the force of his steam, or of the furnace whlichl gives tensioVn to his steam, to this part.icularl purpose. It is not his interest to allow any portion of that force to be converted into another form of force which would not promote the attainment of lis ob)jet. It c does not want his axles heated, (and hence ihe avoids as much as p)Ossible exl)ending his powe\ in heating theml,. aln fiact, lie has obtaincd Ihis force from heat, a t; its fireezing temp)erature, )possesses a vastly greater amount of heat than ice at the same temlperature. (22) )Davy reasoned thus' "If i, by friction, liqucfy ice, a substance will be potduced which contains a far greater absolute amount of heat than the ice; aind, in this ease, it cannot Nwith any show of reason be aflnrmed that I; merely render sensible heat which had been p)rcviously insensible in the frozen mass. l qucfiaction in this case will conclusively demonstrate a lenCr'tion of heat." ice made the experimentCand liquefied the ice b)y pure friction; and the result has bccn regarded as the first which really proved tfle inmmateriality of hleat. (23) Wthen a hamtner strikcs a bell the motion of the haimmer is arrested, but not destroyed; it Ihas bee shivered into vibrations, which impart, motion to tle air and aflect the auditory nerve as sound, So also when our slcdge-hmamimel Works of Sir it. I)avy, vol, ii. p. 11. 0 26 lIhEAT AS A MODEl OF 3MOTION. descended lponitour leadl bullet,, the descending motion of tlhe sledge was firrested; but it was not desitroyed.:l7e motioil was t-ras('ifrred to the atoms qf the letl, anld alnnounced itself to the proper nerves as hetat,'The} theory, then, whlich Rumford so powerfiully tadvocated, and whichl ])avy so ably supp)ortetd,* is that hIeat is a kinld of molecular motionl; and th;at by friction, percussion, and comll)rcssion, this miot ion may be generated, fas well (as by combustion. This is tlhe tlheory which it slhall be my aim to developl until ytour minds attain to perfect; clearness regardingl it. -At the outset you lmust exrcise patticlcc. -re ar c ntering a jlungle, and striking into the brambles in rather a random ftashion at first. 3ut we shall tlhs make ourselves acquainted with the general character of our work, and wvith duc per8sistence cut tlro-ugh all entanllglements at, last, (24) You have ahlrcady witnesssed the effect of 1)rojct ing a. current of compressed air tagainst thlie fhee of tlhe thcrmoelectric pile (p)ag' 14). The instrument was chilled by the current of.air. NTowv he(at is known to be developed when air is compressedl; and Ilt have been asked repleatedly how this hIeat was dis5posed of i tile case of thel condensed air emp)loyedl i tlhe cxperiment referred to. Pray listen to my reply. Supposing the vessel which contained the air to )e forlnmed of a sublstance l)crfectly imperviouls to heat, anll suplposing all tlie helat developed by my arm, in compressing thl.air, to he retained within the vcsscl, tha(t quantity of hecat would be exactly competenlt to undo w\hat had been done, and to:restore th-le compl)rcessed air to its original volunme and tempcerature. B1u this vesscl v (fig. 12) is not impervious to lheat, and it was not )mty object to dr\aw upon the lt eat develo)ped y mlly a1n M; I1:I)avys fi.rst scientific memoir lhe canlls heat a repulsive motion, swhich Ihe says may be ailugmlented in various ways.' Fir-ist, b)y tihe tran-smutation of mechanlical into repulsive 1motion'; that is, by lfictionm or per'cussionl. In th}is case tlhe (callanical motion- lost by thoe, masses of 1-matter iln fiction is the repulsivo motion gained by their corpuscles;i an extremely remarkabtle passage, I have givenl firther extracts ft'om this paper ill th.ie Appendix to Chapter 1ii. 1FIR' SYRRING(tI, 29. after condensation, therlCforc, the vessel wvas allowed to rest till all the hecat generated by the condensation was dissipated, and the temperature of the air withtin and without the vesscl Fle. 12, was the same. W~hen, therefore, t'he air rushed out:, it had not the hcat to draw upon whnich laid tccn developed during comprllession. Th'le lheat from wlhlich it drivtcd its elastic force was only sufitlcient to keccp it at- the timperatl ture of the surrounding air. In doinlg its work,I portion ot this heat, equivalent to othe'work done, wa CtS oltlsced, lid the issuing air was consequently chilled. D)o not be disheartcned if this reasoning shiould not appear quite clear to you. Mre are now ill comparative darkneiss, but as we proceec light will gradually applear, and irradiate retrospectively our present gl0oolm. (25)) Iet me now nmake evident; to yon that lieat is developed by the compression of ail. Ilere is a strong cylinder of gla;ss' U (fig. 13), accurately bored, and quite smooth witliln. Into it this piston fits air-tig'ht, so that, by driving the piston down, the air underneath it is forcibly compressed; and, whlenl tle air is thus compressed, heat is ts suddenly generated. Tindci may be ignited by this heat. Here, moreover, is ma morsel 28 l;ATi AS A MO])DE' OF [MOTION. of cotton wool, -wretted Nwitlh a inflammable liquid, tlhe bisulp)hide of cl'arbon. tr tow the bit of wet cotton into'l:. 1:3, the glass syringe, and instantly eject it. It; has left b e}lindl it a. residue of valor. Onl compressing tile air suddenlyl tihe lheat developed is sulflicicnt to ignite the vaplor, al(nd you se a flash of ligtlit withlin the,Syringe, Nor is it necesslary to eject the cotton; I replace it; in the tube, and urge tlhe piston dow\nard; you see the flash as before. If wvith a narrow..;-fi;t g':::~j~ k. \\: Ti:: manly of you see a coloring of tlhet cloud, as the lig'ht shines tluhrlough it:, similar to that obs'erved sometimes, on a. large scale, around tile m)oon.'Whell thle air is allowed to rceenllter the vessel, it is lcated, exactly as il thle Cexl)rimlnt with ore' FRLIOTION AGAINST SPACE. 31 tin t:ube; the clou(ld mclts away, anl t{he perfect transparency of the air within th}e receiver is restoredt.* (29) Sir I:unmphry Dl avy refers, ill is Chemical Philosophy, to a machline at Schemnlllitz, in ltungary,l in whicl (air was COml)ressed l)) a column of wvater 260 feet in heighlt. Vhtien a stopcock was o)lened so as to allow the air to escape, a dlegrcc of cold wals plroduccd which not only prlcil)italted the aqucouls va11)tor diltlused in the air, but caused it to conge.Al in a shower of snlow, while the pile from which the air issued becamei bearded w\ith icicles. " D1r. ])arwill," writes Da)v, ") as ingeniously explained thle production of snow on the tops of the hlighest mountains, by the prlccilitation of vap)or from thl rarefied (air whtich ascendlts fronl plailns altd vlleys. Theli Andles, plaeed almost unIder the line, rise in thic midst of blurning sands; about thcl midtle heihtll is a plcasantl and mild climate; the summlits -are covered wivitl ullchalling s811tOVS." (30) And now I would request your attention to an eOxperiment, in which heat will be tdeveloped by what;1 must appear to many of you a verye mysterious nagcncy, and indeed the most instructed alnolng us know, in reality, very little about the subject. I wvish to develop lheat b)y hllat might be regardedl s friction tagainst pure space. And, inl(lcd, it may be, and l)rlob)ably is, duIe to a kind of friction agatin-st the inter-stellar medium, to which we shiall have occasion to recfe more fully by-and-by. " A fin more beauttiful mode of demonstratioll was subelqulently resolted to. Removing the lens from tle camera of an electric lampn, tile rays flom the coal-points issuled divergent. A largo plano-convex len.s was placed ill friont, so as to convert the divergo nt cone intoe a cenvergtent one, and caused the cone to pass through t0he receiver. Its track was at first invisible, but two or three strokes of the pif)llp precipitatCd the vapor, and then the track of the beant throulgh tte receiver resei bled a white solid balr. After cirossing thc reciver, thfe light fell 1pon a white scrc-n, and oexhibiited splendid liffitaction colors when tlhe cloud formed. In my recent Cxtlerimtts, clouds of flar greater density, eritnanaccc ll, and splenldor of color tlanl thlse obtaitnable fiomn aqueous vapor have been prodfuced. Aqueous lydrlocelorie acid yield.s such clouds. See Proceedings of Royal Society, vol. xvii. p. 3147. 32 I1MPU l AS A MOD])E OF MOTTIO0. (31) ][eore is a, mass of iro — part; of a link of a. huge clihaill cablle —.\which is surrounded by these multiple coils of coIper wtire c e (fi'. 15), atd whlichl clan instantly be converted into a powerful magnt;Let y sending an elect-ric current through the wire. You see, whcllen thus excited, how powerful it is, This poIer clings to it, and tlhese clisels, screws, and nails cling to the pokce. Turncd uj)side down, this magnct will ]hold a halfhundred-weightl attached to each of its p0oles, and probably a score of thle hcavicst plcople in this room if susp)endcd from the weiglhts. At the proper signal my assistanit will interrupt the electric current,.'Ilhe iron falls and all the magic disappears: the magnet; now is me reC common iron. OJn the enlds of tile magnet are pliaced two pieces of iron, P r -lt. ovable poles, as they are ctllcd......... which, when the magnc t is unlcxcited, can be brought Within any relquir'd distance of each otlhcr. Wllen tihe Cxciting currnt passces, these picccs of iron virtually form parts of the magnet,. Bectwccn tllem I will li)ace a substance whichl the magnet, even wil en exert-ing its utmost power, is inomll)petent to attract. This substance is simp )ly a piece of silver — Ain fact, a silver medal. When it is brought close to the excited magnet;, no attraction cnsues. Indeed, what little forcc-..and it is so little ass to be lltterly insensible in tLhese Cexprimcnts — the magn-let really exerts uplol tile silver is repulsive, instead of attractive. (32) Sulspending this mcedal betwccn tle poles v r of the magnet,, 1. send tlhe current through tile coil, The medal hangsl between thle poles; it is neithler att racted nor repelled, bt)t, if we seek to move it, we encounter resistance. To turn the medal rolund this resistance lust b)e overcolme, tile silver lovting as if it wcere surrounded lby a viscous fluid. This extraordinary effctcmay also be rendered manifest in anothelr way. Causing this rectangtular plate of copper to pass quickly to and fro like at ssaw b)etwccn tlhe pols, 1,, \\thl tllheir points tulrnel toward it; you seemt, t;hough you can see nothling, to be sawing ILLUSTRATION 0lt FRICTION AGAINST SPACE. 33 N~~~t ~ 7~~~~~~~~~~~~~~~~~~2 (jJl11. L-#$ 9j:{ jc I~~j~~~~~ —:i 34 II1qAT AS A 510) ODE OF MOTION. through a lmass of chltcse or butter.* No effect of this khind iqs noticed 1when tite magntc is not active; the copp)er saw then tncotuters nothing ll butt tlhe illftiutesiinmlal Iresistance of the atir. (33)'lhus far you lhave ben compelled to take my statements for granted, tlut an exSperiment ish1crC arranged twhichl will malke t~his strange action of tile maglnet on tlhe silver medal strikingly manlifcst to you all. Above tlte suspended ledal, aInd, attached to it b)y a bit of witc, is a little relectcing pyramid m ~, formed of four trianllilar pieces of looking-glass: b)0oth the lcdal tand thlle reflecto r tare slustpended by at thread whlichl was3 twisted in its malmacturit aInd wllich will untwist it'self whenl the wcightt which it sustains is set free. When froll this electric lamt p aL strong bteam of lif.glt is caused to ftll ulpon the little tyranidt the lilght~ is reflected, atnd you see these lolng' i h luninolls spokes moving, through the dusty air of tihe r0oo0m as tihe mirror turlls. (34) let us start it from a. state of resst Th'ile beam now )passes tlrougt thle room andl strikes against tle white, wall. As the mirror commenccs rotating, tle })pat:c of lighlt moves, att first slowly, over thle watll and ceiling'. T.'he motion lquicklens, and now you call no longer se tlhe h distinct pat:elCs of light, but instead of tclhem you have this splendid luminous bandz filly) twcent0y feet in diameter drawn upon thle wall by htle q(uicl roftation of the reflectcd beams..At the wVord of commlaLnd the magnet will be excited. See the effect: the medal seems struck dead by the magniet, t1h-e lband suddenly disappears, and thero you have the single 1)atchl of light upon thle wall. This st:range result is produced withoult any visible change in the space betlween tli two poles. Observe thle,slight motion of the image on thie wall: tile tension of tilhe string is strupglint g with a un11 seen antagoll nisollit alld prl)oilnig thai:t motion., It is such as would be produced if tl}e medal, instead( of beihlng surroundled by), atit' Lre iler lttl'Secd in a p)ot of treacle. Oin destroying the mllgnetic power, the viscouCs chatir-' Al oAlxeriment. ot l'radtlay's. Ile also wsn tile fistt to ntrst by a,tngnlot tho m1otion of a slpinningt cube of copper. DJ)EVEL,OIMEMNT OF HIEAT, 35 aclter of the space between the jpoles instantly disappears; t he mcdal begins to twirl as before; there are tle revolving beamslls antld there is now the luminous bandiS. I again xcite the Imaget the beamls are struck mlotionless, alnd the band disappears. (35) lBy the force of the hand this resistance call be overcome and the mcdal turned round; but., to turn it, force must be expended.'What becomels of that force? It is converted into heat. qThe mcdal, if forcibly compelled to tturn, will l)ecome heatcd. nMany of you are acquainted with the grand idiscovery of.'araday, that electric currents are' developed \hen a conductor of clcctricity is set inll motion bettween tlhe poles of a magnet. WVe lave these currents Itere, and they are comletent to iheat the medal, hlut what (are these currents? Htow are they related to the space between the inagnetic poles — -low to tite muscular force which is expended in their geeneration? Wc 1do not yet know, bitt we shall knlow )y-anltd-by. It. does not in the least lessen thel interest of the experiment if the force of mlly larl, )revious to appearing as heat;, a)lppears il lnothlr form l...... -int the fol'rm of lctricity. The result is the same' thec heat developed tultimately is tlle exact equivalent of the power employced to move the medal il the excited malgnetic field. (36) I: wish now to make evident to all Iere present thlis development of heat,. HIere is a solid metal cylindert, the core of whvich is comlosed of a metal more easily lmelted t.Xhanl its outer case. t he outer case is cot)per>) and this is filled by a hard but; fusible alloy. The cylinder is set upright between tthe conical pole)Cs, ri i (fig. 16), of tflhe magnet. A string, s s, passes fromll the cylindc r to a wlirling table, by which the cylincder may be caused to spin round. i t might turn till doomsday with the magnc t unexcitcd, and not )produce the effect soutght; butt, lwhen the magnet is in action, an amount of liheat will be developed slflicicnt to melt the core of tlhat cyliitder, and, if successful, T l will p)our thlte liquid metal out before you. T'wo minuttes will sutlice fi:or the experiment.:t5lhe cylinder is n\ow rotating, its ulpper end being- Opl)n. \ 36 }IBAT AS A 01MODF OP MOTION. vill per"mit it to remain open until tile liquid metal is seen spattering over the l)olcs of thle magnet.. Tle metallie spray is already therec, t, houghl a minute has scarcely clapsed Exo. 16. since tile commencelmllenlt of the expcrimcnt, I now stop the motion for an Inoment, cork il l tie cnd of the cy'lindcrl, o as to prevent the loss of the mcet-l, and l lct the action continulle for half a. minute longcir Tile cntine imass of the core is now melted. I witldraw the cylindler, remove the cork, and flius 1pour' ouit before you the li(lqufield alloy.* (37)!:it is now time to consider more closely thanl we have hithelrto done the relation of the ~healt dcvclopled )y mechlltnical action to the force -which produces it.:)Doubtless this relation floated in many minds lbefore it received either correct nunlciatiion or experimental 1)roof. t'lc celebrated en-nXtgolfica' cltcirained thle idea of tile equivalcnce of heat anld mcchainieal wvork; and the idea has )een developed by his nelephew M. S6guinl, illn is workl On Ol tle ^InfluIie oof.tRailways," printed ln t1839. h'lhose vwho reflect onl vital procsses........ onl the changes which occur in the animal l)ody —--—. anld thle relation of tihe forces involved in food to muscullar force are led naturally to entertlain the ideta of interdependencc bletween tilcse forcess. Tt is, therefore, not a matter of surprise t hat tlhe man who wvas one of the first, if not t{he first:, to ra.ise thl idea of equivalence between heat and( meclanicl c energy, a-nd of tlhe mutuall conl Tlhe devClopcnellt of Ileat, by causinl a conlductol to revolve between the poles of a matgnet was first ecfrectecd by Mr. Joule (Phil. Mag. vol. xiiii, 3d Sceries, year 18,43, pp. 355 and 439), and his expeilment was afterward rec vivcd in a strikint fobri by M. Foucault.'Thle artifice above (dcseCibecl, of flsing tile core out of tihe cylilnde, rendellrs til experiment very effcttivc in a lecture-room. MAYJ'R AND) JOULB. 37 vertibility of natural Ipowers generally, to true philosophic learness in his own mind, was a. physician. ]) r.: Mayer, of Icit,1brol1, in Germany, enunciatced in 18423 thle relatiotn wvltich sutbsists tet\ en the forces of inorganic nature. ttll, e first calculated the " mclecanical equivalent of lheat, and followecd iup, as vwill be sltownI in due tilne, the statement of lthe princitple by its fcarless a)pl)ication. luhit the intuitions of Mayer required experimental proof; and to Il)r. Joule, of AManchester', belongs the hlonor of being; thle first to give a decisive demonstration of the correctness of the dynamical theory.t:llntirely indelpendentllt of:Mallyer, land undisml-ayed by thle coolness with which his fitrst labors aippeiar to have been received, hIe )ecsistedl for years in his attml)ts to lprove the invaria.bility of tlhc relaltion of heat; to ordinary mechanical p)ower. J te placed water in a suitable vessel, agit;ated the water by paddles, an-d (letermined both the amount of healt (Icveloped by t},e stirrinFg of the liquid, and the amount of tlabor cx)endedl in its ploductioln. lie did tlle same with mercury and wit}h sperlm-oil. lie also caused disks of cast-iron to rub against each other, and measuredl the heatt produced by their friction, and thie force exptcldced in overcoming it,. Ict urged water throught calpillalry tubes, and tdetermined thte amount of hteat; generated by tlhe friction of the liquid agatinslt the siies of the tubes. And his experimen ts lteave o sh)tdo of doub)t * Liebig's Annalen, vol, xlii. 1). 233; Phil. Maog., 4th Series, vol. xxiv. p. 3'l; anld in r htm, hil. tg, iMag,. vol. xxv. p. 378. I am indebted to Mr. Wheatstone for the perusal of a rare and curious pnamphlet by G(. RIbelnstein, with thle fbllowing (translated) title: Progress of our'Tilme. Generation of I eat.without.l'uel; or, D)eslcription of a Mechnnical rlocess, based on physical and mathematical l)root's, by which Caloric lany be extracted from Atmospheric Air, and in a high degree concentrated. I'he cheapest Substitute ft r Ftel ill most cases whero comhlustlo is ncessarnty.i Rebenstein dedu(ces fromt the experiments of l)unlong tlhe quantity of hteat evolved in tlhe compression of a gfas. No glimpls) of the dynamical tleoiry is, however to e foild illn his paper; his lheat is izatrt' ( }uii-ievfo) which is squleezed olt of the air as water is out of a sponfIe. t PhilT. Ma(tg., Aug. 1803. Mr. Joule's experimtlents onil thle mlechanicalf equivalent of ]heat extend from 1843 to 18i9. 38 IIEAT AS A MODE OF MtOTION. U1l011 the m dilnd that;, unlder all circumstances, the quantitry of ]lat cenleratced iby the same amount of force is fixed and invariable. A givel expenditure of force, in causing' tlhe iron disks to rotate against eacht other, produced precisely the same allount of heat; as when it wa\\ s pl))lied to agitate water, mercur)y, or sp)erml-oil. At the end of the ex)pcerilcntl, -s the teperatrd' s in the respective cases would, of course, be very diflerent; the tempnlcrature of water, for examplle, would be only 3t1th of that of mercury, because, as we w lalready know, the capacit~y of water for hicat is thirty times that of mlercutry. ]D)r. Joule too]k this into account iln reducing his experiments, ans d found, as ale'ady stated, that, howvcer the tcmpl ratures mig'ht differ, in conslequence of thle diferent capacities for heat of the substances employed, t/he absolute a2tount of eat fgeinrated by the same expenditlure of )ower w\as in all cases the s,1came. (38) In this way it was )rovedt that t he( quantity of heat necessary to raise one pound of water one degree ]?ahrenheit in templ)rature, is cqual to that generated by a pountd lweight, falling from a. height of 772 feet against the earth. Convery, tl\e amount of heat nccessary to raise a pou1nd of water one decgrce in temelraturel, woutl if all app)lied mtelhanically, be coml)etenlt to raise at pountl wcight 772 feet high, or it -would raise 772 1bs, one foot high. The term "foot-poundl " thas been introduced to express in a convenient way tile lifting of one pomld to the height of a foot. Thus, the quantit;y of heat necessary to raise the temperature of a p)oundt of water one degree ]Fahrenhit beCill talen a as staldard, 772 foot-pounds constitute wla\ t is called thre mzechanical equtivaltet of hete. If tle di erl e ces e celtigrade,:1,390 foot)pounds constitutte cl e(qutivalent.*i'I IS 18:I3 nl essay entitled I'Theses concerning VForce " wns picsclntcd to thle Royal Society of Copenhagenlll by a t)anisll p)hilosopl)her naledltC Colling. At this uealr (lhtNle I,. Cotlinii soghtlt to ascrtaill the qulnltity of t heat ge6ncrated by t1he i'irtiol of vario its mils again-st eachi othe ia'tld 1twanz)st othLr siubstalnces$, nidlc to dcterminell tle anllonlt of methanical wotk consurned iln its gei'erAti.iotn. In al accoutnt of h}is researelics grivent 1by hiltsel int tle P'liilosophlical Magaziile (vol, xxvii. p0, 5), he states that thle result of his experi 'MEIIIHANIOAL E QUIVALENT1 " OF 1lEAII'. 39 (39) In order to impl)rint upon your minds the thermal effect produced by a body falling from a height., It wlill go tlthrough the ol)eration of allowving' a lead ball to fall from our ceiling) up)on this tloor. Thait the ball is at the pr1)esnt moment slightly colder than thle tair of this room is proved by) bringing the lead into contact with thle thelrmo-clect-ric pile; the dtelcciion of the needle indicates cold., On the floor is placed at slab of iron, intended to receire the lead, andl also cooler than tie air of the room, At the top1 of thIe house is anl assistant, wh]o Iwill pull up the ball by mncalns of a stringl; I[e owill not, touch the ball, nlor will lhe allow it to touch any thlLing' else.'L}Te lead 0now ftls and is received upo)n the plato of iron. The amount of heat generated by a single descolt is very small, bccatusc thLe height is inconsiderable; we wvill, thereforet) allow the btall to be drawn llup -and to descend thlre or four times in succcssion.'\tc have now arrived at the foIrthi collision. I p)lace t1e ball, whlichl at the comml encemlent )rodulcecd cold, again upon the pile, and the immediate deficction of the needle in the opposite direction (declares the lead to be heated; this heat is dute entirely to tIme destruction of the moving energy -which thCe ball possessedi when it; struck tile plate of iron. Accordingl to our tlheory, tl(he common mechanical motion of the lead, as a 1mnass, ha'ts beenl mets, neartly 2)0 it number, Nwas that the lheat discem,:agec d was alw 1ay s inl proportion to the ceehaatiefil energy lost. Independently of the mattlritils by wihich tile heat was gcncrated, M. Colding: found that. an almount of hl't corn. petent to ralise a pound of water 1~ C. would raise a weight of a pouind 1,148 feet high; a most remarkable result,. Coldillg starts from tle principtle thint,: as tile forces of Nature are somietling spiitual and inmaterial-.lti enrtities whereof we tare cognizalnt onlty by their nmastery over NNature. — those entities must of course be c!ry superior to every thing material in tle iworld; andl as it is obviolus thnat it is t through them only that the wisdom we periceive and at(lmire il Nature expresscs itselft these powers )maust evidentl'y be ill re — lation to the spirituatl, immaterial, and intellctual power itself thatt guides Nature inl its progress; ) ut, if suchl i's thle else, it is Conseqtellntlyt lquite impossible to conceive of these tfores s- any thlin mitaturllally mtortal or peritltblfc. Surely, thlcrelfic, the forces ouaght to be renlrded as absolute ly implmislabl)te. The ctase of M. Coldinllt Ishollos how a speuolation, tlhou..hl utterlly unlphylsieal', may, by stilslatilng expelCimiellnt be the mueans of dcvelopinlg inoportalnt. phyical results. 40 IJI.IAT t8 AS t MOt) OF tOTION. transferred to the atomls of tle mass, producing amonlg thcn tile agitation Nwe call hceat;. (40) "V~e can >readily calculate thle atnount of lheiat gncerated in tlhis experiment. The space fallen t ihrought by tihe ball in each instance is twenty-six feet. The heat: gencrated is prol)ortional to the heighlt fthrough which the body fatlls. Now a; ball of lead itn falling tllrough 772 feet would generate hetat sullicient to raise its own tempelrature 300.'an., its " calacity " being -30tl;t of that of water: lncl e, in fidlingl tlhroughl 26 feet, whiicht is in round inunbers -0S-th of f72, thie hieatf gtenirated \Nwould, if all concentlrated in tlhe ltcadl raise its. temperature onre degree. This is tle amolunit of heat generated by a single descent of thIe ball, and fouI t, imes this amount, would, oft' course, be gencrated )}by four descents. The heat genertated is not, however, all concentrated it tlle ball; a smtall portionl of it telontgs to th.e iroll on whichl thle ball Efills. (41) It is Ineedless to say, thait if motlion bte iimparted to a body by othcr means t.lnl grality, the dlestruction of this motion also lproduces heat.. A riftle-bullet when it strilkes at target; is intemisely heated. The mcclanlicl equivalenlt of ]heat enables ius to ctalculate Awit'h accuracy thie amount of hteat gecncrIated by the bullet, whlcn its velocity is knowl,. This is a l)oint wortthy of our atte ntion, and in dealing with it permit me to addr( ss myself to those of my audience who larc unacquainted even w\'it ti1he elenmnts of mechanies. lveryhlody knows tfliat tle greater thie hciglt; is from lwhich a body fitlls, the greator is the force with whichl it; strikes the eatrlt, anlld lttti t}is is entirely) due to the greater vclocit) imparted to thy body in falling from thle greater lIcighlt. Thiem velocity impartcd to the body is not, however, prop)ort1ional to te height from'whllich it falls. If thle 0height 0 be lauglentedl fourfold, thle velocity is augmented only twofold; if tlhe height be laugnc ted ninefold, the velocit;y is augmented only t;llrefold; if tei lhci(ht; be augmienlted sixteentfold, tlle velocity is taugmllente, only fourfold; or, exlressed gclerally, the height, is prooportional to the square of the velocity. TMflATION OF' IEAT TO VELOCITY. 41 (4-2) IBut thce heat generatc d by the collision of the fialling body incrcases simply as the height;; consequently,; the lcat genertated incrt-ces as the stqunare ( the velctity. (43) if, thereforc, -we double thil velocity of a projectile, we augment thte ]heat generated, whcn its moving force is destroy)ed, fourfold; if we treble its velocity, we laugment the hcat3 ninefold; if wve quadrup)l the velocity,' we augment tlhe hcat sixtenfold, and so oln. (4t) The velocity imparted to a body by gravity iln failling through 772 feet is, in round numbelrs, 223 fct la second; that is to say, immediately before the body strikes the earth, this is its velocity. Six times this quanltity, or 1,338 feet a second, would not be an inordinate velocity for'a riflc-bullet. (45) Bu131t a rifle-bullet, if formed of lcad, moving at a vclocity of 2214 feet a second, would generate on striking a target an alllountl of lheat twhlich, if concentrated in the bullet., Nwotlld, as already shown, raise its temperature 300~ 1.; with G times this velocity i;t wouldl generate 36 times tle amount of heat; lhecc 36 tinces 30, or, 1,080~, would represent the augmentation of the temperature of the bullet on striking a target vith a velocity of 1,338 feet a, secolnd, if all ftie iheat generatced wcre confined to thie bullet itself.'lhis amounl t of hIcat would be sutflicient to filse a portiontof the lead.'\\rre' the ball iron instead of lead, thle ieat generated, under the conditions supp)oscld, would be competent to raise the templerature of tlte ball only abolt -3d of 1,080, because thle capacity of iron for ]leat is aboutt three timcs thiat of lead. (46) FVrom these considerations it is manifest that if vwe know the velocity and weight of any p)rojectile, iwe can calculate with; ease the amountl of heat developed by the destruction of its moving force. For example, kno-wNing as we do the rweight of thle e-arth and the vcelocity with which it mloves throughl space, att siml)le calcultation enables us to state the exact amountl of heat twhich would be developed, sut)l)osing the earth to strike against a tatrget strong enough to stop its motion. We could tell, for cxample, the number of dcegrccs 42 3JIAT AS A MODE, OF MOTION. -which this amountt of heat would impart to a globe of vater equal to the eart'h in size. Mayer and I.:l:elnholtz have made this calculation, and found t.lh.t tlhe quantityl of lheat which would be generated by this colossal shock would be quite sufficielent not only to fiuse the entire earth, but to reduce it, ill great part, to vapor. Thus, by t}he simple stopla)age of the earth itt its orbit, "the elements " mighlt be caused " to melt -with fervent heat." Thle amount of heat thus developed would be eqlual to that derived from the combustion of follurteenl globes of coal, each equal to the earth inl magcnitude. And if, after the stoplpaXge of its motion, the earthl should fatll into the sun, as it assuredly would, the amoulnt of heat generated by the blow would be equal to that developed by the combrnlustion of 5,600 wvorlds of solid carbon. (47) Knowledge sutlc as that wvlich you now possess has causedi philosoplher.s, in s)eculalting on tthe mode ill whlich thel sun's pow\'Cl' is mailltainled, to suptlose the solar ]lcal and liglht to be caused by thle showering down of meteoroic matter upon the suln's surfae.* Th111Ce Zodiacal ]ight is supl)os d to be a cloud of meteorites, altd from it, it llas been imagined, the rain of meteoric matter was derivcd,. \Now, whatever be the value of this slpeculation, it is to be borne ill mind that the pouring downl of meteors in t.le cway indicated would be competent to plroduce the light and heat of the sun. I shall develop tlhe theory on 1a future occasion. -%ith ltr egiard to its p1robable truth or faIllacy, it is not necessaIry that I should otffer an opilnion; the theory detals with a cause which, if ill sufficient operation, Awould certainly be concpetent to p)roduttce the effects ascribed to it. (41) l'et, mel now pass from the slunl to something less........ to tile ol)p)site )ole of Nat.ure, if tile exlression be pl)rllittedl. And here that livine plower of the, human intelltect which anMayoer prol':poulndic this hypothesis in 18t8, and wfor;ked it out to a great extent. It Nwas attaerward ot mellatled independently by Mr. Waters-tonl, and adniriablv (levlopeld y Profekssor William tlhomson (Transactioins of thet ioy1al Soe. of tEdillb. 1853). Sco also Chapter I:V. ThlEO(} t" " ATOMt[c r11OJI:C tlf 59E As far back as 1.88, Mr. Joule deduced from this hypothe-.sis the velocity of 1lhydrogeCn-ttoms, and found it to be 6,055 feet p1er second. (65) According to this hypotllhesis, then, we are to figure a gtaseous body as one whose molecules are1 flying in straight lines through -space, impinging like little projectiles utpon each othel, and strikilg agalin"st the boundarics of the splece whlich they occupy. I place this bladder, half filled witlh air, under the receiver of tlhe (air-pump) anld remove thle air ftroml thle Icceiver. L'Phe bladder swells; the air \wvitlin it appears quite to fill it;, so as to remove all its folds and creases. l0owv is this expansion of the b)ladder lprotdTcee? Atccording to our Ilpesent theory, it is produced by thie shooting of atomic )ro'jectiles agcainst its interior surface, driving the envelop outlward, ulntil its tension is able to cope with tlheir force. When air is admitted into t1he receiver, the bladder shrivels to its former size; and here we must: filigure the d(ischarge of t le atolms ag{ainst thle outer surface of the bltadder, drIiving the cnvelop1 inlward, ceausingp, att the same time, the atoms withlin to colnccntrate their fire), until ftnally the force friom wit hlin equals that froml without, Mand tile envlop remains q'1li escent. All tthe impressions, thell, wicll we derive from heated air or v1apor are, according' to this hlypothesis, due to tihe impact of gascous molcules. Tlihey stir thle nerves in their ownNl peculiar way, the nerves transmit the motion to the brain, and the bratin declares it to be heat..hus flhe implll)ressio one receives on enterings the h]ot room of a Tllurkisht bath, is caused by thel atomic cannonade which is thlere maintained against tlhe sllfacee of the body. I would state this as aln hylpotlhesis advocated by clinent men, without expressing' cither assent or dissent myselft (66) If, instead of placingl this bladder under the receivcr of anl air-pump and'withdratwinglo the external air, 1I aug llent, by hleat, tlle lprojcetile forice of tlhe partlicles -wit.hlin it:, these l)ariclets, t hougll ctmparatively few int l1tlnber, will strike with sIttch impetuous energy ag.ainst, tell inner surface as to cause 0G lAlT AS A 3MODE' O MOTION. the envelolp to retreat.: the bladder swells and becomnes apparently filled wiith air; holding the b)ladder close to tJhe fire, all its creascs arc rcmnoved. T''he bladder here intercopts the radiant hecat of tlte fire, becomes warm, and thcen communicates its heat:, by contact, to tlhe air \ith}in, (G7)''This, then, is a simple illustration of the expansive force of heat, and 1: haxve made anl arrangement intelnded to show you the same fiact in anoitelr lmannelr. This flask, (fig. 20), is eml)ty, except as regards air, intended to be lheatid by placing at spirit-lamp undernealcth it. From lthle lask. bcent tube passes to this dish, containing a colorcd liquid, In the Flo. 20. dish, a glass tuie, t t two feet long, is iwverted, closed at the top), but with its open end downward. You kn]ow t:hat the precssure of the atmosl)here is competelnt to kcep a column, of liquid in this tul)e and lhcre you have it quite filled to tie top with the liquid. The tube p)assing from th:e flask( r is caused to turn u11l) cxactly underneath the Ol)Cen end of this uplrighlt tlbe, so that if a bulbble of air should issue from the former, it will ascend the latter. I now heat the flask, and thec air ex 1EXPANSION OF GASES 1BY IIEATr. 61 pands, for the reasons already given; nbubbles are driven f'om the end of the llbent tube, and they ascecnd in the tube t t. le ait speedily depresses the colored liquid, until now, in the coursce of a very fbew seconds, thle iwhole column of liquid Ihas been suplersede(il by air. (08) i:t is perfectly nmanifcst thait the air, thus expanded )by Cheat, is lighlter than thle uncXl)antled air. Our flask, 5at t he collclusion of thlis experiment,, is liglter than it was atf the commencement;, 1)by the w\eight of the tair transferred from it into thle upright tube. Suppt)osing, therefore,t a light bag to be filled with; such air, it is plain that thie bag woIld, wAith reference to the heavy air outside it, be like a dro1) of oil in waterl.'1The oil, teing lipghter tlam the w\ater, wvill asceend through the latter: so also our bag, fitlled with hleated air, will ascend in tile atlosplere; and this is thle principle of the socalled fire-balloonl. My assistant will ignite some tow ill this vessel, over it lie wvill plhce a funnel, and over tlie funnel i: w\ill hold thel mouth of this paper balloon.'lThe heated air Iasccndingl from the burning tow enters the ballool, and causes it to swell; its tendency to rise is already malnlife"slt. On) lett.ing,' it; go, it sails aloft till it st.rikes thle ceiling of the room. (69) But w\e miust not be content iwith regarding these ptenomlnena in a generatl wayS; without exact quanrtitative dcterminations, our discoveries -would soon confound anld bewilder us.'We M must now inqulire, what is the amount of expansion \whicli a given quantity of lheat is able to p1roduc in a gas? T'-tis is an irmportanlt )oint, and demands otr specital attentionl. i3 speaking of the volume of a. gas, we should hlave Ino distinct notion of its real (lquanlltity if its temperatulre were omitted, so largely does the volume var\y with tlhe tCmplerature.'Take, tllhen, a llCmeasure Of gs at tlhe, 1)lCciSiC temll)eraturtC of water vcwhen it b)cginls to frcczc, or of ice whentl it begitns to mtelt, thvat is to say, at a teml)eraturll of 320 ahlli. or 00 Cent.,) and raise that volume of gas one degree in tempe rature, the pressure (i) every Squtare iCih qof t1he envelop which holds the gas beingy pireserved colstantt. The, volunme of the gas will be 62 IIlEVAT AS A MODJE OF MOTION, comXe Ctxlpatled lry a qtuanltitly which we mtay. call (/r; raise it another degree in temperfatturllC its volume will be expanded by 2%, a third degrce vill cetause atit expansion of 3a, and so oln. Thul s we see, that folr every degrce whlich wve add to the tempoer-ature of the gas, it is expanded by tlt same amounlt. \\rlhat is this amount;? No matter what voliumle th;e gas may l)ossss; at: the1 ftreezingl temperature, sby)t raising it one degree -If. 4roeealeit tabove tieh fre'ezingl-polinl w augmelnt; its volume by * -;-ttl of its own amountt; while by raising it one degree eittigrlt/G we auglmentl the volume by -,Ai.d of its \own amounit. A cubic foot of gas, for examnl)le, at 0 C. becomes, onl being' heated to'1~, 1. 3 cubic foot, or expreSsed in decimals —...... I vol. at 00 C. becomes I --- 00366 at C.; at 2,0 C. becomes 1. -I- 00366 x 2; at 30 0. becomes 1 -I-'00366 x 3, alnd so on. The constant number'00366), which expresses thle fractio"n1 of its own volume, which a gas, at tlhe freezing tel)perature, expands on being heated one (legrcee is called tlle Ccoqteicient of exaIpanssion, of tile gas. Of course, if Nwe utse tile legrees of flahrenheit., tite cocfficient w\ill be smaller ill tleC proportioll of 9 to i5. (70) IIt is a very remarkable and significant fact that all pei'rmamlent gascs ct)and by (almost )rcciscly thie same amount for every degrce added to their temperature. Ccl'c can deduce from this with cxtreme l)rol)ability th e importante conclusion, tl hat where heat causes a gas to expand, the tworkl it p)efotrms consists solely ill overcomil'ng tile constant pressure from withouit- -tlhat, ill other lvolds, tlhe heat is not interfered w\ith }by the multual attraction of the gaseous molecules. For, if this were the ease, we should have every reason to expect, in tlhe case of differlllt gases, the same irregullarities of expansion which we observe in liquids and solids. I.. said intentionally " Iy lmost precisely te same amounllt," for mally gases whlich are permat ent, att all ordinary temperatures devia tit li"llt;l from011 the rtle.'his will be sccn from the following tabtle COEFFICOIENT OF] EXPANSION. 63 Nan1ec of Gas. Coelicierlt of Expansion. i lydrotgen.. 0.. 00366 Air.... 0'00367 Carbonlic oxitde... o003 Carbonic acid 0. o003'1 lrotoxide of nitrofgenl... 0'0032'1 Sullphurots acid... 000'90 Ilerc hydrogen, air, and carbonic oxi de agrce very closely; still there is a.sligh;t dilierence, t;he coeflcienlt for hydrogen leing thle least. W'e rentarlk in tlhe othler cases a greater deviation fromI the rule and it is particulally to be noticed that tile gase s which deviate most are those wh\ich are neareslt their pIoint of liquefaction.'t'he first thiree gases in the table never have bccn liquefied, all the othets have. These atre in fact, iMeqe,:/feet gases, occul)ying a kind of interinel cdiate place between1 thle liquid a1tlhe perfect gaseous condition. (71) This nmuch made clear, we shall now anpproachl, iby slow degrees, anl interesting but difficult subject.[ Supplose a quantity of air to be contained in a vcrvy taill/ cylindcr,, A 1 (fit-. 21), tle transverse section whicih is one square inch in area. ]Let the top A of the cCylitnder te open to the air, and let ) be a piston, which, for reasonls to })e exlained illmmediately, I will supp1)OS to weigh two l)ountds oe ounce, and rwhlich can move air-tilght anld wittottfiition ul) or1' down ill theC cylinder. At tlle colmmlencmlnent of the cx)elxpeilt, let; the piston be at tlhe point p of the cylinder, and let th;e height of thle cylinder from its i)ottom n to the point, 1)b 273 inches, the air underleat h the pistoll besing at a temlperature of 00~. Tllen, o heating the air firom 00 to -1~ C. tihe piston will rise one inch; o it will no0\w stand at 274: inch1es a)bove the bottom. If the temperatt' ur)e iS(1 raised wo de'es, tle piston ill stand at 275; if raised three deg-rees, it will stand at 276; if raised ten degtrees, it will statnd atl 283; if 100 deg'rees, it wvill stand at 373 inches ablove the __ 64 11 NAT AS A MOI)l OF MOTION. bottomn; finally, if the temperature were raised to 2730 C., it is quite mtanliest. tlat 273 inches would bo adcded to thle heighlt of the coluimn, or, in other wor(ds, thtat, by heattilg the air to 273 C. i'ts tvolume would be,& doubleld. (72) The gas, in this experiment, executes work. In expanding folm r up ward it has to overcome the downward pr)ssure of the atmosphere, which amllounts to 15 1lbs, on every square inch, and also the weigh~t of thle piston itself, whriict is 2 1)s,. 1 oz. [eecce, the scction of tle'cylindcr being one square inch in area, in expanding from r to v', teho work done l)y the gas is equivalelnt to the raising a. weight of 1:7 lbs. I oz., or 273 ounces, to a height of 273 inchles. It is just the same as what; it would laccomplisih, if the air above p were entirely abolished, and a piston weighing 17 lbs. i oz, were placed atri i. (73) Ie:t us now alter our mode of exleriment, and instead of allowing our gas to expand whlen heated, let us oppose its expansion )yr augmenting the 1resstlure upon it. In other words, let s p s kel its volrume CoiSttntt w\'ile it is being heatcd. Suppose, as before, the initial temperatlure of the gas to be O~ C., tthe pressure upon it., includin tlC teighlt of tle piston r1, being, as formerly, 273 ounces. let s warm thel gtas froml 0O C. to 1~ C.; what, weight must we add at p in order to keepc its volume constant? ]Exactfly onle ounce. But we hlave supposed the gas, at the comImencemnt,C to be under a pressure of 273 ounces, and the pressure it sustains is the measure of its elastic force; ihencel, by Ibeing heated one degree,'thle elastic force of tlhe gas hias augmented l)y -. s3d of what it p)ossessed at 00. If we warm it 2,5 2 ozs. must b)3 added to kccp its volunll consttlant;; if 30, 3 ozs. must C taddc d. And if we raise its temperature 2730, wCe should lhave to add 273 ozs.; that is, we should have to 1 d(ouble the origiral pressure to keep the voltme constant. (]74:) ft is simply for the sake of clearness, and to avoid fractions, that hhave Sul)posdl the gas to be u1n1er the (riginal pressure of 273 ozs. No matter what the lressur1e tmay WORlK 1)ONl1 BY SEXPAND)!JNG GAS. 65 be, thle addition of 1' (J. to its temperature r(loduces an augmentation of 4 3-d of the elastic force wbich the gas possesses'at the freezing temlperature; alld by raising its temperature 273~, while its volume is kept; constant, its clastic force is doubled. ]et uis now compare this experimeint with the last one. [l/ere we heatedt a certain amount of gas from 00 to 273 C., and doubled its volume by so dloig, the doul)e volume being attained by lifting a Nweighti of 273 ozs. thrioughl a lheight of 273 inclhes.,iLrC we ]teat t ie same amount of gas frlom 0' to 273~, but ve do not permnit it to lift any weight. N\\c 1keep its volume constant. The quantity of matter eatedtt ill both cases is the same; the tempieratu.re to whvich it is hleated is tile same; but are thle absolute 2Cqutntities of eat imilparted in1 b)oti cascs thle same? B]y nO means. Supposing that to raise the temperature of tlhe gas whose volume is kept conlstant, 2730, 10 grains of combustible matter are necessary; thln to raise the temperlcature of the gas, whose pressure is kept constant;, an cqual number of degrees, wtould require the consumption of 1l4- grains of the same conl)ustible matter. [Ihe heat prolduced by the combutstiont of the additional 4} griains, in the lattelr case, is entirely constumefd Iin lijfting the weig/ht. Usin g the accurate numbers, tile quantity of ]heat ap)lied when the volume is constant;, is to tile quantity applied whenyc thlC p)resslre is conlstant, ill the p)oportiol of —1 to 1',421. ro. 22. (5)'T'his extremely important fact constitutes thle basis from wnich the lmcehanical equivalent of Ileat was first calculate(d. And here we have reacled a point which is Nworthy. A..W.Y.. of, and whicll will deirmand, your entire atten- C tion. X wvill elndeavor to Imaike this calculation before you. (76) Let c (fig. 22) be a cylindrical yes- sel Nwith a base one square foot in area. Let r v mark tloe lupl)e surface of a cubic foot of air at a teml)er 66 J11AT A$S At MOD E1 OF MOTION. atlure of 0~ 0., or 32~'ahllr.'Thle height A p will be thell one foot. L.et the air be heated till its volume is doulbled; to ectfct thiis it must, as before cxplained, be raisci 273~ C., or 4900~ I. in templ)rature; and, wllen expandcd, its u)pper surftace will stland at' v1, one foot ab)ove its initial I)osition. fB-ut in r1isilng from r r to r' r' it has forced 1,back the atImos1here, whicht cxerts a pressurc of 15 lbs. on every squattre inlch of its ulpper surftace; in othler ords, it has lifted a weight of 14li-x I:::::,160 lbs. to a height of one foot. (77) Tlhe " c)apacity " for heat of the air thus explanding is is 0'24; water being llunity. The weight of our cubic foot of air, is [L29 oz.; lhence the (luantity of hleat retuircd to raise 1.:29 oz. of air 4:900 blrahr would raise a little less th an oncfourth of that weig'hlt of water 4900. The exact quanltity of water equivalent to our -1' 29 oz. of air is:1'29 X 0'24- z:; 0'31 oz. (78):IBut 0'3-1. oz. of water, heated to 4900, is equal to L52 ozs. or 9,: lbs. heated 1. Thus the heat; impartetd to our cubic foot of air, in order to double its volume, and enable it to lift a weighlt of 2,160 Ibs. one foot hirgh, -wotld be competelntl to raise 9f lbs. of water one degree in telmp)craturc. (79) t'hle air has here been heated ui2n(Ce' a constant pjesst'e, and we lhave learned that the quantity of heat necessatry to raise the templerlaturc of a, gas under constant presisure at certainl number of degrees, is to that required to raise the temperature of the gas the same number of degrc'eS -, whefn its volumeih is k'eit consta-Zt,> in the p)roportion of 1'42: 1; hence wte avc thC stat t...... lbs. lbs. 1.'4 1t::1 -: 9.5: 6 7, which shows that tlle quantity of hleat necessary to augment the teml)erature of our cubic foot of air, at constant volume, 4I900, would healt 67 lbs. of Awater 1t. (80) I)cductinlg 6'7 l bs. friom 91'5 lbs., we filld that the excess of heat impal)rted to th1e air, in the (ase where it is* permitted to exl)nld, is comnl)ctent to raise 2'8 lbs, of water t1 in templerature. (81.) As sexllained already, this excess is employed to lift a weighlt of 2,160 lbs. one foot highl. Dividing 2,160 by 2'8> MEiCI[fANtOAI, ]EtQUIVALE\NT,I OF 1IfIIAT. G6* we find tllat a (ltuantity of hleat sullficient to raise I1 lb. of Nwater Flahr. ill twp{eLrature, is compe)tent to rlaise, a weight of 771'4 lbs. a fiot high. (82) This method of calcullatLing' thle mechlanlic-al cquivtalcltt of heat was followed by )Dr. Maye.r, a. physician in I [cilbronn, Germany, in the springl of 1842. (83) AMayer's first paper contaiis merely Can indicaition of the wtay in which hle lhad found thle equivalenlt. In it were elumciated t he convertibilit y and indestructibilit)y of force, alnd its auttlor referred to the mechanical equivalent of heat:, merely in illustraftion of his principles. Tlhe essay \was evidently a kind of )prcli.ilinary note, from which date might be taken. Mayor's sublsequent labors conferred digfnity on the theory wvlhich they illustrated. It:L.t 845 e published fan Essay on Organic Motion andl Nutrition, of extraordillnary merit and illport ance. This was )followed in 1.848 b)y an Essay ont (Cclestial D)ynamlics, inl which, with remarkabllc boldness, s;agacity, and compileteness, he developed the meteoric theor:y of the sun. And this wfas followed by a foturth memoir in L1S., whvich also bears the stamll of intellectual power.'lTakin-g him all in all, the right of l)ir. Mayer to stand, as a man of true genius, in the fi'ont rank of the founderls of the dynamical thecory of lheat, cannot be: disputed. (81) 0nP the 2:{.st of Augl.ust, 1843,,,Dr. Joule* communicated to the 1Britisth Association, thecn meeting at Cork, a. t pal)pr which was devoted in part, to thte determlination of the mellchanical value of heat.~" Joule's publications lhad been preceded by a lonlg course of expetriments, so that lhis filrst work,and Mayer's were relally contemporaneous. This elaborate illvestig'ation'gavo the following weights raised one foot; hight, as equivalent, to the w\armingt of 1 lb. of water FtC ahrenhlcit, the thermlometlric scale employed 1by J)r. Joule 1. 896 lbs. 5. 1.,026 lbs 2. 1,001 " 6, 587 " 3. 11,00''7. 742 4. 91.0 S 8. $0 " * lill. Mag. 1818, vol. xxiii. )p 435. 68 111EAT AS A MOID)E OF IMOTION. (85) From tle passage of water through narrow tubes, Joule dducecd an equivalent of 770 foot-pounds. (86) In:1844 he deducedc from expcrimcnlts on the condensationI of air, the following cquivalents to 1 lb. of water hcated 1~ Fahr. 823 foot-pounds. 7 " 8,20 814 760 " (87) As thc expcrielncc of the exp)erimclnter increased, Vwe find that the coincidence of his results became closer.'In 1845 )Dr. Joule deduced from ex)epriments with wvater, agitated(l )by a p)addle-whcel, an equivalecnt of 890 foot-pounds. (88) S 1umming ulp his results in 1845, and talking the llmean, le found the equivalent to be 817 foot-poulnds. (89) In 1847 hle foiwd the mean of two experiments to give as cquivalent, 781t'8 foot-pounds. (90) Finally, in 18,19, applying all the precautions suggested by seven years' experience, lie obtaineld the following numbers for the mechanical equivalent of heat. T120692, froom friction of water, mcean of 40 experiments. 7F,14'083 " " rcrury; " 50) " 174'98'7 " " cast-iron, " 20 "'hese exlCeriments rank among the most memoralble that have ever been executed in plhysical science.'ll y3 form of themselves a strict demonstlration of the dynlamical theory of heat. DR. JOUE139S INVES'TIGATIONS. 69 (91) F]or reasons assigned in his paper, Joule fixes the exact equivalent at 772 foot-pounds. (93) According to the method pursluelCd by Mayer, l1 18'ii, thie cquivalent is 771'4 foot-poutlds. Such a coincidence relieves thle mind of every shade of unccrtaintity regarding the correctncss of our prcscent mchclanlical equivalent of lheat. (93) The immortal investigations ihere briefly referred to place -Dr. Joule in the foremost rank of physical philosoiplers. AMTayer's labors have, in some mcasure, the stamp of a profound intuition, whAlich Irose, howcvcr, to the cncrgy of undoubting conviction in thfe author's mind, Joule's labors, oil tlhe contrary, are lan experimntal dlemonstration, Mtayer thou/lght hlis theory out, and rose to its grandest applications; Joulle oor/ked his theory out, and gave it forcvcr tfhe solidity of demonstrated truth1. T'rue to tie speculative instinct of his country, Mayer drew large and \eighfty conclusions fiom slender premises; while thle ]'lnglislhman aimed, above all thlings, at the firm establismllent of facts. T\he flturle historianl of science will not11, think, pllae these men in antagonism. To cach belongs a rclputation -which will not quickly fade, for tlhe slhare hle has had, not; only in establishing the dynamical theory of lheat, but also in leading the way toward a right appreciation of tihe gcneral entergies of thl unliverse.* (9,4) Lct us now clhccl< our conclusion regrarding tfle influcnce which the performance of work has on the telmlperature of a gas. Is it not possible to allow a gacs to expand, without performingl workl? This question is answered by the followD )r. Mayer hnas beell recently elected to thle Frendh Academy of Scicnccs, and never, in my opiniona,, was thre rccgnition of that illustirious body more justly bestowed. It is, however, to )e regretted that circuinstances did not permit thle name of tile celebrated mn who firs t made the mechanllical tiheory of heat adcmonlstrated truth to be linked with that of Mayer in the recent clection. Feb. 18'70. '70 11INIT ANS A M0I)1,', OF MOTION. ing imlportant experiment:, which was first madle Iy Gay lutssac. These0 two copper vessCls', A, B (fig. 23), nrc of tthe same size; one of whlich, A, is exhausted, and thle otlher, ii, filled with air. I turn the cock c; the air rushell s out of B ilito A, until the same pressure exists illn both vessels. Now tile air, in driving its own particles out of it, pel1forlis worlc, al(1 cxprimentls whtich I I I I \'we have already made inform us that tile air vwhich remains illn l must be chlilled. Tl'he l)articles of air cntert A withll a. certain velocity, to generate which tte hleat of th.e air illn i has been sacrificedt bult they immediately strike against; thel interior stirface of A), tlheir motion of tatislationl is annihilated, and the exCact quantitly of heat lost by i; appears in A. Th'oe cottents of A.and l mixed together, give air of the original temperature. There is no work performed, and tlhere is no lheat lost. With tile dynamical theory of heat: in view, )1r. Joule made t.his experiment by compressing twenty-txwo latlnoslpheres of air into one of hlis vessels, while the other was exhausted. On strrounlding both vessels by water, kep)t properly agitated, no atgmentation of its temperature was observed, whentl thle gais -wtas allowced to streaml from one, vessel into the oth'er.* I'nl like manner, Supl)ose thle top of the cylinder (fig. 21) to be closed, andl the half above the piston P a perfect vacuum; and suppose, the air in the low\cr half to be latcd llup to 2730 C., its volume being iepl)t constant.l f the pressure were removed, the air would expand x andl fill tile cylindeIr; tile lower portiol of tile columln would thecreby be chillcd, but tile ulpper ortion would b)e iheated, and mixingl bothl portionls torgether, vwe should lave the whlole cotlum at a templerature of 273 t f In X Phil. NMag. 1816, vol, xxvi. p1. 388. 1 have recently tound a casse t mentltionled by F'araday (Researches il Chemnistry and Physics, p, 221), where tihe ftfect referred to iln thl text \was, in: sub CAPACITY FOR tl1lIAT. 71 thiis case, we raise the tcmlperature of the gas from 00 to 273~, andt afterward allow it to double its volume; the temperatinres of the gas at the co enemas aent t h Ol1llttltrt, antld a t tei end, iare t he same, as when thie gas expands against a. eonIstanlt pressurCe, or lifts a constant; weiht; ]lut; the absolutel quantlity of lcat inl tile latter case is 1,t421 times tlat; emp)loyed ill the former, )because, iln the one case, the gas performs mechanical Awork) andll ill thle other not. (95) Wre are taillught by this exlelriment that mere rarefaction is not. of itself sutlcient to produce a lowering' of the mean templerature of a mlass of ailr. It was, ieand is still, a current notion, that tlle mere expansion of a gas p)roduccs refrigceration, no matter how that expansion may h)e ecflected. The coldlness of the hilghler atmosphelic regions was accounted for by referenco to the explansion of theai, ir. \ was thought that whlat we have called the " cl)atcity for heat" wwas greater ill the ease of the rarefied tlhan of thet unrarefied gas, and that chlilling must therefore be the consequence of rarefiact;ion. IBut the rlefrigeration wh\lich laccompallnics xpallsion is, in re1clity, due to the consumption of heat in the performance of work.'Whele no 0work is performed, therel is no absolute refrigeration. All this needs reflection to,arrive at clearness, but every stance obs;ervet. Fllaradaye's explanation of the effet is a most instlrutive instance of thle application of thle material theory of hte at. Thie observation was mna(de at tile'ortable Ga;s Works, in 18'1 " It fiequenttly happons,'' awrites tlraday, " that gas previously at the pressulre of thtirty atmosphe'res is suddenly allowea' S - m f_ O i Y, erF 33 ri o 3;3~' X- C, ICZ; DZ " _5 O "'+ -r Q:~ Zct _;. - 9 ^ -c, o: 0!'.Z - O k S A # -, r O ~~s~0 ~ 0O " 9 Co - e - - -; > ~!v gt a -'.. - v o O C.............. Co w O O -,: nn,C m~ o WI - - < 0). L\n 04 -0 ce- 0J OO c C _ o0~ocntY~ e ce- -. Ct C oog. 033 - 0. A' 0S- - - -r --- - s ( i c C "L Ct3O C Ce-..;r "- 0V YC 00 Cc05-.-Ct-o~ /4 t HEMlAT AS A MlODE1t OF MOTION. Now it stops and commlences edescending, and it will contiilue to do'tso permanently. ]But why the first ascent.? Tt is nlot due to the contraction of the liqulil, but, to the.hemomentary expansiont qf the flask, to vwhich the heat, is first communicated.,'Th1e glass expands before tle lheat call fairly reach the liquid, and hence the column falls; tbut the explansion of the liquid soon exceeds that of tte glass, tand the column rises. T\wo things arie ]lhere illustratld: the expansion of the solid glass by heat, and the fact that tihe observed (ilatation of the liquid (does not give us its t rue augml entation of volmne, but. only the diflerenee of dilatation betxween itself and tlie glass. (98) Itere is another flask filled with water, exactly equal in size to the former, and furnished with a similar tube. I lplace it in the same position, and repeat with} it the experimnent made with the alcohol. Aou see, first of all) tle ttransitory effect due to the expansion of the glass, anld afterward, the pernmailenlt expansion' of the liquidi Ibut you caln observe that the latter proceeds much more slowly than in the ease of alcohol; the alcohol.expandls more rapidly than tlhe -water. Now, we migit examine a Ithundred liquids in this %tway, and find them all expanding l)y hleat, and wte might thus be led to conclude that expansion by heat is a law without exceptionl; but we should err in this conclusion. It is really to illustrate an exception of this hind that this flask of water has been introduced(. Let us cool the flask Jby plunginpg it into a substance somewhat colder than waterl, -whenyl it first freezes.'Thlis substance is obtained by mixing) poundcd ice withi salt;. You see the column gradually sinking, the heat is being given up to the freezing mixture, and the water contracts.'1The contraction is lnowN very slow, and nIow it stops altogether. A slight, -motion commences il the o)pposite direct,ion, and nlow the liquid is visibly c/ etatzdinzy. lBy stirring the freezing mixture, we bring colder portions of it into contact w\ith tlle flask; tihe colder tthe mixture, the quicker the cxplansioil. i tlre, thlenl, Nwe Itave Nature pausing in her ordinallry course, and revelrsing her ordinary habits. T'lle fact is, t}hat thle water D)EPORTMl\ENT 01r WAVATfER IN F1REE1ZINCGX 15 goes onl contract ing till it r1achtcs a t emlperature of 390~ Fahllr., or 4' Cent;., at which l)oint tfle contitaction ccascs.'lhlis is the poidt oqf aximumrt density of water; from thlis (lownward to its ficezing-point., the liquid eXl)ands; and wihen) it is convertcd into ice, the expansion is sudden and consideral)le. Ice, we know, swims ulpon water, bein2g lig;htened by this expansion. If hetat be now al plied, tIle scries of chlangles are reversed; the column descends, showing thlle coltriaetio of the liqytid by heat. After a time the contraction ceases, and pelrmanent expans;ion sets in. (99) The force with;l which these molecular changes are cffected is atill b)Iut irresistible. The changes usually occur under conditions which allow us no opplortunity of observing thie ciiergy involved ill their accomIplisliunent. lBult, to give you an examplle of this energy, a quantity of water is confined in this iron bottle. The iron is fully half anll inch thick, andt the q(uanttiy of water is snmall, though sutlfficient to ll o, he bottle. The ottle is closed )by a scrw tfirmly fixed ill its neck. TIcre is a second )ottle of the same kind, prleparcd ill a similar manner. I1pitcc )otlh of thllem in this copper vessel, and surround them with a freezing mixtture.'They cool gradually, thle water within applroaches its point of mlnt1aximuml density; no dout)t, at; this moment, tile \vater does not quite fill the bottle; a small vacuous sp)ace exists within. ]Bult soon tle contracltioni ceases, andt exl)ansion sets in; the vacuous place is slowly filled, the water grladually clhangcs frTom liquid to solid; in dtoilngf so it 1requires litorc room, which the rigid iton refuses to grant. ]IBut its rigidity is powerless ill tile presence of tile atomic forces..The.cse atoms arc giants inll cisguise, and the sound you now hear indicates that the; I)ottle is shiverld })y thle crystallizingi molecules —-.tlh other bottle follows; and here are t he firagments of tile vessels, shoving tllir tlhickIess, and imnpressing you wvitll the might; of tllhat celcrgy by which tlhey hatve been tlhus riven.* * MAetal cylinderas, an inch in thickness, are unable~ to resist thlo decom)positg force of a smtall galvanic battery. 5M, Gassiot has burst imiany such cylin(ders )by eletrolytie gas. G 1[1,AT AS A MODE]1 OF? MIOTION. (100) You have now no difictulty in underlstanding; the offeet of frosty weatrher upon the water-pipes of your houses. 3Before'you is a ltnumber of piececs of such pipes, all rent. You become first scensile of the damage when the thaw sets ill, bult the mischief is really d(one at the time of freezing; tihe )pipes t. hen burst, and tlhrough the rents the water escatpes, when thlhe solid within is liquefied. (101) It is hardly ncccssary for lme to say a word on the imjportance of this )rop)erty of water in the cconomy of Nature. Suppose a lake exposed to a clear wintry sky; tlhe superficial watcr is chilled, contracts Il )Ccomes thus lheavier, and sinks by its superior weight, its place being sul)Iplied by the lighter water from below. In time this is chilled, and sinkls in its turn. Thus a circulation is established, the cold, dense Nwater descending, and the lighter and warmer water rising to the top. Suppositng this to continue, evn after the first pellicles of ice wereC formed att the surfaCC; theC ice would sink,* and the process would not cease until the entire water of the lalke would be solidified. )cath to every living thting in t]he water would be the consequence. ]lht just when matteis become critical, Nature stcls aside from her ordinary procctding, causes the water to expand by cooling, and the cold water to swim like a scum on the surface of thfle warmer water underneatlh. Solidification eInsucs, but tlhe solid is much lightce tlan the isubjacent liquid, and th;e ice forms a )protccting roof over the living things below. (102) Such facts naturally andl rightly excite t he c11otions; indeed, the relations of life to t.he conditions of life —the general adaptations of means to cnds in Nature, excite, in the profoundest (degree, the interest of the pllilosol)lier. But in * Sir William Thomson has raised a point which deserves the grave colnsideration of theoretic geologists: Su1pposillng tht constituents of theo cart.h's crust to costract on sodlitdiing,' as the experiments of Bischof indicate, a breaking il and sinking of the crust would assuredly follow its forimation. llndcr thltse cilrcumstances, it is extremely difficult to conceive that a solid shlell should be ftormld, as is generally assumtid, rounlt a liquid ntcleus1. Mr. Nasmyth, how ver, infolrms t lo tthat molten rooks e.aadund on solidit)'ing. DI)PORTMFIINT OF BISMUTI[. I7 tlealing with natural )phenomlena, the feelings Imust be carefully watchled.'JIicy often leadt us unconsciously to overstel) the )ounds of fact. Th.lus, I have heard( this wonderful prol)erty of water rcefrred to as anll irresistible )proof of design, uniqiue of its kind, and sultgcstive of pure I)lerolence.'' Why," it is urged, "sshould this case of water stand out isolated, if not for the p1)r1l)ose of protecting Nature against herself? " The fict, however, is, that the: case is not anl isolated one. You see this iron b)ottle, rent from ncck to bottoml; when broken with this hammer, you see a core of metal withii. hllis is tihe ml1tal b1islmthll wh\licleh, whenll ill a 1molteln condition, was )oured into tis 1ottle, and the bottle be ing closed by a screw, exactly as in the case of the water. The metal cooled, solidified, expanded, and the force of its expansion was sutlicient to burst, the bottle. There are no fishl here to be saved, still the molten bismuth lacts exactly as the water acts. Once for all, i would say that the nlatural 1)hilosophlCr, ats such, has nothincg to do witlt purposes and design.s. [is vocation is to inquire what Nature is, not'wh'y s1he is; though; he, like othMes, and he, 1more than others, must stand at times rapt in wvonder at the mystery il which lie d(wells, and toward tlle final solution of whiclh his studies furnish him wAvith no clevw. (103) wrce mliust now plass on to the expanlsion of solid bodies by hlcat,^ w'hilch may be illustrated iTn this way': H lcre are two Nwooden stands, A and t (fig. 25), with p)lates of brass, p P', Iriveted 1against them. These two bars are of eqtual length, one of tlhem is brass, the other iron, and, as you obscr ve, theyale not sufficiently long' to stretch from stand to stand. Phey are therefore supported on two little )ro'jections of Nwood attached to tle stands at p and p'. One of the plates of brass, p, is connected Nwith one pole of a voltaic battery, %, and from the other, p', a wire l)roceeds to the little inst. rumenlt C, in front of the table; and again, fr'om that inst, rument, a vire returns direct to the othcr lpole of the battery. Tlho instrument il frolnt consists merely of an larranglement to 78 lEAT AS A hMiODE 0F MOTION. support a. spiral c of platinum wire, which will g1low wiith a pure white light whel the currtient firom l1 passes through it. At the prcscent mlomenlt the only break ill the circuit is due to the insuffilcicnt lncgthl of thle bars of b)rass -land iron to bridge the space from stand to stand. Underneath thell bars is a row of gas-jcts, which 1: will now ignite; thle bars are heated, the mcetals CXl)and, and in a few moments tlhey %will stretch quite across fiom plate to plate. hllhten thlis occurs, 1km. 26, cr-, the clurent will pass, and thle fact of the gap bleilng bridged will be declared by the sudden glowing of the plzatinumll s)iral. It; is still non-luminlous, the bridge not; being yretl colmpletc; lbut now the spiral briglhtens up), showingl thati one, or both, of these bars have expanded so as to stretch quite across from stand to standl, Which of the bars is it? On removing the irol, the platinum still glows: I restore the iroin, tand reimove the brass; the lighlt disappcars. It; was the brauss, then, that bridlCed the gal. So that we lave here an illustlation, 3EXPANSION 01F SOLAID)S. 70 nlot only of the general fact of explansion, but also of thie fact that diterent bod3ies expand in diffetrcent egrees. (104:) lThe exp(ansion of botlh brass alnd iron is very small; an(d var ious inlst ruments have bceen devised to mcasure the expansion. Such instruments bear tle general name of l)yromoters. 3ttl before you is ta meanst of multipliyilng tle effetct, far more powerful than the ordinary yromecter. On a mirror connected with the top of this solid up)right bar of iron two feet long, is thlrownI a beami of light from the electric lamp, which beam is rcflected to tIle uppt r part of the wsall. If the bnar shortenC the mirror will turn in one d(irection if itt lenltlhen the mirror will turn in the opposite direction. Evcery mtovement of the mirror, however slicght, is mult il)lid bl y thiis long index of light; which, besides its le1ng'tth, has the advantage of moving' withll twice the allgular velocity of thle mirror. ]vvenl the humall n blt reath, projected against this massive b)ar of ironll, produces sensible motion of the beam; and, if it be warntled for a moment with the ftlame of la slirit-lamlp, the luminous index will travel dow\nward, tle tpatcl of light; utpon1 thie wall moving through a space of full thirt.y feet. I withdraw t1he lamp), and allow the bar to co-ol; it contlracts, and the patch of light reascends thle -wxall: the contraction is lhastened by throwingl a little alcohol onl the bar of iron, the light, moves 1more s)cedtlily upward\', nl Ilow it occupies a p)latee near tile ccilingl, as at the commencement of the expel)rilment.* (105) l:t has been stated that different Cbodics possess differcnt )pow+ers of ex)ansion l; that; lrass, for examplle, expands more,01 on beiln eated, thanll iron. Of these two rulers, one is of brass and tile other of iron, and tley are riveted together so as to form, at tlls temlleratl rel a st raightt comptound ruler. But., vwhlen the temperature is changed, tile ruler is no longer straight. If heated, it bends iln one direction'l: if cooled, it bends iln the op^ Tho piece of apparatus wit.h which this experhsilet was lnade, is intended for a totally different, purpose. I therefore indieato its principle merely. t The cooltieients of expansion of a few well-known substances are given iln the Appendix to this Ollapter. 80 HIEMNAT AS A MOIDE OF aOTION. posite direction. iWh1en h1catcd, the brass expsalds most, and forms tihe convex side of thle curved ruler. When cooled, tlhe bral-ss contlracts most, and forms the concave side of the ruler. F:lacts like these ntlst, of course, be taken~ <-~i~u ii1 84 HlEAT AS A nMODE OF OTIONT. At the end of the straw is placed a spcar-sall)cd )icc of paper, w\ich can range over a g'raduatd circle drtawn ol thils black-board. The index is now pressed down at i by ta pltojcction attachcd to the wcight. If the wcight should bott liftecd by the contraction of the India-rubber, tlhe index will follonw, being drawn aftl er it by a sp)ring, s s, which actIs upon the short arm of the lever, Th1e!ndiat-rubbcr tube, you observe, passes through a. sheet-iroln clillmney, c, through which at current of hlot air ascends from this lamp r,. You see tlhe effect: the index rises, showing thllt-; the rubbter contracts, as Silr' nm. l homson antticipated, anId by continuingg to apply the heat for a minute or so, the end of thle index is causcd to describe an arc fully thrce fee;t long.:I withldraw tlle lamp, and, as thte Indiat-rubber returns to its formerl temperature,l0 it lcngthlns; the index moves downward, anld now it rcsts even below the position whicl it occup)icd at first; RIXPANSION. 85 AI)PIENI )X TO CIHAPTE11,R III. FU'XtfE'I',- Rit.lMARlKS ON DIl'ATATION. IrT is i0ot withinl thlte sco1) of )mly present intention to I\dwell in detail on all thie t0iphenomena of expacnsion by ]heat; but, for the sake of my younger reaIders, I will sulpplement this chapter by a few additional remarks.'Phe linelr, superfici il, ori cubic coficiient of expansion, is that fr'aetion of a body's length, sulrfaco, or volume, which it exp)ands on being, heated one degree. Supposing one of the sides 0of a square lplate of 1metal, whose length is 1, to ex>)and, on being leated one degree, by) tihe quantity a; then t he side of thle new sqtare is:I + a, and its area is 1 -I- 2a, -- a. In the oase of expansion by heat, the quantity a, is so small, tlhat its square is almost insensible; the square of a sim-all fiaction is, of cou1rse, greatly less than tfhe fraction itself. ltencee, without sensible error, we may throw atway t he. a in the above expression, and then we havo t. he area of the now1 square 1 -I-2a. 2a, tthen, is tho superficial coefficient of expalnsion; hence we infer that by multiplying the linear coefficient; by 2, we obtain the suplerflicial coefficient. Sulppose, instead of a squaret that we had a cube, having a side 1; and that on heating the cu1be one degree, the side oxpanded to 1- t a; then the volume of tXhe expandled cube would be 1 -- 3 3r a -~- (a. Inl this, as in the fornmer case, tihe square of ta, and lmucht mor trhe cuboe of Xa, may )e neglected, on account of their exceeding smallne-ss; we have then the volume of the expanded ctbe b 1 + 3a; 86 lI EAT AS A MODE, OF MOTION. that is to say, the cubic coefficient of expansion is found by trebling thio linlear oefficient. Tho following tablo contains tle coefficients of expansion for a number of well-known\ substances: Col)per,.. 0000 0'000051 0'000051 ead,... 0'00002 00 000ns8 0000089 Tin,. 0. 0'000023 00000009 0o00000O Iron,... 0'000123 0'00003/ 0'000037 Zinc,.ll. 0-000029i 0'000088 0'000089 Glass.. 0000008 0-000024 0'000024 The firslt col1umn of figures here gives thoe linear coeflicient of expansion for 1 O.; thle second colulu contains this coefflicient trebled, wh1ic0h is the cul>ic expansion of the substance; and the third coliumn gives the cubic expansion of the same substance, deteorminced directly by Professor K(opl.* It will be seen that K(opp's coefficients atgree almost exactly with those obtained by the treobling of the linear coeffilcients. The linear coefficient of glass for 1~ C. is 00000080. That of p)latinlun is 0'0000088. lienlce glass and platinum exp)and nearly alike. This is of the greatest iml)ortanc to ellists \'10ho often find it necessary to furse platimum \wires into their glass tubes. Were the coeflilcients difttrent, the fracture of the glass would le inevitable during the contraction.:The Y'larmomectr. Water owes its liquidity to the motion of heat; when this 10motion sinks sufliciently, crystallizat-io)n, as we have seen, sets in. T he temperature oft clrystallization is perfectly constant, if the water be kept under tlhe same pressutre. -For example, water crystallizes ill all climates at the sea-level at a temtperature of 32~ F., or of 0 0. Theo temllp'ratulre of condensation from the state of steam is equally constant), as long as the pressure 0relmaillns the same. Thoe 1melting of ice and the freezing of water touch each otther, if I may use the explression, at 32~ l.; the condensation of steamn and the boiling of wvater under one atmosphere of pressure touchlt each other at 112': 32~ then is the fieezing-point of water, and it is the melllting-point of ice; 212~ 1 P1hil. Meea. 1852, vol. iii. P. 298. TJIM,' x ^ a v ^X X A. 87 is theo condensing'-point of stenmi- and tle lboiling-point of.water. Iloth are invariable as long,as the )1ressure re1mains t el same. Here, tlhenl, \we have two invalutablel standard points of temlerature, and tlhey have been used for this tlhroughout, the world. The mercurial thermometer consists of a bulb and a stem with capillary bore. The bore ouglht to be of equal diameter tlhroughout. The bulbl) and a portion of thle stemn are filled with melrcury. Both are then plunged into melting ice, theo mercury shrinks, the column descends) and finally comes to rest. Let thle point at which it becomes stationary be mnlarked; it is the fieezzinag-l oint of the thermometer. Let the instrument be now removed and tbhrust into boiling watertt; the mercurty expanfids, the column rises, and finally attains a stationary height. L:et this point be markedl; it is tile boiling-point of the therntmometer. The space betwcen the frecezing-poinlt atl tle boilinginlg point has been divided by Itflaumur into 80 equal parts, by Flahrenheit into t80 eq~'ual parts, and 1)by Cclsius into 100 equal l)arts, called d egCrees. The thelrmometer of Celsius is also called trhe Celntigrado thermometer. PBoth RInltlaum alnd Celsius call the fireezing-point 0~, Flahrenhelit calls it 32~, because he started friom a zero which he incorrectly imagined was the greatest terrestrial cold. F:tahrenheit's boiling-point is therefore 92t2~. 1launimr's )boiling-ploint is 80~, while the boilingp)oinit f Celsius is 1000. The lengtgh of the degrees being in the proportlion of 80 S:100: 180, or of 4: 5: 9, nothing can )e easier tlhanl to convert one into tihe other,. ff you vwant to convert Fiahrenheit into Celsius, multiply by 5 and divide by 9; if Celsius into F'ahrenhelit, multiply by 9 and divide by 5. Thl1us 200 of Celsius are equal to 36~ Flahrenheit; but if we would know what temperature1 by Fahrellnheit's thermometer correspon(lds to 200 of Celsius, we must add 32 to t he 36, which would nmake the temperature 200, as shown 1by Celsius, equal the temperatture 680, as shown 1)by Fallrenheit. ) XTIII mAtTS FISt 8, R 1 r. f)A IR I:8 fA lFST 80i 1t'NTI' FIOI M1S EMOi,R. ABAlI N(1 T( IE,'t'ITLE,I "0N IEATI,(}}iT, IMtAN) TJIFi COM5BINATI'IONS 01F' JXITi[.1T*'.Th'e peculiar modes of existence of bodies —— solidity, fluidity, andi guazity. —-- depend (accordilng to the calorists) on the quantity of the fluid of heat enteringf into their comlposition. This substance insinuating itself between their corlpuseles, separating them from eaelc *; Sir Humtphry ))avy's Works, vol. ii. 88 I1tAT f AS A MODE), OF MOTION. other, and preventing their acttual contact- is by them supposed to be the cause of repulsion. Other philosophecrs, dissqatisficd with the evidences produced in favor of the existence of this fluid, and perceiving the genleratioln of heat by friction and percussionl, have sutpposed it to be motion. CoonsiIdering the discovery of the true cause of the repulsive power as highly implortant to philosop)ty, I Ihave endeavoredtc to illvcstigatd this part of chemical sciencet by experilmennts; from thes eexperiments (of which I am now about to give a detail) I coenclude thlat heant or tho power of repulsion is not matter. /7!e.Phenomenat qf.Rultsiom are i' ot dtpendftt ont a e)Cucti'a elafsti Ifluid for theirJ existence, or Caloric does vot e.ist.'Without considering the effects of the. repulsive power onl bodies, or endeavoring to prove fr'om these effects that it is mo10tio, I shall attemlpt to demlonstrato by expleriments, that it is not matter; and' inl doing this, I shall use0 the method called by m-athematicians),'cduotio ad absurdum. First, let tshe increase of temperature producedl by friction and percussion be supl)posed to arise from a dirminution of t-he capacities of the acting blodies. In th.is case it is evidelnt so11e cllange must be induced ill the bodies by the action, wlhiclt lessens tlheir capacities and increases their temperat1ures. rf,.eriment..i..... I- procured two plarallelopipedons of ice,* of t1he tcemperature of 29~, six inches long, two wide, and two-thirds of an inch thlick: they were fastened by Nwires to two bars of iroln. PBy a lpeculiar mchanisl, their surfaces were pllaced in contact, and kept in; a continued and most violent frict ion for seoine minutes. They )were almost entlirely converted into water, which water was collected, and its temperature ascertained to be 835, after remlaiing in an atmosphere of a lower temperature for some1 minutes. The flusion took p)lace only at the plane of contact of the two lieces of ice, and no bodies were iln friction but ice.'From this experilment it is evident that ice by friction is convet'ted into awater, and, according to the stlupposition, its capacity' is dimintishled; but it is a well-known fact, thlat thle capacity of water *' Thle result of thils ex)periment is the same, if wax, tallow, resin, or any suibstance fusible at a low telnperature, be iused; oven iaron may be tiused by collision. CALORICT D)OS NOT EtX IST. 89 for ]heat is much greater than that of ice; fand ice must have an absolute quantity of heat added to it, before it can be converted into Nwater. tFriction consequently does not diminish the capacities of bodies for lheat, Fliom this experiment it is likewise. cvident, that the ilncrease of telmperature consclqc ent on firiction cannot arise fiolm the decomplosition of the oxygen gass in contactr for ice has no attraction for oxygen. Sinco t.he increase of templ)rature consequent on friction cannlot arise fiom the dilninution of can)acity, or oxidation of the acting bodies, the only remaining supplosition is, that it arises from an absolute (quanttity of hleat adned to themn, N-which heat mulst be attracted friom trho bodies in contact;. Then fr iction must induce some change in bodies, enabling theml to attract lmlet from th e bodies in contact..Yycrzim ct.-.. 1 p)rocured a piece of clockwork, so const ructed as to be set, at wtork in the oexhausted receiver; one of the external vwheels of thifs machine calne in contact with a thin metallic plate. A considerable degree of sensible hteat was produced by friction between the wheel anrd plato vwhen the machine worked, unin-sulated friont bodies capable of communicating heat. T next procured a,small piece of ice; t round the supaerior edge of this a small canal was mado, and filled with wvater. Thle machinle was placed onl the ice, but not in contact wit.h the warter. Thus disposed, tho whole was placed uttder the rcceivcr (whichl had been )proviously filled with carbonic acid), a quantity of plotashl (i. e., caustic vcgetable alkali) being at the same time introduced. The receiver was now exhausted., From tht exhaustion and froml thle attraction of the carbonic-acid gas by the potash, a vacuumt nearly l)erfcct was, I believe, rmade. the machine was now set to work; the wax ral)idly molted, provingf an increase, of tcrpolratturce. Caloric then was collected by fiictionl; which calori, onl the X'l The tempelraturo of the ice and of trhe srlroiunding atmosphere at the colmlencement of the experiment was 3'3, that of thoe mnachline was likewiso 32~. At tho end of the oexperimennt, trhe temperature of the coldest part of theo rmachine was near 83~, that of the ice and surrounding atmosphlere the same as at the can lenltt of the experiment, so that the lheat productl by the friction of tIle difttrent parts of the machine was sufficient to raise the tenperaturo of near half a pound of mactal at least one deagrec; and to convert eighteen grains of wax (the quantity temployed) into a fluid. 90 IBEAT AS A MODIE OF sMOTION. suppositiont, was comnunicated by the bodies in contact w\ith the machine. InI this ex)periment, ice was tht only body in contact witfh the machine. Ilad thlis ice given out, caloric, the, water onl the top of'it must have been frozen. The water on thlo top of it was not fiozen, consequently thle ice did not give outt calorie. Th cttalori could not; come fronl thie bodies in contact with the ice, for it lmustI have passed thlrough the ice to penetrate the malhilie, and1 an addition of caloric to the ice \would have converl'ted it into water. Iteat, when produlced by frietion, canntot be collected from the bodies in contact., 1and it was p)roved, by thle first cxperiment, tlat the increase of temperaturel consequent on friction, cannot arise~ fIroi diminution of caplacity or oxidation. But if it be considered as matter, it mu.st l)e produced in one of these modes. Since (as is d(emonstrated by these experimentts) it is produced int neither of these mode-s, it cannot 1be considtrced as matter. It has therefore 1)een experimentally demonstrated that caloric, or the matter of heat., does not exiSt.. Solids by long and violentl fr'iction becomel expanded, anld, if of a higher teml)eraturo thall our bodics, affect the sensory org.an wNvith the p)eculiar sentsation known by thle common name of heat. Since bodies become expanded by fiection, it is evident thal their corpuscles must move or separate from each otlher. Niow, a motion or vibration of the corpuscles of bodies must be necessarily generated by friction and pertcussioll. 1?Therefore \Nvwe may reasonably conclude that, this motion or viration is heat, or the rclul'siv 1)olter. 1cfeat, tlhen, or that power which prevents tlhe actual contact of the corptIusele of bodies, and w\licht is the c'use of our pecutliar sensations of heat sand cold, may b) tdefined as a peculial motion, proba)ly a vibrationl, of tile corpuscles of bodies, tending to sel)arato them. It may with propriety be called the repulsive motion. Since there exists a v repulsive motion, tle plarticles of bodie.s may 1be considered as acted on by t\wo opposing forces; the apprloximatinlg power (\vhich may, for greater ase of expression, be called attraction) and the rel)ulsivoe motion,. The first of these is tlhe compoundl effect of the attraction of cohesion, by which tlhe p)articles tend to comel in contact -wit, each otherl; tleo attraction of gravitation, by which tihey tend to appl'roximnate to the gret con DAVYyr ON TflE, MtOTION OF I)EAT. 91 tigt0ous masses of matter, and the pressure under which they exist, dependent on the gravitation of the superitfcumbent bodies. Tho second is the efflect of a peculiar motory or vibratory impulso given to them, teniding to remove them fi'ther' fr'om each other, and which can be generatCd, or rather increased, by fi'iction or percussion. The tlfect of tlte attraction of cohesion, the great app)roximating cause, on the corpuscles of bodies, is exfactly siilar to that of the attraction of gravitation on thle great mIasses of mlatter composing the universe, and thl repulsive force is analogous to the planetary projectile force. In hlis " Chemical Philosophy," pp. 94 and 95, Davy expresses himself thus: "]By a moderate degree of friction, as it would appear firom Rullmford's experillents, the samle pice of metal may be kept hot for any length of time; so that, if the heat be pressed out, the quantity must be inlexilaustible. When any body is cooled, it occupies a smnaller volume than before; it is evident, therefore, thlat its )parts must have approached each other; when the body has expanded by heat, it is equally evident that its parts must have seplarated fron each otler. Th'.e' imnlediatet cause of thle phenomenon of heat, thent, is meotion; and tihe laws of its communication are imprcisely thel sameo as thl laws of tihe colmlunication of motion. C' Since all matter lmay be made to fill a smaller space'by cooling, it is evident that thel partiles of matter mtlust have space between them; and, sinlce every body can communlicate the power of expansion to a body of a lower templeraturte- -.that is, can give' an exllansive mnotion to its )lartlicle- — it is a probable inference that its own particles are possessed of motion; but, as there is no chan1ll1ge inll the position of its parts, as long as its tempern ature is unitform, the limotionl, if it exist, must be a vibratory or undulatory mo11tion, or a miotion of the particles round their axes, or a m-otion of tlhe particles round each other. c"it seems lpossibl) to account for all thel phtenomena of hetat if it be supl)osedl t:hat i solids the 1)articles are in a constant, state of vibratory lmotion, the p)articles of the hottest bodies moving with the greatcst velocity, and through thle greatest space; that in fluids and elastic fluids, besides the vibratory motion, which must be conceivedt greatest in tihle last, the particles lave a nmotion round their own axes with diltkrent velocitey, the l)articlcs of elastic fluids nmoving with the greatest quicknecss, and that in ethereal substances ttlh 92 [IlM'UT1 AS A MOD1)El, OF MOTI'ON. particles move round their own axes, and scl)earate friom each other, penct;rating in right lines through space. Ter loratluro may be contceioed to depend upon theo velocity of the vibrations; inlcreas of cnpacity in the motion being performed in gr'eater space; and the diminution of tel)mperature during the conversion of soli1ds into flui1ds or gases, may bt exptlained on the idea of the loss of vibratory motion, in conlsequencel of the revolution of particles round their axes, at the lmomlent when the 1)od,.ty becomIIs fluid or aiSriform, or from the loss of rapidity of vibration in consequence of the metion of thle particles through space." VIBIRAT10N Of' 0 llTXE) Mf'ETATS. 93 CAI iATl 1R tV. T)}I RtV E1.1LYAN NU S' RUN —N C. GO Es P\'Or,J INO } tIN,,S-! IN SFUSOE OF' P}.SUB ON FPS F-: INO ~-I'OST' —O t QUFF'AOTIOAN AN.iD l,\AMIN.lON OF IC BEt' ]]I I'}:-B.JS[~E —- -])SSEO:fION O-' If O n BY A fiALOI, tTI' M I.f —JI.IrIUV D'I.)O\'IER NS DfiEI Cl',NB: llT, fI'I- --— L...l fECSIIO ANIOAt.,'ROP'tR-':S () F WAR I..." OF Alt, It O1L —- IX}-1tPO1 N' OF lIQUIDS'I INfIltUN(,)NO( CIRCUtI' SriANCS:.S-.-.CONYEsION Oit' W: AT INfO VOEK ISN''118 fEAI-ENfIN; tWIt: OGE.YSIES 0 ICIAN I). (.1.1.3,) i31:) l ]'O ]]] tinally quitting tlhe subject of expansionl.) wish to show yoi nlo l csp)exprimelClnt which illustrates in a curious and agrecalble \ay thl conversio n of heat ilto mechanical cnclerg.'. tlhe ftct to bI) rlc)roduced was first obse10rved by a gentleman named Schwartz, in one of thlc smelting-worlks of Saxony. A quantikty of silver which had l)cen ftsed in -a ladle was allowed to solidify, and to hasten its coolingit was turned out upon an anvil. Some time afterward a strange lbuzzingl sound was hc.rd in the locality. T'he sound was finally tracel to the hot silver, which was found quiYvering upon the anvil. Many )ears subsequenllt to this, MrI. Arthur'J'revelyatl chanced to be using a hot soldcring-irtoln, which hie laid, )by accident, against a picce of lcad. Soon afterward, his attenltion was excited,by) a most singular sound, which, after some scarching, was found to lproceed fiom the solderilngiron. Like thc -silver of Schwart;z, the soldering-iron was in a state of vibration. Mr. Trcvcelyan madc his discovery the subject of a verylt i vter stisng invcstig'ation. I t' detcrmined the best; form to be given to the "roccre, " as thie vibrating mass is now called, and tlhroughout i4surop)c this instrument is known as " Trevelyan's ]nstlrument. " Since that; time the sub 94 1 lAT AS A MODE OF MOTION. ject has cmngaged the attention of Pt'ofessor 3. ID. l'orbes, )1r. Seebeck, MAll. Faraday, At. Sondhaus, and myself; but to Trevelyan alnd Secccck wec owe most of our knowlvedge rcgarding it. (1.14) lHere is a rocker made of brass. Its lecngthl, A o (fig. 27), is five inclhcs; the width, A n, [1' inch; and the length FIG., 2?. of tlhe handle, which tcrminates in the kn6b 1%', is ten inches. A g-roovc runs at the i)ack of the rocker, along' its centlre; the crloss-section of the rocker is givlen at Ar. WeV will heat tile rocker to to a tmplra)ture somewhat higher than" thlat of t:mc. ~8, a /d boilig-watolr, and ll ay it onl a block of leadc, allowinlg its knob to rest upon the table. You hear a lquick succession of forciblc taps. ]hit youl Canlluot see the oscillations of the roccer, to whlich the tlaps 1are due. I therefoire place on it this brass rod Av it (fig. 28), with a brass ball at etach of its ends; tlhe oscillations are thcerecly rendeed much slower, and you canll tfJ E TRiEVElIAN INSTRUtMENT. 95 easily follow with the eye the pendulous motion of thle rod and balls. T'[lis motionl will continue as long' as tlce rocker is ab)le to comlumllnicate sufficient heat to the carrier on whlichl it reSts. Thus vwe render the vibrations slow, bult thel can also be rendered qlickl by using a rocker with a widert groove.'.t'hei sides of this new rocker do not overhtlang so much as t1hose of the last,; it is vilttually a. shorter endullum,,ltand wvill vilbrate more quickly. Placed upon the lead, as before, it fills the room with a clear, full note. Its taps are periodic anti reglular, and they have linked themselves togetler to produce this music, Sie re is a thirtd rocker, with a still wider grooNve, and with it we obtailn a shriller tone. You know that the pitch of note augltments wtith the numibe)r1c of thle vibrations; this witde-gwrooved rocker oscillates more luickly than its predecess;or, and tlherefore emits a higher note. lBy means of a beam:of liFht w e obtain an index Awithout, weight, whlich does not retard motion. To the rocker is fastened, by a sing;le screw at its centtre, a small disk of polished silver, on which the beam of the clectric lanmp falls, and from whiclh it is reflected tgaillst the scrlccn. ilhenll thle rocker vibrates, the beam vilnates also, ibut; with twice tlhe anglular vclocity,, and you lnowX see the patch of light drawn out to at band up)On the }wh}ite slurface. (115) \hVlat is the cause of these singular v;ibratioiis and tones? They are due simpIly to the sudden expansion by healt of the body on which the rocker rests. W\lnever the hot metal comes into contact with its lead carrier, a nipple sulddenly jits flIom the latter, bleing' plodced 1by thle lcat comnitunicated to the lead at the point. of contact. l The rocker is tlus tilted lp, and some other pointl of it? comes into conItact with the lead, a firesh nipptle is formed, an(d thle weight is, againl tilted. LeGt.A 1 (fig, 29) be tihe surface of tle lead, anld:t the cross-section of thle hlot rocker; tilted to the right, the nipple is formed as at n-; tfilted to the left, it is formed as lat %, tle nipple in each case disappeairinog as soon as tile contact \twith tLe rocker ceases. The consequence is, that while 96 1JIEAT AS A 0MD1E OF MOTION. its temperature remains sufitciently higlh, tile rocker is tossed to and fro, and the quick succession of its taps against the lead p)rodulces a musical sound. (116) These two pieces of sleet-lead are fixed edgeways FIo. 29,.............................. A 1 in a vice; their edges are about half an inch asunder. A longt bar of heated lb)ass is laid across the two edges. It rests first on one edge, which expands at the point of contact and jerks it upward\' it then falls upon the second edge, which also rejects it; and tlus it goes on oscillatin g, and will continue to do so as long as the b)ar can communl icate sufficient heat to the lead. This fire-shovel will answer quite as well as the prel)ared bar. I balance the liated slovel upon the edges of Fro. 30. i~ji flte lead, and it oscillates exactly as the bar did (fig'. 30)..It may be added, that by 1properly laying either the p)oker or fireshovel. upon a block of lead, and( suplporlting the handle so as to avoid fiiction, you may obtain notes as sweet and musical as any you lhave heard to-day. A heated hloop placedl upon a plate ROTAT'ION BY E)LECTRIC1TY. 9 of leadt may also be caused to vibrate tand siil'; and a hlot penny-piece or half-crown may be casted to do tile lsamle. (11.7) ]L'ooked at with reference to the connection of naturIal forces, this experiment is interesti,g. The'le atoms of bodies must; be riegarded as all but infinitely small, but thell they must be regardeld as all but infinitely nlumerous. The augmenil tation of ithe amlplitude of any oscillating atom by the comnmnicat:ion of leatt is insensible; but thlc summation of anl almost infinite number of such tagmentations )ecomes selnsible. Such a. sumnlation, cfiected almost i anll instant, produces tlhe nipple, and tilts the Iheavy mass of the rockcr. Itcre we have a direcct conversion of heat into commlon mcchatnictl motion. B]ht the tilted rocker fllls aglain by gravity, and in its collision -wit}h the block restores (almost the l)recise amountl of ]heat whlich was consumed in lifting it.:I[ere we have the direct conversion of colmmon gravitating force into htcat. Againl, thle rocker is surrounded by a mledium capable of being set in motion.'I'lle air of this room weighls some tolls, and cvery I,)aIticle of it is shaken by the rocker, and vcery tympanic membrane,l and every tauditory nerve I'csent, is similtarly shakenl. Th's we have the conversions. qf a portion. qf the hAct ilnto sound. And, finlly, cvery sonorous vibration whiclh speeds through tLhe ail of this room, and wlastes itself upon the walls, sLats, and Ceshiot, is converted into thc1 form w\ith which tile cycle of actions commnclt ed.........lnamly, into heat.. (:1:18) There is another curious tffect, for whicoll \e are indebted to Mr. GeorgC Gore, which admits of a similar cxplatnartion. You see this line of rails. Two stripsof brass, s s, s' s' (fig. 31), are set edge-ways, about anll inch lasunider. A hollow ball i, of veryl thin metal, is l)laced upon the rails. If it be pushcl it rolls along theml; but when left alone it is quite still. 1 conlnect the two rails, )by thle wirCes itw', with thle two poles of a voltaic battery. Ai current now passes down one rail to the metal ball, thellnce over the ball to tile other rail, and finally back to the battery. At the two points 98 lIEAT AS A MODE) OF MOTION. of contact of the ball with tlihe rails the current encounters rcsistance, and wherever t current tcollte1rs resistance 1heat is decvcloped. Thlie hcat produccs 1t lc\levation of thte rail at fho. 31. I y v ws v v s v - w w w........................, J~:... -, thcse p)oins. Observe thle cffect the ball, whiche a moment ago was tl'ranquil, is Jnow lneasy.'It vibrates a littlc at first without rolling; now it actually rolls a little way, stops, (and rolls back again. It gradually augments its excursionl, now it }rhas gone farther than was intendedt: it has rolled quite off thie rails, and injured itself by fialling on the floor. (1:19) I:ll t.his other al)tlarattus, for w\hliclh am indebted to tr. Gore himself, the rails form a pair of concenltic hoops.'\hen the circuit: is establishltd, the ball r (fig. 32) rolls round the circle.* Abandoning the clectric ciurrent; Mr. Gore lhas hI. 3?2 obtained the rotation of lightl balls by placing them on circular rails of col)per heated in a fire, the'o1in.g force in this case being thle sate as the m'oeking force in thle Trevelyan instruX-'il. Mi. o.. vol. xv. p. b21. XINJFLUENC OF' PhR1E18SURF, ON 0USING POINT. 99 (1.20)'In thle vast majority of cases tihc passa'ge of 1bodies from thel liquid to the solid state is accomplanied by contraction. Here, for examl)lc, is ia round g'lass dish containiing' hot Awater. Over the water li pour from a ladle a quantity of melted \Nwax. TI'he wvax now foirms a liquid layer nearly IIhallf an ilnchll thick a)ove the w.ater. rc will slfl sfer b)oth water and Nwax to cool, and whend they are cool you will fincl that tlhe NIwax) which now oversp)reads the entire surfeite~ tll is anttcllc all round to the glass, will retreat, and we shanll finally obtain a cakel of wax of considerably smaller area than the dislh. (121) h'11e wax, tlherefore, ill passilng from the solid to the liquid state, expwads.'To assume the liquid form, its )articles must; be tpushed more widely apart, a, cerltain play l)etwcen tihe t)articles being' necessary to the condition of liquidity. Now, suil)pose w\e rsist the expansion of the wax by tan external lmechanieal force; suplpose we have a very strong vessel completely filled with solid wax, and offering a powerfil resistance to the exp)ansion of the mass within it; what would you exp1ect if you sought to liquefy the wax in this vressel?'When the wax is free, the icat has only to conquer the attraction of the molecules of the wNax, but inl the strongl vessel it has not only this to conquer, but also the recsisstance offered by the vesscl. 3y a mere lt rocess of reasoning', we shduld tlus be led to infer that a grcater amount of heat would be requiredl to melt tlhe wax under pressure, than when it is fircc; tor, i other wordls, that the point of fusion of the w-tax will be elevted by pressure. I'hlhis reasonting is completely justified by experimentl. Messrs. ]o:-opis fand Fairbairn have, by p)ressure, raisedl the mcltingpoint of some substances, which, like Awax, contract considerably oni solidifying, as much ats 20~ and 30~ Fahr. (g122)'lThe exlerimenlts ]here referred to conncct thlemselves with3 a very rcmar l kabll c speeulation.'lihe earth is l (nown gradually to augment ill temlerature as we pierce it deeper, and the depth has bI1een calculated at which all known terrestrial bodics would he inl a state of ftusion. 3Mr. toplins, however, observes thalt owing; to thie enormous 1)ressllure of the superinlcumbelnt 100 lE1AT AS A A MODE OF MOTION. layers, the d(eeper strata woull rcquliue a far higher temperatil-c to fuse them thanill would be ecessary to fuse the strata near t;eC eartlh's surftace. It[ece lhe infCrs that tile solid crusst mustl have a considerably grlater thicknelss than that givetn b)y a calculation whichl assumles tlhe fusing-points of the sulperficial and the (ldeeper strata to 1)e the same. Sir Wuillianl Thomson (Proceedings of the Royal Society, vol. xii. 1). 103) exs)rsses tlhe conclusion that " unless the solid substance of tlhe earth be on the whole of extremely r'igid. material, it must yield (be detfrmed) )by thatl, attraction of the sunl and moo1n whNlich generates the tides so as to veiry sensibly diminish the (actual pheDnomlenfa of th-e tides, and of precession tand nutation." Mil I(opkins hiad already rejected the conclusion of geologists that the earthli could be a molten nucleus coverel by a crust, only 1:.00 miles iln thickness. ie concludcd that th e depthti of the crust nmst be at 1east 800 tliles. Sir Villiam Thomson conside's it " extremely ilprobable that any cnrust thilinnc than 2,000 or 2,500 miles could mainfitain its figure vith sutficient rigi(lity against the tide-generat.ing forces of sun andl moon, to allow the phenomena of the ocean-tides and of pi)ecession and unltfiation to be as they now are.' (123) The deporlmentl of ice is o)pposed to thait of wax. Ice on liqulefying contetacts; in the ar-angement, of its tatoms to form a. solid, ]0more room is required than tFhey need ill the neigrhboringl liquid state. No doubt, thlis is duce to crysttlline arrang1lment.; the att','"'"" —.-`Z' t t.'~t~ d' r? ~, C'~~~ ~ 1/ V z~ — ~ "S -~' A uy n~ ~ S r\ A\~L A1c/ A AZ/c YC\,~~~~~~~~~~~~~~ sc,Cil J'7, ~ ~, ~ ~' ~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 6 v~.... LIQUID) FLOWt' ERX AND CENTRAL SPOT. 105 (1.28) T' here1 are two points connected Nwith this experiment, of great minutetne ss, but of great ittcrcst, Ytou see these flowers by trallnsmittced litght — ----- by the liglht, tlthat is, wl\ichl las )assed tlhrougl both t.c flowrls aInd the ice. ttt twhen you examnine thlem by allowing' a, bealm to be rcflcctcld from thlml to your eye, you finl inl the centtre of each flower a spot which shlines with the lustre of burntished ilver. You 1might be disposedl to think this s pot a bubble of air; butt you can, by immersingl it iln lot water, meIlt away tthe circul jaccnt; ice tlhe moment the sjpot is thus laid bare, it collapsels, and no t-race of a bubble is to be Seoel.'/1te spot is (a vucmtOui. Observe how trTuly Na lture orks-....how rigidly she clarries her laws into at-ll her o)eratiouls. \\re knowv that ice in melting contracts, and hcre we filnd the fact making its appcearance.'t'1e Nwater of thlese tloweis cannot quite fill the space of the ice by the fusion of which thlcy are l)roduccd; hence a vacuum necessalrily acccompaniecs time formation of every liquid flowtr. (129) \-lmeli these beautiful figures wcere first observed, anid at thie momenit when the central s)pot aplleared, like a )Oint of lioglht suddenly formed mwithin the ice,, I thought I healrd,a clink, as if the ice had split asunder when the spot was formd., At first I suspected that itit was my imagination whicl associated sound \\ith the aplp)earane of t-he Spot, as it is said that pcolple who see meteors often imagine a rushing' noise \\when they really he"ar1 nonet. 1the clink, however, wais a reality; and, if you allow Ime, I will now coldtluct you from this trivial fact through a series of intercsting phlclllomlena to a ftar-distanlt (qucstion of practictal scielice. (130) All Nwater holds a quantity of air wit;bin it in solutfion; by boililng you may liberate' tlhis ilrisoiled ilr. 01n hceatingl a flask of Nwatcr, air-bubbles are seen crowding on its sides, long before it- boils, rising tll'rough the liquid without condlensationl, and often lthat ol' on the top. The prcsence of t.his air in the water p)romlotes the ebullition of the liquid. It acts as a kilnd of Ceastic s)'rilng, p)ushin thle molecules al)art, anlld thus helping themt to take the gaseous form. 106 tIEAT AS A 5tMO)B1 OF MOTION. (1.31) VWhen this antagonist to their intimate unioll is removed, the molecules lock themselves togetlher in a far tigltoer emlbrace The cohesion of the water is vastly augmented by thoe removNral of the air. Hcrlec is a glass vessel which conltains water pullrged of;air. One cffect of the withdrawal l of the elastic buffer is, that the water falls with the sound of a solid body, and lhence this instrument is called the water-hammer. You hear lhow the liquid rings agaillst lthe end of the tub), when it is turned upside down. T'.his other tube, A B3 0 (fig. Flro 35.. i~-~- C A 35), bent into tile form of a', is intended to show how the coliesion of tle water is aftfected by long-conlinued boiling. Th[he water whihli partijally fills the )ent tlbe is first brought into one arm of thle. And now I. tap the end of this alrm ag;ainlst the t-able. You hear, at first;, a loose and jingling' sound, As long as you hear tha;t jingle the wa\ter is not in true contact with the interior surface of tile tubte. As the tapping conttilcues, you notice an alteration in the sound; the \WA'Vit'1R PUR(GE'I OF AIRt. 101 jingling has no-w disappeared, the impact bcing hard, like that of solid againsft solid. I now r'ais tlhe tub e, and turn the column of water tipside down, but there it stands in the arm A i;. Tts partlices clies g so tcnaciously to thle sidles of the tubl, and lock thelmselve's so firmly together, thlalt it refuses to belhave like a liquid body; it; declines to obey the solute zero (explailed' in ~ 0G). 111hus the tempertlature of meltilng ice reckoned 1frot thlis politt iS 2,73~ C. let.' represent thel initial temuperalture of thle s1teal0, an1d its fimal temlperatulre, bIoth reckoned from the absolute zero; thoen the proportion of the total heat convcrtel into work cannot under the miost avorable condllitions eceed tlhe firnction t lrated can rep roduce its parent;. Look at the planing-tools; look at tle boring instrumelltsl.t-..... streams of water gush over them to keep t}hem cool. T'ake ltup t, he ctled iron stllvingt s which t.he plalnmig tool }has pared off; you cannot; holdl t hem in your hand, th;ly arc sso hot;. Iere t.he mIlvingl force is restored' to its filst form; the energy of the hmaeltilne has been consumed in rceproducinlg thle power from which tlhat energy was derivCed. (142l-) 11 m11ust nIow direct your attention to a natural steamengine, which long held a place amIong tlhe won(lers of the world.- th.c G1reat Geyser of Iceland, Th'le sulrfce of:Icelandll gradually rises firom trhe coast towardl thle cent'le, where the general level is aboult 2,000 feet above the sea. On this, as oni a pedestal, are planted tlhe Jckull, or icy mountains of the island: which extend both h ways in a norltheasterly diircetion. Alontg thlis chain ='D~ 120 liEAT AS A MODE1 OF MOTION. clouds, is projected into the atlmosplhere, and ae have the geyser eruption in all its grandeur. (1347) By its contact with the air the water is cooled, falls black into the basin, partially refills the tube, in whlich it gradually rises, and finally fills th1e basin as before. I)etonatiols are heard at intervals, and risings of the water in the basin. These are so many futile attempAts at an eruption, for )not until the water in tlhe tube coies sutillicntly nlear its boiling tcmperature to make the lifting of thile column cfectivs, can wte have a true crul)tion. (1.48) To the celebrated Bunsen we owe this bcauttifuil theory, andl now let us try to justify it b1y exl)eriment: Hler is a tube of galvanized iron, six feet long', A; (fig. 38), surmounted 1by batsin, c J).:It is heated ~F1(. I a by a tllir underneath; and,l to imitate as far as poSsible the condition of tile geyser, the tube is encircled by a second fire,, att a iheigt of t.wo feet from tlle bottom. )oubltlcss the high temperature of the water, at the corresponldingp p)art of the geyser-tube, is due to the local , 79'3 4~. (or 14t30~ F.), represents whlat thas bccn hitherto called the lateont heat of water; andl the latter number, 53a (20 C. (or 9670 F.) representss the latent ]heat of stelam.:t' was manifest to those who first used these terms, that throughout the entire timl of melting,' and throulghlout ilte enti(re timeC of boiling, heat was communicated;i but, inasmuch as this heat wlas not revealed by tile thenll omete c, the fiction was invented thiat it was rendlered latent. The fluid of heat; wasv supposed to hide itself, in some unknown wayv iin tle interstitial spaces of the water and thie steam. Accorlding to our l)resent tllcory, thle mcat expended in meltingll is consumed in conferring potential energy upon thell atoms. It is, virtually, the l ifting of a, weight. So, lilkeise, ais regarl'ds steanm, the heat is consumed in pulling thle liquid molecules asunder, conferring ul)On tlhem a still greater anmount of potential en1ergy. When the heat is Awithtdrawn, the vapor Condenses, tie molecules again clashl with a dynamic elrgy, equal to that wlhich wats empl:oyed to separate thlell, and tlhe prccise jtuantity of lIcat then consumed reappears. (1.78) The act of liquefaiction consists of interior work-'1..... of work exptendcd in moving thle atoms into ICnew )ositions. h'I'le act of vap)orization is also, for the most p)art, interior vwork; to which, however, must be ladded the exterior work of forcing back thle atmospherer, when tlle liquid becomes vtapor. (179) \ e indebted to an eminent mlan to whom 1 have already often ref1rrcd, for tle first accurate determinations of the calorific power of fuel. " 11 Rumford estimatce( the calorific p)ower of a body }by the number of parts, by wcight., of wNater, wh~ich one part:, 1)y weight, of the body would, on cerfect cotibustion, raise one d egree in templlralturc. Thus, one p)oundf of clharcIoal, in combiningr ith 2.2 lbs. of oxygen, to form carbollic aeid, Cvolves hea.lt sullicient to raise the telnl)era 136 I EAT AS A MODEl)' OF MOTION. t ure of about 8,000 lbs. of water 1~ C. Similarly, one poutid of hydrogell ill comblining with eight pounds of oxygen, to )lform water: lf, generates an amlount of heat sltlicient to raise 384,000 lbs. of water 10 l. The ce1loriftic powwers, therefore, of carbon and hydlrogen are as 8: 34'.)" Th e reeelnt refilled 1csearches of'Ftavre and Silberlall oentirely confirm the (leterminations of Imuntford. (180) Let us, then, fix our attention upon this wonderful substance, \water, and trace it throughl tltc various stages of its existence. ]l'itst, -we have its constituents s free atoms of oxygen and hydrog'en, which atrac t each othier and el:tsl together'. Th'le mecha1lnical valueic of this atomic lact is easily determitred,'.h'e heatinit of 1 1). of water 1~ O. is equivalent to 1,390 foot-pounds; lence the lhelctinllg of 34,1-000 lbs. of water O1 (J. is equivalenlt to 34>000x 13900 foot-pounds, WVe tinls find that the concussion of our 1 lb3. of ]hydrogeln with 8 lbs. of oxysctl is cqual, illn mechfanical value, to the raitsing of fortty-seven million potulls one foot hbig}h. It was 1no overstatement -which -affirmed th}at the force of gravity, as exerted near1 the ellarth is al most a vallnishing qtntitXllill in l comparison with these molecular folrces. The distances \which sepl)aate the atoms before combination atre so small as to be utterly ilmmnasurable' still, it is inl passing over these distances tlat the atoms acquire a vYclocityr sufficient to cause them to classh with the tret(mdo us cn ery indicatl ed tabove. (181) After com lt,iation, the suistantce is ill a statet of 0-apor, wh\ich sinlks to 1000 C., and afterwlard condlenses to water. Iil the fiLst instance, the atoms ftll togetheIr to formt the comi)oiundl; in thle nlext instant the molecules of the compound fatll togcether, to folrm a liquid. t'he mechlanical value of this act; is also easily calculatced 9 lbs. of steam, inl falliqng to water, gencirate aln.amounlllt of heat sufllicient to raise 537'2, x 9 —:4,83ai lbs. of water it C., or 6 x 9:::::::8,3 l)s. 1Z l. Multiiplitng the forlter number ly 1,390, or Ithe lattr bt b T), we haett, ill rounid nm1tl1be'rst a l)rodct. of (f,720,000 Itbot;-ponllds, as the lt tc: Percy's Mttallurgy, p. 63. LATENNT H1EA't' OF LIQUIDS. 137 chaniical value of thle mCre act of condensation.* The next great fall is from the state of liquid to thait of ic, ad the icchltanical value of this act is equal to 993,564 1 ootpounds. TIlulls, our 9 lbs. of vwater, at its origin a lnd during its 1)rOgress, alls d(own tllree great; precipiecs' tlle tirst fill is cquivalent, in energ;y, to tlhe descent of a ton weight down prlcipxice 22,320 feet high; the second fall is equall to thlat of a ton down a i)recipice 2,900 feet high; and the thlird is cqtual to the fill of at ton (Iown a t preciplice 433 feet high. I have seen thec \wild stone-tavalanchles of thle Alps, which smolke and tltundtt down the d(eclivities, with a vellcmene almost sufficicnt to stun tLhe observer. TI have also seen snow-falakems descendin' so softly as not to hurt tlhe fragile spangles of which they wer\e colmp)osed; yet to produce, f'om laqueous valor, a quantitr, tvwhlict a child could carry, of that tender material, deiands ai exertion of enelry competent to gather u1p the sllattered b)locks of tle lXrgest stone-avalanc he I avl e ever seen, and pi)tc}l thlem to tw\ice the height from which they fell. (182) A few exp)eimtettal illustrations of the calorific efFoe. 41. feets which accompany the change of aggrlegation will not 1be out of place here. I place the thermio-clctLriC pile wlith its back Ul)Onl the table, and on its naked fitce this thin silver basin, iJ (fig. 41), which contains a qualtity of water slightly warmed1d; the needle of tile;alvanonleter moves to 90"1 and remainsl permanentlty deflected at; 70~. I now put a litfle ~ In Rtumtt rd's experiments thle heat of condensation was included ini his esti01mate of calorifie power; deductitlg the above numbe1' fi'om that found for tihe chemical union of the hydrogen and oxygen, firty millions of foot-ipounds would still remain asi tie 11mechlanical valut of tie act of combination. 138 1At AS A8 t01MO) 1 OF MOTION. po\\wdletd nitre, not more than will cover a thlreepelnny-piceC, il thle basin, and allow it to dissolve. Tioe nit.re vwas prcviously tplaced before the fire, so that not only was the liquid warmtNl, but also tlhe solid powder. Observe tle offect of their mixture. llThe nitre dissolves in the wnater; and, to prodluce this change, all the heat which bo)0th the water and hfie nitre )ossess, in excess of the temperaturc of this room, is consumled, and, indlee, a great deal more. The nccdle, you scc, Inot only sinks to zero, but mloves strongly 1 ) a at the othcr side, showing that the face of t.he pile is now powerfully chilled., (183) P'ouring out tihe chilled liquid, 1iand replacingf it with warm1 wate1r, introdu ce a )inch of common salt. The needle was at 70Y wihen tile salt was put in: it is now sinking' it reaches zero, and moves u1) on tlhe side which indicatc s cold. Butt tlhe action is not at all so strong:as in the case of saltpetre. As regards latent heat, then, wc have diferencees similar to those which wie have l already illustrated as regards tspecific heat. Putting a little sugar, instead of salt, ill tilhe wvater, the amount of heat absorbed is sensible; the liqcuid is chilled,)tbut the amount of chilling is much less than ill citlher of the former cases. Thlus, when you sweeten your hlot tea, you cool it in thte most!)hlilosop1hical lmanner; 1 whecn )you put; salt in your soup, you do tile sanlt; and if you wvere concerned with thle act of cooling alone, and careless of thle flavor of your SOUll), you miglht hasten its refr'igeration by adtding' to it satlctptre. (181) Onl a former occasion a mixture of )pounded ice and salt \was employed to obltain intense cold. 13oth thle salt and th1e ice, when they are thus nixed together, change their state of aggfregation, andl as a conselquence, the tcmperature of thle mixture sinks many deg'rees b)elow the freezing-point of water. -Here is a nest of watch-glasses wrapl)ed ill tin-foil, and immersed ill a mixsture of ice and salt.. Into each wattch-gla'ss \vas pol'cedI a little wvater, in wvhich tile next glass rest(ed. They are now all frozen together to at solid cylinder, by tlhe cold of this mixture of ice and saltt. LATlJNT l EtAT OF LIQUIDS. 139 (185) 1 will now reverse the process, landt endeavor to show you the heat develolped, in ptassilng from the liquic to the solid state. ]But first let me show you tha eat h is rell(ered latent, wlhen sulp)hate of soda is dissolved. Testing the sublstance as the initre was tested, you see thlat as the crystals melt int the water the pile is chilled. Anid now for the complementary experiment;: ThLis large glass vessel, in (fig. 42), with its lonlg neckl, is filled with a solution of sulphate of soda, Yestertllay, my assistant dissolvcd the sul)stance inll a pain over our laboratory-fire, and filled this bottle with the solution. Hie theni covered the top) careflly with a piece of bladder, and pl'aced the bottle bclhind this table, where it lhas remained undisturbed throughout thle nioght.' The liquid is, - at the prcsent moment, supersatturated with sulphlat of soda. \'Whcp the watcr'was hlot it melted more. than it could melt when cold; but now the tempclrature has stunlll mucllh lower thaln that which corresponds to the point of saturation. This 7 V~';t:.: stateo of things is sec1urd by k]ee)- ing tlhe solution perfectly still, and permittinlg nothing to fall into it. ater, kept 1thus still, mlay be,: i cooled many degrees below its; freezing-p-)oint.: S omel of you mIay.... have noticed thlec water in your jugs, after at cold winter nightt, s-uddenly freeze, on beilng poulred out in the morning. In cold climates this is not uncomlmlnrl L. The particles of sul)phate of soda, ill this solution, are o0n the brink of a precipice, and lmay be pushed over it, 1by siml)ly dropplingl a small ciystal of the substancell not l arer than a grainl of sand, into the solution., Observe what takes plGace — thme bottle now contains a clear liquid; I drop the bit of crysttal in; it does not sink; the molecules have closed 140 JlhElAT AS A MO0]D) OF' MOTION, ro'und it-, to fo.rm a, solid, in which it is now embedded. Thi'e passagze of the atoms, from a state of frseedom to a state of bOondagelC goes oil quite grtaduaIt ly; you see the solidifieation extelldingt' dow the neck of the bottle..Th }e naked face of the thlcrmo-clectric pile rests against the conlvex surl'tace of the ottlc, tand the needle of the galvanometer lpoints to zero. The procss of crystallization has not yet reachd the liquttid in front of the pile, but; you see it approachling. The salt is now.solidified ol)posito the pile, and mark tlle cffect-.'lThe atomls,* in falling to the solid form, dtvelop heat; this lheat communicates itself to the glass envelop, which, in its turn, watrms the pile, andl the needle, as you see, flies to 90~.'.l'}lo (uantity of healt thuls rendered sensible by solidilication is exactly cqual'to that whlich was ndcrced latent by liqtuefaction. (186) W e l ave, in theseex!periments, dealt with tGhe latent heatt of liquids; lot me now direct your kattenttion to a: few experiments illustrative of whallit has been called the latent lheat of vapors...... -.in other worlds, the liheat consumed in conferrling potentialn elergy, when al body passes from the liquid to the gaseous state, 2As before, the lpile is laid upon its bxack, with its nakled face uptwaird; on thlis fiace is l)aced tlle silver batsi already usedl, anlld wlich now conltains a, snmall quantlity of ai volatile liquid, 1)url)osely war\ned. IThe, needle moves, indicating heat.. But scarceely has it attaincd 900, when it turns prom"ptly, descends to 0~0 andit lies uI) with violence onl tlhe side of cold. The liquid here utsed is sullhuric other; it is very volatile, and the speed of its evaporatinio is such, thlltt it consumes rapidly the heat, ( at first coimmnicateCd to it, and then abstracts ]eat t from the face of the pile.:l 110remove the etlher, and supply its place by alcoh1ol, slightly warlm; the needle, as befo)re, ascends oln the side of heat. ly thesle small bellows we can ptrotmlotte theo evap)oration of thie a(lco1hol; you see ttlc nteedle dcseending, anl now it is 1up at 90~ onl thIe side of cold. \Wrater is not nearly so volatile as alcohtol; still, witll this larrangement, the absorption of heat by the evato J1,ATENTf IIfAUT OF VAP)ORS. 141 ration of wvater may be shown. Wre se somllctjimcs unglazCed )ottersy for holding water, whichl admlits of a slight; p)rcol.tionl of the liquid, and tllhus causes a dewiness on the esxternal iurface. Frolll t:hat turfac evaporation goes oin, and Ltie heat necessary for this molecular work, b)eing drawn in great part from thc \wvater within, (eeps ilt cool. Butttcr-coolers are made on the same plriciple. (187) The cxtent to whicih rcfrigeration may be carried by the evaporation of wvater is illustrated by the fact tlhat Awater mlay be frozen, thlrougtl t~he simple abstraction of healt by its own vapor. The inst rumentlt which ceffects this is called (t ciy'l/pwoi'zts, or ice-earrier, which was invcented by Dtr. 1,V ollastoln. It is nd in t(hlis \wa.. a liRUtt, le wa"ter is put. into one of these bulbs, k (fi;. 43); the other builb, ii, while softened by heat:, had a tube drawn out from it., with a minute a)pert.ure at thle cnd. T'lhe water was boiled in, Aand steam wass plrodulceld, unltil it had chlased all the tair away through tile small aperture in tlhe distant butlb. Wrhen the bulbts, and connect.hing tube, Fao. 43. were filled wilth pure steam, the sm-lall orifice was scaled witi a blow-pipe. Hcrc, thcn, we have wvater andlt its \rvapor, with scarcely a trace of air. rYou hearl' how tihe liquid rings, exactly as it d(lid iln the casei of the water-hailmner (188) i. turn all the liquid into one bulb, A, which is dipped into an empty glass, to protect it fromi air-currents. The c.3ptf /bulb, )I, is plunged into a freezingt mixture; ttlus tlhe vapor which escap)es from the liquid in tlhe bulb A iss con 142 IlF:A'T AS A tMODE O0 MOTION. dconsCd, tby the cold, to Awater intl.'.his conldellsation periits of the formlation of niew quantitics of vapor. As thle evaploration continues, the water which.suplplics the vapor becomes more and mlore chillcd. In a quarter of a11l hour, 01'r twenty minutes, it will ble converted into a cake of ice. Xt1re, inl fact, is the opalescent solid, formed in a second instrumentl whlich was set, in (action about half an hlour ago. Thc )whole process of refrigeration consists in the uncompcensated transfer of atomic motion from the one bulb to tlCe other. (189) ~But the most, strilinlg' example of tlhe conlsumltion of heat, iin changing the state of atggregation, iStfiurnislhed by a substance which is imprisoned in this strong ironl bottle. This b)ottle contains carbolic acild, liquefied by enormous pressure. Th;ll substance, you lknow, is a, gas underl ordimary circemstances. This glass jar is full of the fgas, which, thoug14 it manifests its nature by extinguishing' a. taper, is not to ibe distinguished, by the eye, from common air. When }the cock attached to tlhe iron bottle is turined, the presslre uponl tlhe acid is relievedl; the liquid boils..tlashes, as it were, suddenly into gtas, which rushes from tlhe orifice with im)petluoulls force. Yrou can trace this current thlrough tle ail; mixed with it yoi u see a white substance, \whlich is blown to a distance of Cjghltor ten feet.'What is this white substance? it, is i rbe acidl sMow. the cold p)roducc, in passing' from the liquid to the gaseous state, is so intense, that a portion of th e udlio a cidl is actually friozen to a- solil, whictl mingles, in small tlakes, Nwith thle issuing stream of gas. 1This snow may be collected ill a suitablle vessel. Here is a cylindrical box, with two 0hollow- handles, througllh which the gas is allowed to isslue. ltight tvd l eft yout see the turbid current, but l a large portion of the frozen ma,;ss is retained in the box. On b eing opened, you see it filled with this perfectly white solid. (190)'Tlhe solid dfisappears very gradlmlly; its conversion into vapor is slow, because it cani only slo\wly collect, from strrollunting substalnces, the llheat nlecessary to vaporize it. You can handle it freely, but not press it too mttch, lest it SOLID) OARO3NIC ACID. 143 should burn you. It is cold enouglh to burn tile handl. \Whlen a piece of it is )lunged into water, and lheld there, you see ubbel)ls rising through te watel........these are pure carbColc-acid gas. It possesses all the ploperties of the substance as conmonly prepared. 1 put a hit of the acid into mly mloutt, talking care not to inhale, while it is ther,. B3reathing against this candle, my breathl extilguislhes the flame. U[ow it is possible, to keep so cold a sulbstance in the mouth witllout injury vill be immediately explained. A piece of iron, of equal coldness,:would do serious damage. (191.)'Wato will not melt this snow, b1ut sulphlluric ther will; and, onl pouring a. quantity of this ctller ol the snow, a pasty mass is obtained, lwhich has an enormous p)owcr' of refrigeration..lcre are some thlick and irrelgular masses of glass...hc fcet, int fact) of drinkidng-glasses. I llace.a port.ion of the solid acid onl them, and wcet it with etherl; lyou hear the glasses crack; they lihave been sattcrced by the contraction produced by the intense cold. (192) In this basin is s pread a little )ap)er, and over the 1aper1' is poured a pound or two of mercury; on thle mcrcllury is lplaced some solid carbollic acid, and over the acid is poiured a little ether. MN:ercury, you know, requires a vel y low tempelatture to freeze it; but here it is firozen. It is now before you as a solid mass; the solid canl be hammlered, and also cut with a. knife. t'l enable me to lift tlhe mercury out of the basin, a wire is frozen into it; by tlle wire I rlaise the mercury, and plunge it into a glass jar containinig wtater. It liqufis, anId showers downward througtl the water; buit every fillet of mercury freezes the wrater with which it comes into contact, and t.hus', round eachi fillet is formed a tube of ice, through which you canl see the liquid mectal descending. These exlerimenCts might, be multilplied almost indefinitely; b)ut enough has been dlone to illustrate the chilling effect of vta)orization. (193) 1 have now to direlct your attention to another, and very singultar class of phenomlena, connected with the prodltuction of vapor. On the table is a broad porcelain basin, lB (fig. 144 HIUTI' Aj A MI1ODE) OF AOWtION. 44-), filled with lhot \water, and over this tlamp is a light silver basinl, heatedt to redness. I' the silver basin be placed onl the water, as at s, what will occur? You might naturally replly, that the basin will impart its Im 4t:., heat instantly to the water, and lbe cooled down to the....... tenper'atue of t]le ]atter. Bl]t nothing of thlis kitnd occurs..lThe silver for a time ~ ii'-:!j? n i; develops t sufftlicient amount of vapor underneath it, to lift it entirely out of contact with the water; or, in tile languagc of thle hypothesis dcxcloped oil a formert' occasion, it is lifted by the disceharge of molecular projectiles againlst its under surlface. This will go on11, until thl temperature of thle k)asin sinlks, and it is no longer able to produce vapor of suffiti'lt tensol to supp)ort it. Then it comes into contact- with thle water, and tle ordinary hissing of a hot mectal, togetoler withl ithe cloudt which forms olvcread, declares the fact;. (1941) LeXt us 1tnowN reverse tile experiment, allnd, instead of placing the bansinl in water, place watcr in the basinl —first of.all, however, heating tmle latter to redness by a lamp. You hear no noise of ebullition, no hissing of the water, as it falls into thle hot basin; the dro) ro lls about on its own vapor. tlat is to say, it is sustained by the recoil of tCme molccular projectiles, discharged from its under surface. I withdraw the lamp, and allow the basin to cool, until it is no longer able to )rodulc vapor strong' enough to suplpot tlhe drop. Th'e liquid then touches the metal; the instant it does so, violenlt ciuIllition sets in, and the cloud, whllich you now observe, forms al)ove the basilln. (195) You cann11ot, from your I)eseint position, see tlnis flattened spheroid rolling about in the hotf basin; -but it may be shown to you, and, if we are fortunatc, yout will see somlething very beautiful, There is, underneath thle drop, an inces LIQUID SUPPORTfED BY ITS OWN VAPOR:, 14 sant development of vapor which, as incessantly, escapcs fi'om it laterally. If thle drop rest upon a flatrtish surface, so that the lateral cscape is very difficult, t1he var1por will burst iup through the middle of tlhe drop. But imatters are hlcre so arrangcd, that the vapor shall issue laterally; and it somec timlcs lap)l)cns that the escape is rhythllmic; the vapor issuc s in regular pulses, and thlen we have our drop of Ywater mouldcd to a mIost beautiful rosette. It is there r ow —— a ro und mass of liquid, two incles in diamcter, wtith a, beautifully crimped border.'Tlrowing the beam of the electric lamp 7o...5. upon11)01 this drolp, so as to illuminate it, and holding this lens over it, I hope to cas its image on the ceiling, or on the screen. It is now p)crfectly defincd, forminig a figure (fig. 45), eighteen inches inl diameter, wvith the vapor breal:ing, as if in music, froml its edge. If a little ink be added, so as to darken the liquid, the definition of its outline is augmented, )ut the p)carly lustre of its surface is lost. I withdraw the ]eat; ta he undulationl contimms for some time, diminishing gradually the )border finally becomes unindented, The1 drop is 10now p)I'fectly motionless-a liquid spheroid; and now it suddenly 71 14( IJAT AS A MODE0)] OF' MOTION. spreads upon the surface, for contact }has been established, and the "s)pheroidal conditionl" is at an enld. (196) lWhen this silver basin is placed, with its bottom upward, in front of the electric lamp, by means of a lens in front, thle rounded outline of thel basiin may l)e b)1rought to a focus on tie screell; 1. dip tlhis bit of sponge inll alcohol and squCzce it; over the cold basin, so that the drops fall upon thle surface of thle metal: you see their magnifiel images upon the screen, and you observe that when fley stritke tlhe inverted basin tlhey spread out and trickle down along it. Let us now liheat the basin by) placing a lamp underneath it. Observe what; occurs: whlen the sponge is squeezed thle rop1)s descend as before, butt, when they come in contact; with the basiln, they no longer spread, but roll over thel surflace as liquid sp)heres (fig. 46). See how they bound and dance, as if they had fallen upon l lastiA springs; and so, inl fact., 2m1, 46, they ha-ve. E]very drop, as it strikes tile hot surlhe, and rolls along it, develops vapor which lifts it out of contact, thus destroying all colhesion bIetween the surface and the drop, INTERVAl, BE.JTW`EEiN DROP? AND HOT SURFACE. 147/ and enabling the latter to preservse its spherical or sphcroidal form. (197) thce arrangement now before you was suggested by Professor Poggendorff, and shows, iln a very ingenious mannt'r, the interruption of contact betwccn the lspheroidal drop andt its supporting surfalcc. 3From this silver basin, it (fig. 47), intended to hold the dropi, t wirc, w, is carried round yonder magnetic needle; the other cnd of the galvanometer wire is attached to one end of thlis battery, A. Fromn the opposite pole of the little battery a wire, o2', is carrictl to the movable arm, a b, of this retort-stand, Ti. I hcat the basin, poulr in tfie fIo4, 47. _ A water, and lower the wire till the cndi of it dips into the spheroi dal mass: you see no motion of the gt'v nometer nectdle; still, the only gap in the entire cir cuit is that which now exists undernc ath the drop. If the drop were in contact, the current would pass.'Tlis is p)rovdci by ithdrf winig the l mp,; the spheroidal state will soon end; the liquid will touch the i)ottoir. It now does so, and the needle intstantly flies aside. (1.98) You can actually see the interval betwcceen the drop and the hot surface upon which it rests. A private CxpCeriment Imay ie made in this way: Let a flattish basin, n (fig. 48), be turned upside down, and let the bottom of it lbe slightly indented, so as to be able to bear a drop; heat the ibasil 148 UlBAT AS A tODl 01t MOTION. with a spirit-lamp, and )lace upon it a drop of ink, d, with which a little talcohol has bccn liset. Strctch a platinum wire, a b, vertlically behtind the drop, and endcr the wire inlFit. 48.S candescent, by sending a currcnt of electricity through it. Bring your eye to a level with the bottom of the drop, and you will be able to see the red-holt \wire through thle interval Fo. 4o. between the drop and the surface whivich supports it. TLet me show you thlis interval. I place a heated basin, ii (fig. 49), as before, with its bottom upward, in front of the electrit lamp1); 'IIIRY OR1DA Ih —30 4l19R —eX PLOSIONS,4 9 and lbring carefully down ulpon it a drol), d/, dependent fiom a, pi)cttc. Whle it seems to rest uponl thle surface, and when the lens is b)rolught to its proper position in front, you sce between the drop and the silver a bright line of light, indlicating that the beam lhas passed, underneath the drop, to the screen. (199) The scpheroidal condition was first observed by L idenflost, and fifty other illustrations of it might be given. Liquiids can be made to roll on liquids. If this red-hot copper b)all be ltunged into a vessel of hot water, a loud sputtering is produced, due to the escape of the vapor generated; still, the contact of the liquid and solid is not established: llut we will let the ball remain until it cools, the liluid at lcngth touches it, and t{hen the ebullition is so violent as to project the water from the vessel on all sides. (200) AT. Ioutignly has lcnt ncew interest to t.hlis subject by expanding the field of illustration, and appllying' it to the explanation of many ext.raordinlary effects. If the hand be wet, it may be )assed tflrough a stream of molten mlietal without injury. I have seen T, 3Boutigny )ass his wet ihand through a stream of moltenl ironll and toss with his fingers thl fused metal from a crucible: a blacksmith will lickl a whitchot; iron vithout fear of burning his tongue. The tongue is effectually l)reselrvetd from contact with the iron), by the vapor developed; and it wias to the vapor of the carbollic acid, whlich slhicldcd me from its contact, that -I owted my safety whven the subst-ance vwas put into my mouth. Tl'o the same protective influence many escapes from the ficry ordeal of ancient times have been attributed by Mi. loutigny. It may be addcd, thlat the explanation of the sphelroidal condition, given by M. 3Bo1tigny, has not been accepted )by scientific men. The foregoing experiments reduce its cause to ocullar demonstration. (201) BIoiler - explosions have also been ascribed to the water in the boiler assMlming ithe sl)hroidtal state; the sudden development of steam, by subsequent contact; wit~h thle heated metal, causing the explosion. We are more ignorant of these things than Awe ought to be. Ei)xperimcntal science has brought 150 HlEAT A$ A MODE OF MOTION. a series of true causes to light, which irmaly produce these terrible catastrophes, but practical science has not yet determlincd thlle extont to which they actullll lly come into opertio. Thll efifct of a sudden genelation of steam has been illustrated by an experimentt which may be made in your presence,. Here is a copper vessel, v (fig. 50), -with a neck stopped with Fro. 0. a cork, through which half an inch of fine glass tubing pxases. Whenli the vessel is heated, a little water pourted into it assumes the spheroidal state.'The vesscl is thell corked, the small quantity of steam developed, while the -watcr' remains splherloidal, escaping through the glass tube. On removing thlle vessel from the lamp, and iwaiting for a minutet or two, tlhe water comt s into contact with tlhe copper.'V1hen it does so, thile cork is driven, as if by the explosioni of gunpowder, to a conlsiderable height into the air. (202) The spheroidal condition enables us to perform thil extraordinary expleriment of freezing a liquid in a f d-hot vessel. 1M. B]outigny, by means of sullphurous acid, fir-st iroze \water in a red-hot crucible; and Mr. Faraday subsequentl y froze mlerc:ly, by means of solid carbonic acid. This latter result may be rel)roduced; but firtst let me oplerate with wNater. Icere is a1 hollow spheroe of briass, about two inches in dliamet'er, -now accurately filled with water. Into the sphr1)e is screwed FREIi3ZSIjNG( IN REIIt). —lOT ViESSELS. 151 a w\ire, wuhich is to serve as a handle. This platinuml crucible is hcatced to glowiing rcdncss, and in it are placed some Imnps of solid carbonic acid, th1en ethIer is poiured on the acid, nceither of tllcm comes into contact, with the hot; crucible; they are protected from contact by the elastic cushion of vapor which surrounds tlhem, I lower the sp)leret of -water down upon01 the umass, and carefully pile fragments of carbonic acid over it, adding alsso a little ether. Th'e )pasty substance, %wit;hin the red-hot crucible, remains intlensely cold; cand now you hear a crack! rYou are thereby assured tlat the eXl)eriment; htas succeeded. The freezing water has burst the brkass sphlere, as it burstl the iron bottles on a fortmer experiment., Raising the sphere, and peeling off the shatter d brass shell, you hlave a solid sl)ller'e of ice, extracted from the red-hot cruciblc. (203)! pllace a quantity of mrculry in a conical coptper spoo00 and dip) it into the crucible. Thle ether in the crucible has taken fire, which was not intended. The ex)pericment olglt to be so made, that tlhe carbonic-acid gas. —.the cholke-damp of mines —-oughit to kceep the ether ftom ignitioll. But the melrcur' will frieeze not\withstanding. Out of the fire, and through the fiamc, I draw the spoon, andl the frozen metal is now beforet you., 152 lI1EAT AS A [MOD)h OF MfOTION. COIAlPTE't VI. CONVYEO'ItON o' OP}tIAE!l) AIR-lVAlI'SNl) —-SII I1'PI' AN t ) LONVW} A'ITA:.)3't:'P:'O O't'l if FAIT'lI 8 ROTA'ION ON'11Wt1: PIREO1ONS OP' ISVND- — INFLJEUNUC! OF AQUEOUS YAPOlt l'PO N UJ.MA'I'.E-UROP} 1TH}E CONDE)NSESR OP TitlME W})iSA' ERN ATIN —'fl-,lAL..IN ]RtIAN')D-TtlI: (OUI'Y St.W IEAM. —.t.Ot.IAT'ION 01' S NOY -.}' —ORMA,'ION OF ICY, OP ROM F110 O SNO.. OGA(,IRP-R l —..II}:SO3I:gEt N OP O1[,ACtIR NOtfION - GXL}I'Et:f,,llON-..OU)1)DN OP ICE B3Y 1',E.'LU:E ANIl E 1NT OACI tRElS (20) %1 PIROPOSEI devoting an hour to-day to the considcr-... ation of sonce of the thermal phe nomenaa which occur, on a large, scale, in Nature. And first, with regard to winds. You see those siblurners, intended to illumlinate this r0oo0m, whcen the daylight is. intrceptetd, or gone. Not to give light alone were, they placed thlere,!)ut, in part, to pr1olnote ventilation. The air, lhcated by the gafs-flames, expands, and issues in a strong vert;ical current into the atmosphere.'IThe air of the room is thereby incessantly drawn upon, and a frcsh supply must be introduced to make good the loss. Our chimlney-draughlts are so many vertical Nwinds, due to thl heating of thle air by our fires. (205) VlWhen this piece of brown paper is ignited, the flame ascends; and, when the flame is bllow\ln out, the smouldering cdges xwarm the air, and produce currents whlich carry thce smolke upward. I dip the smokitng paper into at large glass vessel, and stop the ncckl of the vessecl to )rcveit t he escaple of the smnokec; t.he smoke ascends with the light air inl the middle, spreads out laterally above, is cooled, and falls, like a cascade of cloud, along tIhe sides of the vess-el. WVhen this heavy iron spatula, heated to dull redncss, is held thus in the )ERIVXATION OF ATLL OUR WINDS. 153 air, you eallnnt see tlhe currents of air ascending from it. B3ut they reveal themselves by their action on a strong light. Placing the spatula< in the beam of tile electric lamp, its shadow is tfrown oni t.le screen, and those waving lines of ligvht and shade milark the streaming upward of the heated air. If this fragment of sulpllr, contained in an iron spoonl, be heatctd tmtil it ignites, and then plutnged into a jar of oxygen, tle combustion becomes brilliant and energetic, and the air of tlhe jar is thrown into intense commotion. The fulncs of thile sull)hur enable you to txrack the storms, whichl the hcatilng of the air produces within the jar. Ii use the word " steorms" advisedfly, for thie hluricanc s which desolate tle cartht are nothing more than large illustrations of tixe effectr produced inl this glass jar. (206) From the heat, of the sunll our winds are all derived.'We live at the bottom of ain aerial ocean, in a reimarkable degree pl)rmeable to the sun's rays, and but little disturbed by their direct actiont. B]ut those rays, when they fall upon thle eartl, heat its surface, and, when they fall upoll te ocea1n, they provoke evaploration. The air in contact with tle surfatce shares its heat., is expanded, and ascends into the upper regions of tle atmosphere, while the vapor fromn the ocean also ascends, because of its lightness, carrying, no dloubt, air along with it. Where tile rays flll vertically onl thle earth, that is to say, between the tropics, the heating of the surface is greatest. Here aUrial currents ascend and flow laterally, north and south, toward tlhe poles, the lieavier air of the polar regions streaming in to sul)p)ly the p)lace vacated by thte light an(lt wtarlm air. T'lhs, we have anl incessant circulation, A few days ago, the following experiment was made in the hot room of a Tur.ltish bath': I opened wide the door, and held a lighted taper in the door-way, midlway between top and bottom. The flntame rose straiglht from the taper.l) When placed at thle bottom, t;le flame was blown violently inward; when placed at the top, it was blown violently outward. Here we hlad two currents, or winds, sliding over each other, and mov 1 54 DFIAT AS A MODEl' OF MOTION. itng' in olpposite directions. Thus, alSo,.as regards a 3portion of our hemisplhere, a. cirrcnt from the equator sets inl toward the north, and flows in the higther regions of thle atmosphere while another flows toward the equator, inI the lower reg'iolns of the atmosphere. Thelsse are the upper aind the lower T.radt Winds. (207) Were the cartlh motionless, thse t\wo currents iwoultd run dircctlly north land south,, but the eartll rotates from weSt to cast round its axis, once in twecnty-fourt1 hours. nll virtue of this rotation, the air at. tlc equator is carried round with a velocity of 1,000 miles an hloulr. You have observed what takes pltace when a person ineautiously stcps out, of a carriage in mot1.ion. I..e shlares the motion of the carriage, anld whlc n his feet touch the earth het is thrown foi'wtrd il the direction of tlle motion. This is what renders leapiqng from a railway earria'c, when te train is fat utllt speed, almost, always fatal. As we wit;hdraw from thle eqxuator, the velocity duce to the earthl's rotation diminishes, anid it becomes nothing at the lpoles. Tt is prolportional to the radius of the parallel of latitude, and diminishes as these circlcs diminishl in size. Jmagine, then, an indlividlual suddenly t ransferrc d from the equator to a place where the velocity, d(te to rotation, is only 900 mniles an hlour; on touching the carth lhere he would be thrown forward in tan casterly direction, with a velocity of:1.00 miles an hlour, this being the difference bctween the equatorial vclocity with which he started, and the velocity of the eartlh's surficc in hbis now locality. (208) Simlilar collsiderations -apply to the tralnsfer of air from t~he equatorial to the norlthern regions, and vice vesat. At the equator the air possesses the celocity of the earth's s1urface there, anld, on quitting this position, it not onl]y ]has its tendency northward to obey, but also ~a- tendency to tlhe east, and it mutst take a resultant direction. T.'lh\ farthler it goe8s nlolthl, the more it, is deflccted from its original coutlre; thle 1t1oe lit. turnis towcard thle east, and tends to becoime what we should call a. westerly winmd,'.The opposite holds good for DISPXLACMENT OF AIR. 155 tile current procceeding from tle lnorth; this passes from places of slow motion to pltaces of quick motion it is mnet by the earth; hence, the wind which startet as a nor il wind becolmes a nlOl'theaSt win u as it approaches twie Cquato r, it becomes more and more easterly. (209) It; is not b)y reasoning alone that we arrive at a knowledge of the existence of thle upper atmospheric cullrent, though reasoning is sufficient to show that complensation must tlake place somnehow.-that a wind cannot b1low in any direotion w\ithout an equal displacement of air takingi place, in the opposite direction. 3But clouds are sometimes seen in the tIrolics, high in tlte atmostphere, and movig inll a direction opplosed to that of the constantt \ilnd below. Couldl we dischlarge a lightt body with sufflicient force to cause it to penetrate tihe lower cutrrnt, and reach the highCr, thle direction of that body's motion would giNves us that of the wind above. uliTanl st1'rength cannot perform this experiment, but it has neverthcless been umade. Ashes have beenl shot through thle lower current by volcanoes, and, flom the places w\here they have subsequenitly fillen, the direction of the wind which carried thelm halscc been inferred. Professor D)ove, in his " WVitterungs Verlhliltlnisse von 30eliln, cites tile following instance: " On the night of April 30th, explosions like those of heavy artillery wvere heard at ]3arbtdoes, so thattt the g-arrison at Fort St. Anne remained a-1 night under arms, Oln May l st, at dayrealk, the eastern portion of the horizon appeared clear, while th}e rest of the firmamentl was covered by at black( cloud, which soon extendlltd to tie cast, quenched the light: there, and at length prodtuced a darkness so dense that the windows ine the rooM}s could not be discerned. A shower of ashes descended) under which the tree branches bent and broke. Whence came those ashes? From the d(irection of the wind, we should infer that they came from the Peak of the Azores; they catme, however, frol;I the volcano AtOl'rne Gtarou in St. Vincent, which lies about 100 miles west of Barbadoes. T'he ashes had been cast into tlhe current of the upper trade. A second examllle of the 1i6 111AT' AS A MD01)lE 1OF MOTION. same kind occurred oil Januiary 20, 1835. O n the 24th and 25th1 tlhe stunl wavs darkened ill n Jalmaica by) a shower of fine ashes, which lad been discharctged from thle mountain Coseguina, distant 800 miles. Thle people learned ill this waly that thlle explosions previously helard were not those of artillery. lThese alshes could only tave been carried by the upper current, as Jamaica lies northeast from the mountain. lThe same erup)tion gives also a beautifutrl proof that the ascending aircurrent divides itself above, for ashles fell upon them ship Conway, ill the itacific, at a distance of 700 miles southwest of (toseguina. (210) "4 venl on the highest sumllits of thle Andes)," contillnues ove, "no traeller hats as yet reached the upper trade. f1lrom this some not1;ioln mnlay be formed of the force of the exp)losion1s; they were indeed trclmendous in both instances. TIlIo roaring of Cosegui l was hea rd at San Salvador, a distance of 1,000 miles. 1Jnionl, a seaport on the wvest coast of Conchlagua, was in absolute darkness for forty-three hours; as light began to dawn,t, it w\as observed that the sea-s-hore had advanced 800 feet upon the oce1an, through the mass of ashes which had faillen. The eruption of Mornellc Garoll forms tlhe last link of a clain of vast. volcanit actions. In June and July; 1811, near St. Miguel, one of flie Azores, the island Sabrina rose, accomplanied by, smoke1 and flame, from thle bottom of a sea 150 feet deep, and attainedl a height of 300 feet, and a circu mference of a mile. The stmall Anltilles w\ere after\ward shaken, and subsequently the valleys of the Mississippi, Arlqansas, and Ohio. But tile elastic~ forces found i1o ventl; they soughtl one, then, on the north coast of Colombia. Marclh 26th began as a day of extraordinary hleat ill Caracas; the ai was clear and thle firmamentd- cloudless. It wyas Green Thursday, and a regilment of trool)s of the line stood tunder tarmils in the barracks of the c(tuartter San Carlos, ready to join inl the procession. Thle peo)le streamed to the clurches. A loud subterrancan tlnlder was fheard, atnd immeditately afterward came an earthquake shock so violent, that thle church of ATIMOSPIIW tl; 0 CONDWNSATION. 157 Alta Gracia, 150 feet inl ]height, borne by pillars fifteen feet thick, formed a, lleap of crushed rubbish not more thlan six feet high. In1 the evening' thealmost full. moon looked down with mild lustre upon the ruins of the town, under which lay the crushed bodies of upwalrd of 10,000 of its inhlabitants(?)]Blt even here there was no cxit granted to the elastic forces unlderncatfi. Finally, l on. A)ril 27th, they succeeded ill opening once more the cratcr of Mornc G arou, which had been closed for a century; and the eartt, for a distanlce equal to tthat from'Vesuvius to P1aris, rang8 with the thunder-shout of the liberated prisoner." (.211) On} this terrestrial globe, I trace with rmey lhandct two meridilans. At the cquator of the globe thley are a foot apairt, -which would corrcespond to about 1,000 miles on the carthl's surfiace. But these meridians, they proceed'north1ward, gradually al)proach each othelr, and meet at the north-pole. It is manifest that the air which rises between these meridians, in thle cquatorial regions must., if it went direct to the pole, squcczc itself into an evcr-narrowing b)etd. Were thle eartht a cylinder, instead of a. s)here, we,might have a circulation from the middle of the cylinder quite to ceaclh end, and a return-current from each end to the middle, But this, in the case of tle earth, is impossilble, simply beca-use the sp)ace around the poles is lnab)le to emlllrae the air from the equator. The cooled equatorial air sinks, and the return-current sets in, before the poles are attaincd, and tlhis OCcurs ml'r or 1' less irregularly. ]Thel two currcllnts, moreover, instead of flowing one over the other, often flow beside cach other. They constitute rivers of air, with incessantly-shiffing beds. (212) These are lie great winds of our atmosphere, wlmicl;, however, ar' materially nmodlified by t.fe irregular distriblution of lan ad and water. Winds of minior importantce also occur, through the local action of heat-:, cold, anld eval)oration. There are win(is )rodlluced by local action in the Alps, whlich sometimes rush vith suddenl and destructive violence downl the gullies of the mountains: gentlelr diow-flows of air are 158 fla AT S A. MOMDEJ OF 0MOTION. 1prodluccd by the presence of glaciers upon the hcioh)ts, Thelmero tal'r tlhe land-31reez and the sea-breeze, due to the vrary ilng teml)erature of the seaboard soil, by dlay and nig'ht. The. morniillg sunl heating the l andt produces vertical displacement, 1 Cand t he air from the sea moves landward. In the eveninhg the hitla is more chtilled superficially, by radiatt;ion, than the sea, and tihe conditions are reversed; the leavy tait' of the land now\ flows sceawar(l. (2113) l.Ihus) tlhen, a portion of the hleat of the tr'opics is sntl,.>y an aerial messctnger, towtarl thle poles;, a more' cquable) diStrit)ution of terrestrial warmth'l being thusl secured. Bult illn its flithit; nortlthward tlte air is accolpanied )y anotlher substance..t. —.by the varpor of water, whichl, yout lnlow, is perfectly tranls-.pIarent, Jnlllt'ine the ocean of the trol)ics, g;ivinpg; forth its va)o'1, which plromotes byt its lightness tile ascent of the (associated air. ]Both expand, as they ascend: at a height of 1(6,000 feet thle air and vapor occupy twice the volume which they cll)r'aced at tlhe sca-lcvcl.'o securc this space thely must, by thcir elastic force, p1ush away\ th}e airl in all directions round them; thely must p)erfolm work; anld tllis work cannlot he b)crformed, save lat thte expeltnse of tle wlarnmth with which thcy 1were, ill the first; instance, charged, (21,I) The vapor, thius chilled, is )no longer colnmpltelnt to retain the gaseous form. I[ is l)recil)itatedl, as cloud: tlhe cloud deseemds, as rainl; and in the region of catlms, or directly under thle sunIl, \where the air is first drained of its aqueous load, the descent of rain is enormous. The Still does not tcmaiin always vertically over the samll parallel of latitudt. —.l. he is sometimcs norath of the equator, sometimes soutlh of it, the tivo trol)ics limiting his excursiont. Whenl he is south}1 of thlt equator, thlle earthl's surface, north of it, is no longer in the recgion of calnms, but in onel across which thle arital curcrent from the norith flows towalrd tihe region of calms. The moving air is but sligttlJy chattrged Awith va})orl and, as it ti'avels from north to southt it beceomes ever warmer; it constitutes a dry wind, and its capacity to retain vapor is conti\ually augment 8OUlMVEj'T,f'ILRY WtINDS IN LONDON. 159 ing. It is plain, from these considerations, that each p)lacet between the trop)is must have its dry season and rainy season; dry, when the suin is at the oplposite side of the cequator, and wet, when the sun is overhead. (215t)) Gradually, however, as tc he upper streamt11, whichl rises fiom the (quator, and flows toward. the poles, becomes chilled and dense, it sinks towa'Atrd the earth; at the )Peakl1 of lenerltlofe it uha-s already sunk below the summit, of the tlmountain,'\\ithl the cotrtary wind blowing at the basce, t t ravellcr finlds thle wind fr'om thle equator blowing strongly over the top). larither nortt the eqIuatorial Nwind sinks, lower still, and, finally, quite reaches the surfaice of tahe earth. IUtlUrole, for thle most palrt, is overflowed by this equaltorial current. IHere, inl,London for ciglt; or ]nine l11months inl the year, southwesterly winds prevail. 13tit mark wlhat an influence this must have 0upon our clilmate. The moisture of the cqutatorial oceanl comes to us, endow'ed with potential cnerg.y; with its moleclules sp)aatic, and thercftore competent to clash atld develop bheat by tlheir collision; it comles, if you will, cllharged wNvith latent helat Int our iorthern atmosplhere condensation takes 1)lace,.nld th1le h}eat generated is a main source of warlmth to olr climatlc., rcre it not for the rotation of thel elarthsl, we should have over us t;lhe hot, dry blasts of Af'rica; but, owing to this rotatiol, the wind which starts northward fi'om the Gulf of MTexico is dteflected to ]iro)pc. Iutlrarope is, therefore, the iccipicnt of those stores of latent heat which were amassed in the western Atlantic.'lhe 3British Isles come in for the gr(eatest share of this moisture -afnd heat, and this circunmstance tadds itself to th1at already dxwelt uponl-.t}flc high specific heat of Nwater. —to pres'erve our climate from extremes. It is this condition of thlings which makes our fields so green, and which aIlso gives thIe bloom to our maidens' checks. (2tG6) Another property of this wonderful substance, to whichl is probal)ly ldu its lmain inftlence as a meteorological agent, shallt b)o extunined (n12 a future oeeasion.* *x See Chalptl XI. 1060 lIEAT AS A aMO)E OF] MOTION. (217) As we travel castward in PIXurope, the amount of aqueous prccipitation grows less and less; the air becomes more 1and more dlraincdl of its moisture. ]Jrlven bet\cve the east and wcest coasts of our own islands, the dilfternce is sensitle; local circumstances, also, have a powerful inlttclnc on the amount of p)recil)itation. Dr. Lloyd finds the mean yearly temp)crature of the western coast of Ireland to be about two degrees Faihr. higher than that of the eastern coast at the same clevation, and in the same 1 arallcl of latittude. The total amount of rain which fell in tlhe year 1851, at various stations in tlhe island, is given iln thle following t-able Station. 1iafin in Inches..Pota'rlington,... 21'2 Killough,..... 28'2 Dublin),... 2-sA Athy,..... 20 )onaltglhade,.... 27'9 Courtown:'. *'' 29'6 Kilttlsh,,... 8. 32'6 Armagh,. 1.. 3 I(illybegs,,,... 33$2 ullmor,..... 3'5 Portrus'lb....... 3l/'2 lrdinncrana:,,.... 39'3 fMarklee,....... 403' Castletownase1d,..... 425 WesAtportt),.. 45'0 Calirciveenl,..... W\Tithi reference to thlis table, I)r. Lloyd remarks: (218) "1. Thalat there is great diversity in tile yearly amount of rain at the difibrent stations, all of'wh}lich (excepting four) are but aL few feet above thle sea-level; the greatest rain (at Cahircivccen) being nearly tlhree times as great as the least (at P)ortarIlington). (219) "2. That the sta;tions of least rain are eitherl inland or on the eastern coast., whbile those of the greatest, rains are at or near the vwestern coast. CONVEOY ION IN LIQUIDS. 161 (220) "c 3. That the amountl of rain is greatly dcpendc' nt on tlhe proximity of a mountain chain or groutp, being alwys considerable in suct neighborhood, unless the station lie to the northiast of the same. " Thus, Portarlington lies to the northeast of Slievcbloom; Killoug'h to the northeast of the Mourne range; D)ublin, northeast of Wicklow range, and so on1. On thle other hanld, the stations of greatest rain, Cahireivccn) Castletownscnd, Westport;, etc., are in the vicinity of high mountains, but> on a differentl side.)" * (221) This distribution of hecat by the transfer of masses of heated air from place to pllace, is callcd "coveection," in conFo,. 51. tradistinetion to the process of conduction whichlt will be treated in its proper place. e ieat is distributed in a similar manneor tlnrough liquids.'This glass cell, o (fig. 61), contains warm water. Placed in front of tihe leetrie lamp, by means of a converging lens, a magnified image of the cell is thrown upon thell screen. I now introduce the end of this I)ilpette into the water of the cell, and allow at little cold Nwater gently to enter it. IThe difierncc of refraction between the two enables you to see the heavy cold water flling thrlough the lilghter The greatest rainfall recorded by Sirt John Hlersehcl in his table (Mete,orology, 110, etc.) occurs at Cll erra Pungee, where the ammual fall isk 59a inch es. It is not my object to enter fir into the 5l)ject of mcteorology tor tile fullcst andi most accurate informatio n tho reader will refer to tihe excelltent Vworks of Sitr John l1ticr ach e and11 Professor I)ovMc 1G2 iXTl.', AS A MODE, OF aMOTION. warm wtater. The experimentl succeeds still better when a fragmentc of ice is allowced to float upon thle surfiace of the water. As the ice melts, it; sendls lonug heavy strhit doiwnivward to the )bottom of the]l cell. I nowN reverse thoe experiment, placing cold wlater in the cell, and hot; vwater in the pipette. Care is here lnecessary to (allow the warm water to enter without any momentum, which would carry it mechanically down. AoTu not;ice the effect.. The'l point of the pil)ctte is in the mid(le of the ccll, and ts the varm waater enters, it speedily turns u)pward (fig). 52) and spreands out at tlhe topl, almost as oil would do, under thie same circumnsances. (22M2) When a vessel, containilg water, is heated at; the bottomn, the warmIth communllllicated is difilused by convection. You may see tlhe direction of the ascending warm currents by meanrs of the clccti-c I lnamp, nd also thatf of the currents which descend to occupy t.hc 1)lave of the lig-hter water. Icre is a, vessel) conttailling cochinclal, the fragments of which) being not much heaviicr t0han the watr, fircel follow the dlirection of its currents. T'hl:e iieccs of cochineal break loose fromil the iheated bottom,,,scen(ling. along t el middle or tela jar, and descending again by tle sides. In tlhe Geyser of Iceland this convection occurs ofn a. grIand sceale. A fragt'enti of papei. thrown upon the centre of the vwater wlich fills tIhe p)ipl, is instantly drawn towardl the sidtl, and there suckcd down by the dcscending cur'rentt. zblt }}S ~l\('i)l Ot)tl'S()1t:,'t~t1sE'tt. 2\{lt:,l:4ltO 0CAUS E OEl0 E'UROPEAN MILDNEISS. 163 (22;3).Partly to this cause, and partly, perhaps, to the 4actionl of winds, currents cstablish tlhemselves in the occan, and powverfilly influence climate, by the lheat whNich they distrilbute. The most remarkable of thesec currlents, and by far the most important for us, is the Gulf Streamll, whichl sweeps across the Atiantic, from the equatorial rergions, through the Gullf of Mexico, whence it derlives its name. As it quits the Straits of ]lolida it has ta tcmperature of 830~ aItln'., t henice it follovs the coast of America as far as Cape t'car, whelnce it; starts across the Atlantic, taking a noritheasterly course, and, finally, washing the coast of Ireland, anld the norlthwcestern shores of 4tt'ope generally. s might be expected, the influence of this body of warm water malk-es itself most; evident durling our winter. It- thietn entirely abolishes the difttrencc of templeratur'e, due to to te diflerenc of latitude of north and south 3Britain; if we walkl firom theit Chamnnel to the Shetland Isles, in January, we encounter everywhrllere the same temperatur. The isothe;lrmal line runs, then, noirth and southl. The presence of this wNater renders the climate of VWestern lVurop) totally difilrent from that of the o)pp)osite coast of America. The rivcer I[udson, foir exam ple, in the latitude of Rome, is frozen for three months in t.he year. Starting firom lBoston in January, and procecdilng roundl St. Jolll's, and thlence to Tcelalnd, we meet eveIywhlerc the same telmpierature. ThJei harbor of 1 lanmmerfc st derivcs gr'eat valtlue f'0om tlle fLact, that it is clear of ice all tle year round. This is due to the Gulf Strceam, whlich sweeps round the North Cape, tand so modifies the climate there, that at some places, by p)roceeding nolrthward, you enter a warmer region. The contrast vbcetwcet Norlthmn E]1uropt3e and tthe east coast of America caused Halley to surmise, that t he north-poloe of the earthll had shifted; thiat it; was formerly situate somewhere near B3ehring's Straits, and that; tho intense cold, observed in thlmso re'iolns is really tlhe cold of thoe ancient pole, whichl had not been entirely subdued since the axis chalnged its direction. Bult now we know that; fthe Gulf Stream, and the ditffutision 164 IHIlAT AS A,OD, OF? MOTION. of iheat by winds and vapols, are the real causes of Tihnropean mlildness. On t he western coast of America, betweenl the 1.ocky Mountains an1d the ocean, we iind a, IJuropean climate. (224) E]lurope, then, is the condenser of the Atlantic; and the mountaint s are tlhe c1ief condensers in lEurope. On them,) moreover, when they are suttilcietly high, the condensed vapor descends, not in a liquid( but a solid form. Let us look to this iwater in its birthplace, and follow it tflrough its subsequent course. Clouds float in tihe air, and hence has arisenl tie sutrmise t1hat tllcy are composed of vesicles of bladders or water,~ thus forming shells instcad of spheres. It is certain, however that if lthe particles of water be sufficiently small they w\ill float for an indefiIite perliod without b)eing vesicular. ]it is also certain thati water-particles at high ele cvations possess, on or after preecipitatioln, the O power of buildlinpg themselvces into crystalline forms; they thutts bring forces into l)lay which we have hitherto been accustomed to regard as molecular, and wvllich could not be ascribed to thle aggregates necessary to forml vesicles. (2g~5) Snow, perfcctly formed, is not an irregular aggregate of ice-particles; in a calmh atmosphlere, the atoms arrange t}hemselves, so as to form t}he most exquisite figures. You have seen thlose six-petalled flowers, whNich show themselves wit;hin a block of ice, Awhen aIt beam of rheat is sent, through it;. The stlow-crystals, formed iln a calm atnmoslphlere, are built upon the same tyl e; the molecules arrange themselves to form hexagonal stars. Fl- rom a centrlal niucleus shioot six spicuil, evetry two of which are separated by an angle of 60O.!irom thellse central ribs smaller spiculm shoot right; and left, Awith unerring fidelit;y to the angle 600, and from t.,hese again other smaller ones diverge at thle same angle, The six-leaved blossoms issumcn tlle most wtonderful varicety of form; their tr'accry is of the finest frozen gauze; ald round aboult their corners othelr rosettcs of smaller dimensions often cling. Beauty is superposed upoen beauty, as if Nature, once committed to her t ask, took delight in SNOW —0R1YSTA1LS. 165 showing, even within the narrowest limits, the wcealthl of her resources.* (226)'These frozen blossoms constitute our mountain snows; they load the Alp)ine heights, where their fr'ail architectutre is soon destroyed by the weatlher. very wvinter they fll, anld every summer they disappear, but this rhythmnic action does not perfectly compensate itself. 3Below a certain line, warmth is predomintant, alnd the quantity which falls every winter is entirely swept away; above tlhis line, cold is predominant; the (quantity which falls is in excess of the quantity melted, an( ant annual residue remains. In winter the snows reach to the plains; in summner they rctreat to tlhe snow-iztne-to that particular line where the lsnow-fall of every year is exactly balanced by the consumption, and above which is the region of eternal snows. But, if at residue remains annurally above tlhe snow-line, the mountains must be loaded with a burden whlich increases every year. Supposing, at a particular point above the line rcferrcd to, a layer of tbhree feet.a year to be added annually to the mass; this deposit, accuimulating even thlrough the brief periodl of the Christian era, would produce an elevation of 5,580 feet. And did such aceumulations conitinue throughlout geologic, instead of historic ag es, we canniot cstimate the height to which the snllows would pile themselves. It is manifest that no accumulation of this kind takecs placo; the quantity of snow on thlec moiuntailns is not augmenting in this way. By some means or other the sun is prevented from lifting the occan out of its-basins, and piling its waters perminanently upon the hills. (227) HIow then is this annually augmenting load taken off the shioulders of the mountains? Thec snows sometimes detach ftiemselves, and rush (1own the slopes inl avilanches, melting to water inl t:he wvarmer air below. ]But the violent rush of t.he avalanche is not their only motion; they also creel, by almost insensible degrcees, down the slopes. As latyer, * See flg. 63, in which arn copied aomno of tho beautiful drawings of Mr. nlaishcr. .................. ~~~~~~~~~~~ ~ ~ ~ ~ ~.............. K......J.. ~!~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~r........' q....,~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~,'I,;,- t < Al vin: *: ~~~~~~ Nv io o. C ft Co nf igu ra ionall, GLJACIE'tRS. 1I V6 moreovelr, heaps itself upon layer, the decper portiols of the mass becomle squeezed and consolidated; the air, first entrapl)ed inl the mesles of the slH\o, is squeezed out, and the compressed mass approximates more and more to the character of ice. You klnow hlow the granules of a snowball will adhlere; lland you know how hard you can make the ball if mischievlously inclined. The snowball is incipienti ice; auglment your prcssure, and you actually convert it into ice. 13ut even after it has obtained a compactlness which w\ould entitle it; to be callcd ice, it is still capable of yielding more or less, as the snow yields, to pressure. Whlcn, tlerefore, a sufficient; depth of tile substance collttccts upon te earth's surface, the lower portionls are squeezed out by thie pressule of tile upper ones, and if thle snow rests upon0 a slope, it will yield principally iln tlhe direction of tile slope, and move downward. (228) lThis mlotioll is incessantly going on along thle slopcs of every snlow-laden mountain; in tile l[iimalayas, in tlhe Andes, in tihe Al)s; butt in addition to thlis motion, which depelids upon thle power of tlle substance itself to yield to l'essutre, thlere is also a sliding motlioll over tile inclined bed. Th'le consolidated snovw movcs 1bodily over tlte mountain-s.lope, grinding off thle asperities of tile rocks, and plolishing thleir lard surfaces. The undlter surface of tile miglhty polish er is also scarred and furrowed by thle rocks over whichi it; has passed; but., as the colmpacted snow descends, it enters a warmer region, is 1more copiously mnclted, and sometimes, before thle b)ase of its slope is reached, it is wvlolly cult oft' by fision. Somctimes, ilowever, large and deep valleys receive tlhe gelid masses tlhuls senit down; in these valleys it is furthler consolidatcd, anld: thlroughl them it moves, at a slowr )ut measurable pace, imitatilig in all its inOtiOlls thlose of a river. Timhe ice is thuts carried far beyolld thle limits of perletual snow, luntil, at leng th, tle consumpt ionl below equals thle supply above, and at tiis p1oint ttle glacier ceases. ]From thle snlow-line downward il sullnmer we lhave ice; above thle snow-line, bothl summer and winter, we fthave, on tile surface, Sno. The pr 168 lI lAT AS A MODE1 OF MOTION. tion )below the snow-line is called a glfcaier"; that above the snow-line is called the neo. Thle n6v6, then, is the feeder of th1e glacier. (229) Several valleys, ttis filled, may unite il a single valley, the tributary glaciers welding themsclves together to form a trunk-glacier. Both the main valley and its tributaries are often sinuous, and the tributaries must ehalnge their direction to form tlhe trunk,. The Nwidth of the valley, also, often changes: the glacier is forced througlh narrow gorges, w(idening after it has passedl them; the centre of the glacier moves more quickly thfan the sides, and tile surface more quiclly tilnlt thoe bottotm. The point of swiftest motion follows tilhe same law as that observed inl t he flow of rivers, changing from 1one side of the centre to the otherl, as the flexure of tlhe valley lhaul,'es. Most of tile great glaciers in the Alps have, in sum11111er, a central rvelocity of two fcet a day. There are points on the Mer-de-Glace, opposite the Montamnvert, w\lieh lhave a daily motion of thirty inches int summler, and in winter ]1have been found to mnove at half this rate. (230) T'hle power of accommodating itself to the chlnnel. thriough ywhich it moves, has led eminent men to assume -that ice is viscous; and tlhe plhenomena, at first sight seem to enforce this assumption. The glacier.widens, bends, and narrows, and its centre moves more quickly than its sides; a viscous mass would undoubtedly do thie same. Blut the most delicate experiments on the capacity of ice to yield to stainl, to stretch out like treacle, hloney, or tar, have failed to detectt thlis stretching powver. Is there, then, any other physical quality to which the power of accommodation, possessed )by glacier ice, may be referred? (231) Let us approach this subject; gradually. W'e know that vaplor is continually escaping fr'om the free surface of a liquid; tlhat the particles at the surface attrain their gaseous liberty sooner than the )articles wYitihin thle liquid; it is natural to expect a similar state of things Awith regard to ice; thfit wltn the temperature of a mass of ice is uniformly aug REGEfLAJT, O. 19 mxented, the filrst )particles to attain liquid libert.y will b)e thlose at thre surfIace; for here tile are entirely free, oil one side, ffi'oml the controlling action of the surrounding p)articles. Supposing, tlell twto pieces of ice, raised tlroughoult to 32~, and neltilng, at this telcmperalture, at their suirfaces; \vwhat may be expected to tale place if we place the liquefying surfaces close together?,VWe thereby virtually transfer these surfaces to the centre of the ice, whereC the motion of each molecule is cottrolled, l roun11d, by its neighbors. As might reasonably be expected, the liberty of liquidity, at each point where the surfattces touch each other, is arrested, and the two p)icces freeze together at these points. Lct us niake the experiment: liere are two masses justscut asunder with a saw;. place their flat surfaces togethler; a second's contact will suflice; tlhey are now frozen togetheri', and by taking hold of onle of them I thlus lift them both.} (23,2) Tlhis is the effect to which attention was first directted by,Mr. Jl'aradtay, in June, 1850, and which is now known under the name of.I'eelation.* On a hot sunml er's day I have gone into a shop in the Strand, where friag'ments of ice were exposed in a. basill i the window; and, with the shlopmlan's perlliission, have laid hold of the topmostl piece of ice, and, by means of it, have lifted t he whole of the pieces bodily out of the dishl. l'louglil thle thlermometer at the time stood at 80%0 the pieces of ice had frozen together at. their points of junction. Eveon uWnder hot water t1his cfifct takes place. This basin contains water ats hot as my hand can bear; I plmge into it these two pieces of ice, anId hold0 them togetlher for a moment: they at(re now frozen together, n1otwithstaldilng the presclice of 1 the heatc;t liquid. A )pretty cxperimc nt of AMr. Faraday's consists in pltacing a number of small fragments of ice in a dish of \ater deep enough to float them. Wlhen one piece touches the otherl even at a single point, regclation instantly sets in.'l'lhus, a train of pieces may be caused to touch each other, ~ A trnim uggested )by Dr. HIook r to Mr. HIuxley and myself on the publieation of our first paper upon glaciers. 8 170 ITEAT AS A MIODIE OF [MOTION. and, after tlhey have once so touchled, you may take the terinatl piice of the trailn, and, by melns of it, draw all the other1s,after it. W'lhen we sCek to bend two'0 pieces, thus unitcd at their poilnt of junction, the frozen points suddenlly separat eby fracttre, but, at the same moment, other points come into contact, and regelation sets in betweenc them, Thus a wheel of ice might be caused to roll on an ice surface, the contacts being incessantly ruptured, wtith a, crackling noise, and others as quickly establlished by regelation. In virtue of this proporty of re-gclation, ice is able to reproduce *many of the plhenomena which are usually ascribcd to viscous bodies. (233) Here, for example, is a straight bar of ice: by passing it successively through a. series of moulds, each more curved than tie lat;t, it is finally llturnd out as a semi-ring. TIle straighlt bar, on being squeezed into the curved mould, breaks; but by continuiilg tle pressure, new surfices comeo into contact, and thle continueity of the mass is restorced. A handful of those small ice-flagments, when squczet d together, frccze at their points of contact, and form one aggregate. The rmalking of a s)nowrball, as remarked by Mr. "alraday, illustrates the same pincilnlle. In order that t.his fi'eezing shall take place, the snowv ought to be at 32~, and moist. When below 320, and dry, on beiing squcezed it behaves like salt;. Thle crossing of snow-bridges, in the upper regions of the Swiss glaciers, is often rendered possible solely by the regelation of tihe snow-granules. Thle climber treads down the mass carefillly, and causes its gratnules to regelate: lhe thus oltains an amount of rigid(ity whiclh, withoutl the act of regelation, would bl quite unattainable. To those unaccustomed to suchl work, the crossing of snow-bridges, s)annting, as they often do, fissures of 100 feet, and moret in depthi, must appear quite appallingl. (234) When this mass of ice-fragments is still flirther squeezed, they are brougl{ht into closer proximity. AMy hanld, I)owever, is incompletent to squeeze them very closely together-. Placing them in this boxwood mould, lwhiclh is a shallow cyl )MOUXDING OF IOE 1BY PRESSURE. 171 inder, and inserting na flat piece of boxwood ovcrhlcad, I introducc both between the plates of a small hydraulic press, and squcteezc tihe mass forcibly into the mould. The sutbstance is convltttd by thle pressure into a cohcrcnt cake of ice. We Fro, 54.t can place it ini this lenticular caYiity and agaiin squcteze it. It is crushed by the p)rcSSulrc, of course, )utL new contacts are establishlicd, and now the imass is turned into aL lens of ice. Iet us now transfcr the leAns to this hemmispherical cavity, it (fig. 54), bring down upon it a hcmisphcrical protub)erancc, P, whichl is not quite able to fill t he cavity, and squeeze the mass: the ice, whict at moment ago was a lencs, is now pressed into the space betwccn tile two spherical surfacCs: Oln renoving the protuberance, yo0u see the interior sulface of a culp of glassy ice. When detached from the mould, it is a hlcmispherical cup, which may be filled wilth cold win, nwitllou; the Fro. 65. escape of a drop. I scrape, with a. clhisel, a quantity of ice from this blockl, a, a lacitnl tihe sp)ongy manss within this splherical cavity, o (fig. 55), squeeze it. and add to it, till, finally, by l)ringing down upon it another sphlerical cavity, i), it is cn a1 1 A IIMi'V AS A [MOD1) OF MTOTIOXN. closed as, a sp)here between both. As t.he press is worke(l, lthe sublstance b)ecomes more and more compact. I add more material, and again squeeze; by every sucll act the mass is made hardeor, and now you hlave a snowball before you such as you never saw before. It is at sphere of hard, translucent ice, ii. Thus, you see, broken ice can be compacted togetlher by presstire, and, in virtue of the 1property of regelation, whIich cements its touching surfaces, the substance may be made to take any shapec we please. Were the experiment mworfthl the tt'ouble, a?opme of ice might, be formed from this block, and afterward coiled into a knot;. Nothing, of course, can le casier than to lrodulce statuettes of thle suibstance from suitab)le molds. (235) It is easy to understand hlow a substanllce so elndowcd can be squeezced through the gorges of the Alps —— can bend so as to accommodate itself to the flextltes of the Alpine valleys, and canl perm it of a differential motion of its parts, Nwithout:, at the same tilme, possessing a sensible trace of viscosity. The hylpotlhesis of viscosity, first; started by Rlcndu, zand worked out with such ability by Prof. Forbes, accounts, certainly, for lalf the facts. Nihtlere pressure come1s into p)lay, thle deportment of ice is, applarently, that of a viscous body; where tension comes into play, the analogy with a viscous bod y ceases.* (2.36) 1 have thus briefly sketchled the phllenomena of existing glaciers, as far as they are related to oiur present subjeter; but tlhe scientific exl)lorer of mountain-regions soon meets tith aplpearances,'whiich arry his minld back to a state of tbings very dilifrentt from that of the plresent day. The unmistakable traces which tthey have left behind them show that vast glaciers once existed, in places from whlich thle) have fo ages disappceard.c Go, for example, to the glacier of tlhe Aai in the Iernese Alps, tand observe its pr1esent performancllcs; look to thle rocks uI)pon its flanks as they alrc att this momentl, rounded, polishledi, and scarred by the moving ice. Andf, havx For futrthler information regarding the glacial phenomena, I must recfr thle'radcr to the " (laciers of the Alps," Murray, ondilon, ANCII:NT GI, ACIERtS OF SWITZER1L,\AND). 1'13 ing 1)3y patilent and varied exercise educated your ey)e and judgmetntt in these matters, w\alk down th e glacier towfard ifs cldm kccping always in view the evidenccs of glacier action. After (quittinlg the ice, continue your wall( down the valley toward the GOrimsel: you see evcerywhere the same untistakable record.'JThe rocks which rise from t)he bed of tie valt Icy nire rounded like hogs' backs; these are the " roi s lmoutomni6s" of Charpntlier ancid Agassiz; you observe upon them the larger flutiltngs of thle ice, and also the smallcr sears, scratched by pe)cbblcs, which the glacier held as a kind of emery on its tinder surface. All the rocks of the G rimsel Ithave been thus plnltcd downr. Walk down the valley of -Itsli and examine the mountain-sides right antd left,; witlhout; the key, which If now su\ppose you to possess, you would be in a land of enigmas; but %with this key all is plain —.you see cverywit re the well-known scars and ilutings and furrowings..fin the bottom of the valley you have thle rockcs liled down, in some places, to dome-shaped mtsses, and, in othlers, polished so smoothly that to pass over them, even when the inclination is moderate, steps must be hewn, All the way dovwn to Mteyringcn, and beyond it, if youl wish to t)ursuie thle illquir,, these evidences abound. Fi{or a p)reliminary lesson in recognisziJnIg the traces of ancient glaciers, no better ground than t his can be chosen. (237) Similar evidences are found in t}e valley of the Ilhone; you may trace them thlroughl the valley for eighty miles, and lose them at length in t. he Lake of Gcneva. ]3But on the fll'anks of thle Jura, at the oplposite side of the Canton de Vnad, the evidences reappear-. All along these limestone slo)tes arte strewn the granite bowlders of Mont B]lanc. R.ight and left, also, froml tle great tlhonle Valle), the lateral valleys show that they were once filled wiith ice. On tle Italian side of thle Alps the remains are, if p)ossib)le, more stupendous t. han thtose on the northern side.'Grand as til present glaciers seem to those who explore them to their full extent, they are 1mleore pigmies in comparison with their predeccessors. 17,1 1 it EAT AS A MOI)DE OF] MOTION. (238) Not;in Swit, zcrland alone —not alone in proximity -witlh existing glacirs-are these well- known vcstiges of the ancient ice d(iseelrnile; on the hills of Cumtlerland they are almost as clear as tamong the Alps. Where the bare rockl has been exposed for ages to tlhe action of weather, t.he finer marks have, in most cascs; disapl)arled; land tlhe mammillated forms of theL rocks are the only cvidenccs. ]h lt the removal of the soil which has protected t, hem often discloses rock siurfaces, scarred as sharply, and )polished ats cleanly, as those which are now being scratched and polished }by tihe glaciers of the Alps. Round about Scawfcll, the traces of ancient ice apl)ear, both in'rocites )'nottones and blocs perchAs; and there are ample facts to show that ]Borrodale was once occupied by glacier ice. In North Wales, also, t;he ancient glaciers have pl)aced -their stamlp so firmly ulpon the rocks, that tihe ages which have since elapsed have failed to obliterate even their superficial marks. All round Snowdon tihese evidences al)ound. On th e southwest coast of Irteland rise tlhe Reeks of MaIcgillicuddy, whicll tilt; upward, and catch uponl their cold crests tle moist winds of thle Atlantic; precipitation is copious, anld rain at KillarneCy seems the order of Naturelo. T1 this mloist region every crag is covered with rich vegetationl; but thle vapors, which now descend as mild and fertilizinl rain, once fell as snow, which formcd the material for noble glaciers. The.-Black Valley was once filled by ice, which p)lancd down the sides of thle ]1urple MXountain, as it moved toward thle [lper ]ake. Th'le ground occul)ied by this lake was entJirely covered b)y thle ancient ice, and every islanid that now emerges froml its siurface is a glacier-dome. The fantastic names, vwhich many of the rocks have received, are suggested by the shapes into which they lave been sculplitred 1)y the mighty moulding lane which once Ipassed over them. North America is also thus glaciated. ]uat the most; notable observation, ill connectioln witfh thlis slbject., is on1e recntly made by l)r. ITooker during a visit to Syria: he has found A PROBThE1St AND ITS SOLUTIONS. 175 that the celebrated cedars of I,cbanon grow uponl ancient glacier moraincs. (239) T.o determine the condition, lwhlich permitted of tlhe formation of tlose vast masses of ice, has long been a problem wvith pi)loslpehrs, awd a consideration of the solutions whicht have been offered, from timne to time, wvill not )be uninstructive. I have no new hypothlesis to offer, but it seems possible to give a truer direction and more definite aim to our inquirics than they can at preseint boast of. The aim of all the writers on this subject, Nwith whom I am acquainlted, has been the attainmltent of coNl. Somne cmintnt men haver thought, and some still think, that the reduction of temperaturet during the glacier epoch, was due to a tcmporalry diminution of solar radiation; others lhave thought that, in its motion thllrough space, our systcnm may have tr aversed regions of low temperature, and that, during its )passage through these regions, the ancient glaciers were l)rodulced. Others have souglht to lower the temperature, l)y a. redistribution of land and water. If I understand the writings of the eminent menl whto have proploutndd and advocatcd the ahove hypothcses, all of lthem seem to have overlooked tihe fct, that the ctormous extension of glaciers in b)y-gone ages demonstratcs, just as rigidly, t.he operationl of heat as the action of cold. (240) Cold alone will not produc1e glaciers. You may have the bitterest northeast wvinds here in London throughout thle winter, without a single flake of snow. Colt must have the fitting object to operate ul)on, andl this objcct.......the aquc ous vapor of the air........ is thime direct product of heat.. Let: us put thlis glacier question in anothler form:l the latent heat of aqueous vapor, at thle temtleratture of its productionl in the tropics, is about 1,0000 Pahrl, for the latent heat augments, las the temperature of evaporation descends. A pound of water, thfen, vaporized at the equator, has absorbed 1,000 times the quantity of hicat whtich would raise a IpounI1d of the liquid one degree in temlerature. But tlhe quantity of heat which would raise a pound of water one degri-c would raise a pound of 16 IIEAT' AS A MODE, OF MOTION. cast.itron ten deg'rees hence, simply to conivert a poundl of the water of the equlatorial ocean into vapor, w\ould require a quatltity' of heatt, suftlicient to impart to a pound of cast-iron 1:0,000 degrees of temperature. Htit the fusing-roint of cast-iron is 2,000 iFahr.; - therefore, for every pound of vapor produced, a quantity of iheat has been expended by the sun, sufficient to raise 5 lbs. of cast-iron to its. meltilg-point. Imagine, tfhen, every one of those ancient glatciers with its mass of ice quintupled; and ilmagine the place of the mass so augmenlted, to be taken by an equal weight of east-i'on raised to the white ]eat; of fusion, we shall thel have the exact expression of the solar action, involved in the )rtducltion of the ancient g'laciers. Substitute the hlot iron for the cold icc —our speculations would inst antly be (lirected to laccount for the yigh, temperature of lthe glacial epoch, and a. complete reversal of some of the hypotheses labove quotedl would probably clansue. (241) It is perfectly manifest, that by Sweakening t he sun's action, either through fa defect of lemissioll, or )by thi steepinig of the entire solar systeml in space of a low temperature, we should be cutting off the glaciers at their solurce. ast masses of mountain-ice ilndicatt, infallibly, the existence of commen-ll surate masses of atmospheric vapor, and a )roplortionately vast action oil the part of tlhe sun. In t a listillintg appl)aratus, if you rlequired to augment tle qualtity distilled, you \would not surely attempt; to obtltinl t.he low templerature, necessary to condensation, by takillg the firc from un'der your boiler; bnt thllis, if I understand them arighlt, is whyat: has been done by thlose )hilosophcrs who have soughtl to produce the ancient glaciers by diminishling the sunll's hlaelt. ^It is quito mallifest that the thing most needed to produce ftle fglaciers is all improveda conledSCer; we c cannot afllrd to lose an iota of solar act;ion; we need, if any thlling, more va)or, but we needl a condenser so 1)owerful, tlat this vapt)or, instea(l of falling in liquid sho+wers to the carthl, shall be so tt frduced in temlll)ratur as to dlescend in s1ow. The )'rob)letm, I thilnk, is thus narrowed to tlhe precise issue onil wh}ich its solution depenids. MOULI)NIG ICNl. 17 NoTr,- -......In 0noldillg ice, it is advisable to first wet the Vmould -with hot water. This facilitates theo removal of the conp)ressed subsltallee. The ice-cup, refcrred to ill ~234, may be firolm M&- to 3 inchels in cxternml diametiCer, but the thick'Cess of the cup ought not to cxceed a qtarter of an inch. A conical plug is inserted into my own moulds, the tal)ping of which soon detaches the ice. 1s8 hlEAT AS A MO0,DE OF,MOTION. CII APTE dt VII. CONDUCTION A'R. ANSttIIMION ON NOfON'- (. —001) CDONDC.Gt'ORS AND 1AD) CONI)UGOS-T -. CON' DIlVOt'tI'l Y ONF'fTle M)-;I'AS FOR IIAlAT: i},AI'{ON It: WR}':N t'IlE CONi)U1Gil t't'I ON' DIEAT ANND'IlfAT Of T:I,E~t'}.t"I'~...-I NfII.UEN: } OF'IP:i'ERA'IU}E: ON tllEt CONDUotlION OF EtECj,'(',IY. - INFltUNC- OP,MOECULAR CONSI'IIt ON't'lE CONDUGIION O1F iEtAT. — REl,'ATION O' 8PEt'fIO r i:NAT'IO CONDUCION -... - l'lltOSOPlt o 0N CI.Ol'tlE:,tl RUtIORI)'8 }IXPNEI:'l'IN ISTS-I -.N.!,U}.:(CE O',MECI CAbestos a red-hot iron ball: the ball can )bc supported without inconvenience., The asbestos intercepts the lheat. That this division of the substlance should interfere \itfh th transmission might reasonably be infcrred; for, heatt being motion, any ftiing whlich disturbs the contiuimty of the molecular charin, 204 IlSAT AS A MODE OF MOTION. along which the mnotionll is colveyed, mulst affect tlhe trlansmission. uIn tile case of the asbestos, thle fibres of the silicates are separated from each otl'er l)y spaces of air; to p)ropagate itseltf iCmrefore tile motion has to pass fr'om tle solid to the air, a very light body, and agEain fi'om the air to tlhe solid, a, colmparatively heavy body; and it is easy to see that t;le tlransmissionl0 of motion throlught this comll)osite texture 1must be very imnperfect. In thie case of anll animaPls fur, this is Imore especially the case; for here, not only (1o spaces of tair intervene betiween the hairs, but; thle 1hairs themselves, unlike the fibres of the asbestos, are very bad conductors. LJava has bcen known to flow over a, layer of ashes, underne ath which wl s a bed of ice, ttand the non-conductivity of the lashes has slaved tle ice fr'om fulsion. 3Red-hot cannon-balls may be \wheeled to thle g'tun's moutlh in Nwooden barrows p)arttially lillcld with salud. Ice is packled in sawdust:, to prevent, it from melting-; tpowdered charcoal is also an eminently bad conductor. 13ut there are Cases where sawdust;, clhaff, or charcoal, could not be used with safety, oIn atccount of tltir collmbustible nature. EInt sauch cases, powdered gyp)sum may be used wifl advantage; in t1he solid crystalline state, it; is an incomparably worse eonlduct1or than silica, and it may be safely inferred that, in thel povwdered state, its imperviousness faitr transcends thalt of sanlld, each griaill of wvhlich is a good conductor. A jacket of gypsumpowvder, rounld a steam-boiler, would materially lessen its loss of heat. (286) VWater usually holds certain minerals in solution. T1 percolatingl through the earth, it dissolves more or less of the su8;bsltances with which it; comes into contaet. 3lor examp)le, in chalk' district's, the water atlways contains a quantity of carblonate of lime such water is called (r7'd water. Sulphate of litlme is also a, common iltngredientt of wrater. it evaporating, tile water only is driven off, the mineral is left, behind, oftenll illn quantiies too great to be held in solutiol by the water.. Many sp)rings are strongly impregnated w\ith carlbonate of lime, and tile consequence is, that ywhen the wNaters of \VIi''ill)ItA\V.A O1F ItEAT i3Y COND)UCTO1RS. 205 sucl slprings reach the surface, andl arc eX)OSCe to tile ilr, whllcre they canl partially cvaporatl, tehe miner'al is precipitatc(l, an1 forms incr'ustations on the surfaces of plllnts alnd stonces, over whtich the water trickles. it boiling water, tlhe same occurs; thlte minerals are prcoipitated, and there is scarcely a kettle in L.ondon vwhich is not internally coated with a mincral incrustation.'.This is Lan cextremely serious diftichult3ty, as regards steam-boilers; the crust is a bad conductor, andc it may become so thick as materially to interccept the pa ssage of heat to tlhe Awater.:IsBefore you is Xan example of tlhis misc hicf. This is a portion of a boiler belonginglo to Ia steamer, whvichl was all but lost throughl the cxhaustion: of hier coals: to bring this vessel into port, her sp)ars, and cvcry other piece of available wood(t, wcre burnt,. Onil xamtlulation this formidable incrustation wavs found witllin the boiler: it is mainly carbonaite of lime, whvich 1by its nolnconducting p)o\wcr rendered a prodigal expenditure of fucl nccessIay, to gcncrate the required quanttity of steam. iD)oubtless, tlhe slowness of many kettles ill boiling wNould be tfound due to a similar cause. (287) One or tvwo instances of the action of good conductors, in!)rcvcnt.ting the local accumllation of iheat, w\ill not be out; of place here.'Thecse tNwo spherel s are of the same size, antd arc both covered closely with white paper. One of them is co)per, thle otlher is wood. I place a. slirit-llamp undernela th each of thcm.'The motion of heaIt is of course, comlinun.icating itscelf to each ball, but, in one, it is quickly conducted away from tlhe place of contact wit: the flame, throulgh the entire m1ass of the ball; in the othler, th}is (tiuick conduction does not take ptlace, the motion therefore accumulates at the )point where the flame )plays upon tlhe balll; and ere you have the resullt. O)n turning u11) thle w\ooden ball, the white lp)pcr is seen to be charred; the other ball, so far from bteing charred is Iwet, at its under surfllce, by the condensation of t:he aqueous vapor gcncrated by t;ic lalml). I ecre is ta cylinder covered closely with paper; I hold its ccntre, thus, over thec la1mp, 206 1IIlAT AS A Mt01)I OF Mt11rION. turning it so that the flame shall play all round the cylinder: you see a well-defined black mark, on one side of whicht the paper is chalrreld, on tlhe other side not. The cylindr is h1alf brass and half ood, and thllis black mark shows their line of junction; T-where the paper covers the vwood, it is clarred; where it covers the brass, it is not; sensibly laficted. (288) If tle cntlirc. moving force of a common riftc-bullet were communll icated to a hlca\ty cannon-ball, i t wotuld produce in the latter a very small amount of motion. Supllesing the ritlc-bldlet to wNeidgh two ounces, and to lhave a velocity of 1,600 feet a second) tlhe moving force of ttis bullet, commulll icated to a 100-lb. canlnon-ball, would impart to the latter a velocity of only 32 feet a secondt. Thullhu with regard to a flame; its molecular motionl is very intense, butt its wcight is cextremely small, and, if communicated to a theavy body, the intensity of tlec motion must; f all Ilere, for example, is a sheet of wire gaulze% with meshes Nwide Cnough to allow a ir to pass freely through them; and here is a jet of gas, burning brilliantly. -I bring down the wire gauze l)pon thle flame; you would imagine tlhat the flame could readily pass througll the meshes of t1he gauze: butt no, not a flicker gets throughl (fig. 65). tlhe combustion is cntirely confined to the space Flo. 05. Flo. 66. under the gauze. I extinguish the flame, and allow the unlignited gas to stream firom the burner. I place the wire gauze, thuls, above the burner: the gas is 1now freely passing through tlhe mieslhes. On igniting the gas above, you have the flame, but it does not propagate itself downward to the burner (fig. TIlE' SAF1TY- AMl'. 20'. 66). You see a d(lark spalce of four inches, blt\\ween the burner and the gauze, a space filled with glas in a condition eminently ftavorable to ignition, but stlill it does not ignite. Thus, you see, this met;allic gauze, whicl allows the gas to pass freely through, intercepts thle tlame. And why? A certain lheat is nececsanryl to cause tlhe gas to ignite; but -by placilng the wire gauze over the tlttue, or the flame over thle wire gauze, you transfer tflh mlotion of that light and quivering t, hing to the comparatively heavy metal. The intensity of thie molecutlar motion is greatly lowered: so nmutc lowered, indeed, that it is incompetent to propagate the conlbustion to the oplposite side of the gauze. (289) 5We are all, unlappily, too well acquaintcd with the ter'riblte accidents tihat occur, througl explosions in coal-mines. Yout know that the cause of these explosions is the l)lresence of a certati gas-...-.a coimpound of clarbon and hydrogen:-generated in t he coal strata. rlWhen this gas is mixed with a suflicient q(luantity of air, it explodes on ignition, the carbon of t he gas uniting with the oxygen of the air, to produce carbonic acild; the hhydrogen of ttie gas uniting with t, he oxyg'en of the air to produce water. By thle f-lame of the expl)osion the miners are burnt; but, cvcen should this not destroy life, they are often suffocated afterward, by thle carbonic acid p)rodtced. The original gas is thte miner's " fire-damp," the carbonic aei(t is his" chokce-damp." Sir Jlumphry ]avy, aftcr ha ving assurcd himself of tlle action of wire gauzel just exhibitcd before you, applicd it to tfhe construction of Ia lamIp, which should enable the miner to carry his light ilnto an explosive atmosphere. Previous to th he introduction of thle sa.fietylam~p, thle miner had to content himself with thle ight; fr'om sparks plroluccd by the collision of flint and steel for thlcse sp)arks were found incompetent to ignit the fire-damp. (290) D)avy surround'd a common oil-lamp by a cylinder of wire-gauze (fig. 67). So long as this la1mp is fed by)r pure air, tlme flame burns with tte ordinary br-ighitness of an oilflame; but~, when the miner comes iinto ant atmosphere coln 208 11PAT AtS A 5IMOt) 0, OF M(OTION. taining ire-dca(llmp," his flamen enlarges, and becomes less luminlOus; insteladlof being fed by the pure oxygeon of the air, it is nlo%1 in pa)rt, siurrouinded by intllammable gas. Tbis cnlargement of tile flanie h6 oughtl to take as a warningllg to retire. Still, thlough a colntilmous explosive atllosplhc re may extend froml tle air outside, through the meslhes of t.he gauze, to thie flante Nwitbhin, ignition is not propagated across the gauze. The lamp may be filled with an almost lightless i'mo. o7. flamel; still, explosion does not occur. A defect in t, he gauze, tile destruction of the wtir at any point bly oxidation, hastcnced b=y the flame playing' against it) would cautse exllosion. Tlhe lmotion of the lanmp thiloughl the air migiht; also fore, mechlallt. I ically, the flame throug'h the Illeshles. itn short;, a certain amount, of intelligence and!caution is necessary in using the lamp. Th;. ltis inltclligncc, unhallpily, is not atways i;S!;~i lpossessedC, or1' this cautlionV always csxrcised, by the miner; and thlie consequence ii - ii is that., cvnc witlh the safety-laml), cxploN?~ji i-:~ sions still occur. Before permitting a main or boy -to enter a 1 mine, wotld it' not be I Ii- li wtei"ll~ to p)1ace thoSC'results, by expoeriment, - Ir Bi iiIt visibly before himn? Mere advice will not n orce caution; but llet the miner hlave:- if thtle )physiclal image of what lie is to expect, clearly and vividly before his mind, and lie will fined it a. rsti raining and a monitoryl influence, long aftcr the ffect of cautioning words ihas passetl away. (291) A word or two Y, now, on the conductivity of liquids and gases. Rumfoird mtade numerous experiments on this subjcect; showino a t once clearness of conception, and sklill of execution. [He sulpposed liquids to b)e non-conlductors, clearly distintguishlini te t antllsl)ort " of heat:, bl y conxvection, friom true conductionl; and, ill order to prevent convc ction in his C00OLING ACTION OF AILR 1Y CON\VEJOTION. 209 liquids, lhe heated them at the top. In this way, hte found tile heat of a Warm'tl iroln cylinder illCOmpetent to pass downward, tilrough} 0'2 of all ineht of olive-oil; lhe also boiled water in a glass tube, oveYr ice, without menclting the latter substance. The later experiments of M. ])esprctz apparcnltly show thlat; liquids p1ossess true though extremely feeble powers of conduetlion,.umfolrd also denicd the conductivity of gascs, thoilugh lhe %was well acquainted Nwith their convection.* The su)tejcct of gaseous conduction has been recently taken up I)y Professor M-Tagnus, of B ierlln, and thi dist di inguishled hilosop)her considers his exp)eriments prove tllat hydrogen gas conducts lheat like a metal. (292) The cooling action of air by convection may be thu;s illustratedl On sending a voltaic currentc tlroughl this coil of latintml wire, it los brl'ight r'd. I now stretch out the coil, so as to form a F1' 6 straight wire; the glow instantly sinks. —-you can hardly see it. Thlis effect is (Ie to th le frccr access of the cold air to thle strctched wire. Htere, again, is a receiver, it (fig. 68), T which can be exhautsted at pI1asture; att achcd to the bottoml is a vertical metal rod, li n, annd tl1rough the top another rod, a b, passes, which can be moved upll and down through an air-tight collar, so as to bring the en ds of thl two rods wit.in any required distance of each other. At piresent, the rods are united by two inclhes of platinium wire, tb M,,:, i ic mlay be Theated to any required degree of intenlsity bya voltaic current;, On establishing connection w\ith tbhis small battery, thle wire -. i; —i -. is barely) luminous enough to be soeen in ll fact, the currenl t from a single cell only is \o\wr setl through it. it is surrotunded by air, which is carryingt off a portion of its 1heat. 5thenlc the * Phil. Tralns., l92,; Essaiys, vol. ii.,. t5. 210 111A]T AS A AMODE O4 5[0TION. ccceiver is cxhausted,.te w ire glows more brightly than before. l: allow air to rcintler......-the wire, for a time, is quite tlquenclicl, in fact., rendecrd p)erfectly black; but, after the air thas ceased to entcr, its first feeble glow is restord, The current of air hrere passing over the wire, and dcstroying its glow, acts lilke the current established by the wire iitself, by heatting the air in contract with it.. The cooling of thle wire, in botht cases, is duie to convection, not to trul coldutction. (293) l)the same cffcct: is obtainled in a gractly incrleased dcgree, if hydrogen be used insteadR of air. We owe this interesting observation to Mr. Grove, and it formed the startingpoint of AML. MatgnLus's investigation. The rccciver is now exhausted, the wire being (almost whlitc-hot. Air cannot (lo more tlhan reduce that wihitcness to bright redness; but observe whtat hydrogen can do. On the entrance of this gas, the wire is tottally quenched, and e cveaftcr the reccceivr has been filled wvith the gas, and thoe inlwayrd current hlas ceased, the glow of the wtire is not restored. lThe elect-ric current;, now p)assing through the wire, is from two cells; ] try three cells, the wire glows feebly; five cause it to glow more brightly, but:, even witih five, it is b)ut a brighlt red. Aere no hydrogenl thlere, the current now passing th;ro'ugh the iwire would infallibly fuse it;. Let us see whctcer this is not tthe case. On cxhautsting the rccciver, the first few strokes of the ptump plroduce a scarcely sensible cffcct; but the effect of rarefaction soon begins to be visible. The wire whitens, and a)pcears to thicken.'To those at a distance it is now as thick as a goose-qutill; and now it glows, upon the point of ftusion; I continue to work thle p)ump.l.- tle. lig'ht suddenly vanishes; the wire is fused. (29f4) T,'his ext raordinary cooling power of hydrogen has been usually ascribed to thoe mobility of its particles, which enablcs currents to establishl themselves in this gas, with greater facllit~y than in any other. But Professor Magnus conceives the chilling of the wire to be arn offect of conduction. To impede, if not to prcvent, thle formation of currentshlel SIXII.N..TS, 01'OF MAtGNUS. 211 )passes his platinum wir'e along' the axis of a narrow glass tul)C, filled wit }hydrogen. Althougth, ill this case, the wiire is surrounded by a mere ilhn of the gas, anld the presence of currentsl in the ordinary sense, is scarcely to 1e asstuned, the fi11lh shows itself just as competent to quench the incandescence as whenll the ire iis caused to pass throulgh a lhargoe vessel cotntaining, the gas.'Professor Magnus also lheated the closed top of a vessel; and found tQhat the heat, was conveyed more quickly fromi it to a thlrmomelter, placed at some distance below the source of iheat, when t;he vessel was filled wvith hlydrogenl, than whbnlli it was filled with atir. I ie found this to be the case even when the vessel was loosely filled with cotton-wool or eider-down. HIere, he contends, currents could not; be formedncd; the heat must lI) conveyed to the thermometeor by tihe ttrue process of conduction, and not by convection. (295) l 3cautifill and ingenious as these experimentts are, I do not thlink tihey establish tihe conductllivity of hydrogen. Let us suppose thge wire, in Profssor Magnus's first cxpeiiment;, to be stretched along, the axis of a wide cylinder containling ltydrogenl we should have convection, in the ordinary sense, onl lhcating the \wire. W, hcre dtoes the heat thuts dispersed ultimately ~g'o? Itt is manifestly g'iven up to the sides of thle cylinder, and, if we narrow our cylinder, we simply hasten thle transfer. Thle pwrocess of narrowingli maly contitue, till at narrow tube is t.he rcsul~ t.... —the convection bctwccn centtret and sildes will continue, and tlroduce tihe same cooling' effect was before. The htlcat of the gas being instantly lowered, by cotnmunlicattion to the heavy tubc, it~ is 1)rep'ared to reabstiract the lheat from the wire. Witlh regard, also, to the vessel heated at t-he top, it would requirle at sutrface matlhemat.ically horizontal, and a perfectly uniforlm a)pplication of heat to that suftiace - w d im; wot tld, lloreovc, be necessary to cut thle heat; sharply off from thie sides of thi vessel- — to l)reven lt conlvec, t;ion. l1cii in the interstices of t.he eider-down land of the cotton-wool, the convective mobility of Ihydrogen will make 212 111I5AT' AS A MO)DE OF MOTION. itself felt, and t, taldia)g every thinlig ilto accout, i [ tlhill the experimental question of gnscous conduction is still an opten t * In mty opilion, thte question of litquid conduction also denanlds fl thcr itnvcstigation. Thils opi)lion is folndcd oil numeoltsl experimclets which I hanve inyselt' rmade in connection with this question. COOIING A LOSS OF MOTION. 213 CI APTIIR VI IL CO001(NO A.OSS OF' MfOOS.: TO \W'IIAT 1'StIlS MOTl)N 1T iPAT};: 1?. -!SXPFRIIMENT S ON SOUND IMARIN'I ON'1111 QUEitTION EX.'.. -— Xt:1MiNt'S ON lt1t IBEABRINS ON'S111S QUIT:ST'ION.S-*-Tl. TIqlIFEOIitS Or' YEMUi..SSION AND UNiUlAlION-.lXOGYt1: OF VAVES,1 AN NUMBR stit' OP IMPULS.ES (SF' I,I(}11T...'.lYSIOAl, CAUSE OF COl,OR t —.NVl5,1'}1: }RAYS OO''tlFE1 SI'ilRU7}M:(t:f.TIl —EM: OAIORITFO EAYT Bd IOND)'t11Y EEl T —-'Itl: CEMttIAI. }:itAY BEYOND T'rltE UJIIUE...-. D3)EFINI'tONe' }OF ADIANT IEATw. -RE.RFIEOT'ION OF tADIAN' It tEAT YFROM I',ANE AND OURVED SURF'.ACE8: ]AW.SV'iRl:E SA.M5E. AS 1'fIOSE: OF' 1t}l'i -CONJUGOATe MItOROS,. APFSN DI S..:-. SOIO FlIAME.. (296G) X fl/4] have this day reaclhed the boundary of one of VY tlhe two great divisions of our subject. I:l:thcrto wNe have dealt with h eat., whlile associated withl solid, liquid, or gaseous bodies.'W'e have found it competent to produce changes of volume in all these bodics, 5We have also observed it reducing solids to liquids, and liquids to vapors; we have seen it transmitted through solids, by the process of conduction, and distributing itself th'rough liquids and gases by the process of convection. reC have now to follow it into conditions of existence different from anly which wse have heretofore examined. (297) This heated copper ball hanligs ill the air; y)ou see it gl0ow, the glowt sinks, ftle ball becomes obscure; inll popular lanunage, the btall cools. ]Bearing in mind w\hat has been said on t nature of heat, wve must regard this cooling as a loss of miolecular mtotion. ]3But motion cannot be Iost: it must;be imparted to something: to what, then, is thf molecular miotion of this ball transferred? You would, )eCrhaps, answer, to tlhe air; 21XL4 IEAT AS A MODE OF MOOTlION. and this is partly true: over thCe 1ball air is passing, and rising in a heated column, quite visible aogainst te screen, if we allow the electric beam to pass thrfough the warmed air. But not the whole, not even the chlif part, of the molectular motion of the blall is dissipated in this way. If the ball were placed int vacuo, it would still cool. tumfolrd, of whom we lhave hecard so 0much, contlived to ihang a small thermometer by fa si-ngle lfibre qf silk, in the middle of a. glass globe, exlhausted by mealns of mrcur, anid hic found that the calorific ray) s passed to and fro across th;le vaillum; tlthus Proving thle transmi ssion of thle heat to be indeplendent of air. I)avy, with thle apparatus now before you, sh}owed thait thl heat-rays from tihe electric light p)a ss frely through an air-p1ump1 vacium; ant d we can repeat his experiment substantially for ourlsclvcs. It is only nceissary to take t:t receiver already employed (fig. 6(8), and, rem(ovi1ng' teim remains of tih platinum wire, thell desstroyed, attach to each end of the two rods, m nI andt a b) a bit of retort carbon. I now exhaust[ the receiver, bring flic coal-points togetoier, and siend a. current from point, to point. T:he lmolment the points are drawn a little apt)rt; the electric light shines foLrth: and here is the thlermo-electric pile ready to receive a portion of the rays. Tie galvanlometer-needle at once flies aside, and this has been accomplished by rays whitich have crossed the vacuum. (298) B]ut if not to air, to what is thIe motion of our cooling ball communicated? AYe must reach by easy stagres the ans;wer to this question. len land tfaken a very considerable step) in science, when they first obtained a, clear coicep)tion of tlhe way in which sound is transmitted through1 air, and a ve:r imnporltant exp'erient; was 1ma8de by]r I aulitsbee before thle Royal Society in 1r705, when he showed tlat sound could not rolpaag;ate itself through a vacuum. Now, I wish), in te first illnstance, to make maniftest to you this conveyance of thie vibrtations of sound by t.he air. This b)ell is turned upside-down, and supported by a stand. Wlen a fiddle-bow is drawn across the edge of thle bell, you hear its tonel; the bell is now vibrat VJI3RATION COMMUNICATITM ). 2 5t ing,' andtl when sland is thrown upon its flatfish bottom it arralges itself there, so as to form a definite figure; or, if it were filled withl water, wNe should have the surfiace fretted with befaiutfiful:crispations. These crispa tions would show that thle bell, in emitting this ntote, livides itself into four swinging' parts, separa-ted firom each other by lines of no swingingl. [elre is a sheet of tracing-ptaper, drawn tightly ovcr a. hoop, so as to form a hind of fragile drum.'When hleld over the vibrating bell, but not so as to touch thle latter', you ihear the shivering of tilhe membrane. It is a little too slack; I fighten it by wttNar1ming it beforec the fire, and repeat the experiment. You no longer hear a shivering, btut a. loud musical tone, superadded to that of the bell. W\hen the cmembtrane is raised and lowered, or moved to and fro, you hCear the rising and tle siningl of tlhe toll. tHere is a smaller drum, 0which.l I: pass rounld tihe bell, holding tihe membranle vertical; it actually blursts into a roar, whllen brought within htalf an inchl of thle bell. IlThe motion of the bell, communicated to the air, has been transmitted to the membrane, and the latter is thus converted into a. sonorous body. (299) The t;wo plates of brass, A B (fig. 69), are united together by a metal rod. The plates have been darkened by bronzing, and on both of them is stlrewn a quantity of fine white sand. T now take the connecting brass rod by its centre, between thell finger and thumll of ly left hand, and, holdingl it utprighlt, draw, withl my right, a piece of flannel, over whvich a little powdered resin has been shaIlken, along the rod. Yout hear the sound; but olbserve thle behaNvior of the sand: a single stroke has caused it; to jump into a series of concentric rings, which must be quite visible to you all. Operating more gently; you hear the clear,, wreak musical sound, and ace the sand s hivcring, and creeltring by dcegrees, to tile lines whlich it formerly occupied. The curves now there arle as shlarply d(rawn upon the surface of the lower disk as if they haid been arranged with a camel's-hair lpencil. On the upper disk, you see a serices of conclentric circl s of the same ki(nd]. The vibra 216 IEAT AS A MOD1E OF MOTION. tions here imparted to the rod have communicated themselves to both the disks, and dividced each of them ilnto a series of vibrating scgments, se)aratcd from eacth other by lines of no vibration, oni which lines tile sand finds Ieace. (300) Nolvw let lme shlow you tle transmissioll of these vibrla'tions firom t;le lower disk through the air. 1On thie floor is a paper drum, D, with dark-colored sand st'rewn uniformly over it; i mighti stand on the table........-or, indeed, as high as the ceiling, and produce tile Oflcct which youl are now to witness. Pointing the rod which unites thle plates, in the direction of the papler drum,l I draw th]e resined rubber vig'orously over the rod: a single stroke has caused the sand to spring into a reticulated pattcerl,. A lprccisely similatr effect is produced, )by sound, oil the drum of the car;;the tympanic nmembrane is caused to slliver, iIn the same man-, ~ C9> 1w ncer as tlat drumni-head of,-c-'.. ":'.a..e.. pal)r, and its motio1n conlu0~-' veyed to thle auditolr newrves, and it,-asmittedd t1hence to the btain, awakes in uS hfle soensatiion of sound. (301) tHerc is a still more strikillng examr1ple of the convey TIlE1EORIES OF EMISSION AND) UNDULATION. 21?/ ance of thle motionl of sound through air. B3y permlittting a jet of gas to issue thllrough a s1tlll orifice, a, slendlcr tlame is olbtailncd, andii, by turning the cock, th}e flame is recduced to la hcight of about h1alf an inch. The flame is tfhen inltroduced into this glass tube, A ri (1fIg. 70), o.0 which is twelve illnches long. Give lte your p)ermissiol to A 4address that flame. If 11 be skilful elnough to pitch ily v(ice to a crl'tain note, the flatme will respond by suddenly startinlg into a melodi- CA oils Son', anld it will continutle singing, as long' as ttlhe gas conltinues to burn., Thte burncl' is now arranged within the tubc, whlich covers it to a depth of a coul)le of inclics. JIf the tube were lower, the n flame wouldl sing of its own{ll accolrd, as in the well-known case of tle hydrogen hlarmonlicca; but, withl the p)esnl; arll:lrangement, it canllot sing ulnt.il ordered to do10 so. I emit a. sound, which you,0 will pardoi, if it be not musical. T heo flame does not respond; it htas not been spokcn to in ftie proper la-lnguage. 1But a note of somewhat higher' pitch causcs tll, flame to stretch, and every individual in this large audience now hears its song. I stop the sound, and stand at a greater disttanice from t:ihe flame: now tlhat the Ipro')pe' p)itchl has been ascetained, th;e exi)erimnent is sure to succeed, alld, from a distance of twenlty or thirty! feet;, the flame is caused to sing. I turn my bac tupon it;, alt strike t ele note as bcto 218 JIXEAT AS A MOD1I OJF 3O0TION. fore: when call ed to, it answers, and, with a little practice, one is able to command a flame to sing andt to stop, wlhile it strictly obeys the injunction. Icre, then, we have a striking examp)le of the conveyance of the vibrations of the organ- of voice through air, and of their communication to a body cminently sensitive to their action.* (302) Why are tfhese explriments on sound performcd? Simply for the purplose of giving you clear conceptions rcgardling what takes place in the case of heat; to lead youl from the tangible to the intangible; from the region of sense into that of theory. (303) After philosophers had becomell aware of the manner in which sounld was produced and transmitted, analogy lcd someic of them to suppose that light mightt be Xoductledt andl t ransmittcd in a somcwhat similar mannr., And pt)erhapls, in the whole history of science, there was ncvcer a qucstion more ]hotly contested than this one Sir Isaac Newton supposed light to consist of minute particles, darted out fr'om luminous bodies: this was the celebrated Emission Theory. Iulyglhns, the contemporary of Newton, found great difficulty ill colneci\vitlng this canonad of particles; or in realizing tlhat thcy could shoot with ihnconceivable velocity through space, aind yrt, no; disturtb each other. This celcbrated mani centertained the view that; light was 1)'rodutcct by vib)rations, similar to tlose of soulld, lE~uler supported H[uyghens, and one of his argumentls, though not truly plhysical, is so quaint and curlio0us, that, I will repeat it here. Ile considers our various senses, and the mannerlll in which tlhey arc'affected by external objects. ~tSVithl regard to smell," lie says, " tve know that it is produceed by material )articles, whichl issue from a \volatile body. In the case of hearing, notilng is detached from tile sounding Though tnot h)lonnging to our prielent subjeet:, so many persons hanv evinccd an inltecaSt iin tlhis cxpcriIncnt that I havoe beet1 induccd to reprintt two shtort papers in the Appendix to this Chapter, in whicht the experiment is more ftllty dlesceiltoed. Fitim'es snld jets of smoke ue acoustic tests of astonlslhin' sentsiilitt. Watttc-jcts maty als o he rendere lt d excedingly sensitive. For ia full account of their actiolt, sce 7t/ndott on S'oul, Iccturte IV. iLolgtans, INTElRSTE.LLAR MeDI)lUM. 219 body, and in thle case of feelinlg we must touch the body itself. Thlle distance at. which our senses perceive bodies is, tll tile case of touch, no distance; in the case of smell, a small (istance; ill the case of hearing, a considerable distance; but, ill thle canse of sight, greatest of all. It is, therefore, more 1)robable that the same mode of propagation subsists for sould and ligllt, tlhan that odors and light shlould be )ropagatcd iln tlhe samoe lannerl.......lthat luminous bodies should behave, not as volatile substances, but as sountldilng ones." (304) lthe authority of Newton bore these men down, and nlot until a man of genius within tlhese walls took iup the subject., had the Theory of Undulation any lchance of coping with thie rival'Theory of Emilission.'Jo D)r. Th'ilomas Young, formerely Professor of Natural P)lilosophy il thlis Institution, belongs the immortal honor of stemtnitng this tide of autllority, and of establishing, oin a safe basis, the Theory of Undulation. Greatt things lhave been done in this edilice; but scarcely a greater thing than thliS. And Yoiung was led to his conclusion regarding light, by a series of illnvestigationls onl sound. He, like ourselves at th1e 1 )eseCt momentl, rose from the known to tle 1iunknown, from the tangible to the intangible. This subject has been illustrated and enriched by the labors of genius ever since the time of.Young; but one name only wvill I lhere associate with his —a namell which, illn connection wvith this question, can never be forgotten: that is, the nlame of Augustinl Fresuiel. (305) Accordling to thle theory now universally rcccisved, light consists of a vibratory motion of thle particles of the lu1inous body; but howr is this motion transmitted to our organs of sight? Sound has the air as its mncdium andl a close examination of thie phlenomena of light, by tihe most refined and demonstrative experiments, has led )hlilosophecrs to the conclusion, lthat space is occupied by a substance almost infinitely elastic, through which tle puldses of light make their way. IHere your concepltions must be perfectly clear. T'le intellect knows no difierence.between great and 220 I1EAT AS A MODEtI oF M3OTION. small: it is just as easy, as an intellectual act, to picture a vibratint' atomn as to picture a vibrating cannon-ball; anli tlhere is no more dtificulty in conceiving this ethwr', as it is called, which fills space, than imagining all space filled with jclly. You must,s then, i maginei the atoms of luminous bodies vilrating, and their vibrations you must tligure as com)nllmltlluicated to tite thle in which they swing, being propaglated tflrouglt it ill wavc s; thcse waves enter thle pupil, crloss the ball, and impinge upon the retina, at the back of the eye. The act, rememberl is as real, and as truly mechanical, as' the strl)ok e of sea-waves upon the shore. The motion of tle ether is commtunic ated to the retina, tranlsmitted thence along' tl o})tic nerve of the brain, and there S 1announIIces itself to consciousncss, as light. (306) Oni the screen in firont of you pr oject an image of t:he incande(scent coal-points, wlbich p1roduce the electric light. Thle p)oilts art fiirst brought togrether, and then separated. Observe th e ffect. Youl halve first thfe place of contact rendered lumlinous, then you see t,}C glow conducted dowlnwarld, to a certain distance along the stem of coal. T'is, as you Iknow, is, in reality, the conduction of motion. tlhen tie circuit is interruplted, the points conltilnue to glov for a short tinme. Their light is now subttsiding, and nowx thc)y ar' quite dark;t but have they ceasced to radiate? ]3:y no imeans. At the l)rcscet moment, there is a co)iouls emlission from these points,'which, thoultgh incompetent to affect sensibly th:e nerves of vision,0 iS quitCe Complctcnt to affect other nerves of thle human system. T o the eye of the philosopllher, who looks aIt sucl matters Nwithout reference to sensation, these obscure radinttions are Irccisely tce sate in lhilld as those which produce the impression of light:. You must, tlheefore, figture tilhe pliarlicles of the heated body as in a state of motion; you must figure that motion as commnunicated to the surroundingl ether, and transmitted throughl it with a velocity whvtich wte have the strongest reason for believing to be the same as thlat of light. Thus, whln you turnl toward a fire o a cold day, and expose EIBtIT OF1? SPIGT1RUA. 221 your chilled hands to its influence, the Awarmth which you feel is due to the impact of these ethereal billow\s upon your skin; they thl'row the nerves into motion, and the consciousness, corresp)ionding to this motion, is what we p)opularly call wAarmth. Our task, durino; the lectures which reml ain to us, is to examinoe heat thuts prolpaatett through the ether. In this for it is called Wacfrtat.Hie'at.. (307) For the investigation of this subject, weC p)ossess our invaluable thermo-electric pile, the face of whlich is now coated Awith laml-black, a. powerful absorber of radiant licat. I1 hold the instrument btfore my helck; it is a radliating body, and the pile drinks in the rays. They) generate electricity, and tle needle of the galvanometer moves up to 90~. W\3itldrawing tile pile from tle' source of heatc, and allowing theo needle to come to rest, I. now place this slab of ice in front of the pile. Srt\ou have a dellection inl tle ol))osite direction, as if rayls of cold were striking on the instrument. Blut, in this case tlhe )ile is thle hot body; it radiates its hbeat against the ice; the face of the pile is thus chilled, and the needle moves up to 900 on tihe side of cold. Our pile is, tllherefore, not only available for the examination of bheat communicated to it by direct contact, but also for thle xaminattion of raldiant. heat. ILcet us apply it at once to a most important investigation, anid examine, by means of it, tile distribution of thermal power in tile electric spectrum.l (308)) -Let; me, in the first 1l)tace, shlow you this spectrum. It is formed by snding at slice of pure white light from tlhe orlifice o (fig. I7), throughl a double convex lens, and through a l)ism a b, a 4 c, built up of plainl glass sides, and filled with the liquid bisulphide of carbon. This liquid grives a richer display of color than glass does, and this is one reason for its employmeilt in trcfcrelnce to glass. Thle white beam is no'w% reduced to its complonent colors. —. —-rcdl, orang'e, yellow green,g and bluc; the long blue space being usually subdivided into blue, indigo, and violet. I will now cause a thermo-elcctric plile of p)articular' construction to pass gradually thlrough all these colors in 222 IIlAT AS A MI0)DE OF MOTION. succession, so as to test thleir heating powers, and you shall observe the consequent action of the galvanometer. Pto. 11. (309) Th'. cx)eriment is made wiith this beautiful piece of al)parnattls (fig. 72), designed by Melloni, and executed, wiith his accustomled skill, by t. tRutmPo *' Ic'kortf.* You observe here a poliished brass plate, A n, attached to a stemn; this stem is mounted onl an V I lhorizontalt bar, whichl, by mieans of a screw, may lave motion implarted to it. By turning this ivory handle ill one dircotionl the plato of brass is caused to approach; by turning it ill tie other, it is ca\lsed to recede, the motion being so fine and gradual, that we can, with; 8ase atnd certainty, pisl the screen throlugh a space less itlanl --- thl of an inch, You observe a narrow verticaI slit in the middle of the l)late A n, a1nd something dark behind it.'I'llat (dark lin is the blackened face of a thermloelectric pile, r, the clements of whvlich are ranged in a single row, and not in a square, as in our other instrument. W'e will * tLent to imo by imy friend MI.r Gassiot, ULTRA IRE) ANI) ULTRA VIOLET RAYS. 223 atlloiw distinct slices of t he spectrum to fall on that, slit; neach will impart whlatever heatt it possesses to the pile, and the quantity of lcat will be marked b)y the needle of our galvatOlll{ttC'r, (310) At 1present, a small but brilliant spectl.rum falls upon the plate ak i, but the pile is quite out of the spectrum, On turnincg the hlandle, the slit gradally app)lrpaches thlle violet; the light now falls upon it, but the nccdle does not move senisibly. In thiC indigo, the needle is still quiescent; the 1bue also show\s no action. Nor does the green: the yellow falls upon the slit; te h motion of thle needle is now )perhaps for the first time visible to you; but the deflection is small, tholugh the pile is exposed to the most btmiZnos part qf the speetrutn.* I )ass onl to the oranllg, which is less luminous than tle yellowv, but you observe, thought the lighlt diiniiish-les, the lheat increases; the nCeeCdl moves still farther; w\\ile in the rc(l, w\\ich is still less luninous than the orange, we have thle ~greatcst thermal power of thle visible spectrum. (311) The appearance, however, of this burning red mig)ht lead you to sul)lpose it natural for such a color to be better tlhan any of the others. 13it now pay attent.ion. causcll the pile to pass eltirely out of the slecttrtum, quite beyond the extreme rcd. Th.to needle goes proml)tly upl to the stops. So that we have helre a lheat-spectrum which we cannot see, and whose lthermal power is far greatrc tllan tlhat of any part of the visible s)pectrln. Int fact, tihe electcric ligiht, with whicll we d(eal, emits an infinity of rays converged by our lens, rcfracted by our prism, forming the prolongation of our spcctrumll, but utterly incompetent to excite the optic nerve. It is the same with thle sun. Our orb is rich in these obscure rays; and though they are, for the most part, cut off by our atmosphere, multitudes of thclml still reach us. To the celebrated Sir William }ecrschcl we arc indebted for this discovery. (312) This is sufieient for our present. pur)tose. I ptropose, iln a future lecture, to sift the complosite emission of our -' I am )ioro dealing with a largo lecture-roomn galvanonietor. 224 IhEAT ASL A MOiDE OF MOTION. electric lamlp, detaching' the visible from the invisible rays, and illustrating thle discoverics which h]ave been recently made in connection Awith the subject; of obscure radiation. (313) Th'.e' visible spectrtmn, Ien, simplly m-arks an interval of radiant action, iln which the rays arc so relatetl to our organization as to excite the imlpression of lighLt ]Beyond this intterval, hi both, directions, radiatt powver is exrted —-obscue rays fall......... those flling beyond tthe red being- l)owcrfiti to produce heat, while thlose falling beyondl tie violet are powcrful to promote chemical action. These latter rays cant actually be rendered visible; more strictly speaklting, the undulations or waves whlichl are now striking here beyond the violet against the scccen, though they are illcompetellnt to excite vision, may lce caused to impinge 11pol another body, to i-ln plart their motion to it, land actually to convert tle dark space beyonld the violet into a brilliantly illuminated one. The mealns of making- thlis cx)eriment are at hand.''ttec lower half of this shcct of whlite paper has been washed with a solution of sulphate of quinine, the upper half being left in its naturall state. Hlolding tle shcet so that trhe straight line dividing its plepared fr1om its unprepared half shall be horizontal, it will cuit thle spectrum into two equal )parts; the ul)l)per half will rcmain unaltered, and youi will be able to compare Nwith it thie tinder half, on whllich you will finld tlle spectrum clongated. You see the effect,.r~ have here a splendid fluorescentl band, scvcral inches in width, wherc a momen t ago thicre w\as notling b)ut~ darkness.'Whcn ttt e preplarlcd pl)ar is removed, the light (disappears; l)ut, on 1ceintroducing it;, the li-ght flashes out again, showling youl, il tl he most empllatic malnner, that the visible limits of the ordinary spectruml by no lmeans mairk the limits of radiant; action. I dip my brush in this soluttion of sulplhate of quinine, and dab it agatinst the paper; wherever the solution fills, light flashes forth. The existence of these extra violet ray'n s bes bccl on'g known; it was known to Thomas Young, who actually experilmented on them; butt to thle excellent reselarches of Professor Stokcs wve arc indebted PIYtOtAL C(AU81, OF COLOR. 225 for otr 1presellt compnllcte knowlcdge of thist subject;. ite relldecd thlc r1ays tints visiblc. (314) flowv, then, are we to 1)icture to oursclvcs thle ra)s, visilble and invisible, w 1ichl fill tlis large space tupon the screen? tlWhy arc sonie of tihlle visible tand others not?'Whty are the visible oncs disting'uished by var1iouts colors? Is there any thing' that: - we can lay hold of, in the undulatlions of thel ctlther, to w\lich, as a, plhysical cause, Awe must assignt tile color? Observe, first, that thtc ntire bcam of Nwhite light is dratlwn\ asi(le or refracted by the prissm, b)tt the violet is pullcd atsidt more than the indigo, the indligo more thall thc blue tho lule ore than the grcceen, the green more than the yellow, the yellow more t han the orange, and the orange more t;han thte red. The colors are differently refirangible, and upo)0n this deplclds tite possibilityr of their sepaltationl. LTo cvcry P)articular (legree of refraction belongs a definite color, ani nio other. Butlt whyfl should ligh;t of Olte dcgree of refrangibility produce thie scetsation of red, and light of ataothcr degree t]he scnsatiol of green? This leads lts to consider more closely thte cause of these sensations. (31.5) A refermnc to toilc plitnomucna of souind will materially hel1p uts here. Figure clcarly to youtr minds a harl)-string vibratilg to and fr o; it advancesx and causes the particles of air in frott of it to crowd together, trlhus prodItcing a. cont(ts(tnion, of the air. It retreats, and tle tair-particles behind it scparate mnore widely, thlls lproducing a irarc/jCtion of the air. The string again advanlccs and )rodtuces a condensation tas before; it again ret reats, and lprocluces a rarefaction. in tis way, the air, through which the soundl of tile string is prop)agated, is moulded into a regular sequence of condensatiols antid rarcfactions, whi\tl travel wvith1 a velocity of about 1,100 fet; a sccond. (316) The condensatiion and rarefact, ion constitute w\tat is called a sonorous ipldse or wave. The lclgth of the iwvave is measurtel ftom tlhe centrc1 of one condensation to the centre of te lnext one. Now, thle quiticker a string vibrates, the more 226 1UItA AS A MIODF4 OF MtOTION. quictly wtill these pulses follow ach'.1 other, and tihe slhortel,', t;t the same time, will be the length of each indivi\dual wave. Upl)on these diflel'ences the 1pitch of t note ill music depends. if a violill-l)layer wislles to plroduc a higher note, hle shorltens the string by pressing his fillnger ol it; thereby augmenting the rapidity of vibration. If his poihnt of prcssure cxactly htluves the leongtll of his string, ]he obtains tlhe oct-ve of the note which the string emits, when vibrating as a Whole. "lBoys Care chosen as, choristers to ploduce the sihrill notes, men to prloduce thle bass notcs; the reason being that thoe boy's organ vilrates mnore rapidly than the man's;" so, the htlt of a glnat is shlliller than thlnat of a beetle, because tlhe smttallcr insect canl senld a greater number of impulses per second to the car,' (317) Wer have now cleared our way toward th]e full cotll)rcl0c1ision of the )hysical cause of color. t his spectrum is to the eye what tfle lmusical scale is to the car; its dliirent, colors represenft nots of diilerent )itch, Thte vibrations whlich prod(cel the impression of red are slower, and the ethereal waves which they generate are longer, tllan those which prodluce the impression of violct, while the othecr colors are excited by wavtcs of some intermlediate lecgth.'The length of thet waves both of sound and liglht, and the numberl of shocks which thlcy respectively impalrt; to the ear and eye, have beeon strictly determined. Let us here go through a simple calculation. light travtels tlhrlough space att a velocity of 1.92,000 miles a second. IReducing this to inches, woc fintd thoe numlberl to be 12,4165,120,000. Now, it is found thatt 39,000 waves of red light, lplaced end to end, would l)make tiup a inch; multiplying tfle nmllber cf inches in 192,000 miles by 39,000, we obtainll thle number of waves of red light embraced in a distalnce of 192,000 miles: this lsnumber is 474,439,680,000,000..AIl these -waves e)lter the eye in a si2gle seeond. T'o tproduce thle implressionl of violet, a st.ill greater number of impulses is necessary. It would take 57,500 waves of violet to fill aln inch, anld the number of shoclks to plroduce the impression of INVISIJ3ITE RAYS OC1 THE l SPECTIRUM. 22 this color amounts to six hundred ant ninety-1nino millions of millions per sconld. The other colors of th;e specttnrum, as tlready st.ated, rise gradually in pitch from red to violet. (318) Bu3t beyond the violet we have rays of too highll a pitchl to be visible, and b1)yond the red w\te lhave rays of too low a pitch to be visible. The phenomena of light are il this case also p)arallcled by those of sound. If it did not involve a conltradiction, we itight sy that there are musiceal sounds of too high a pitecl to be heard, and also sounds of too low a 1pitch to be heard. Sl)caking strictly, there are wav\s trallsmlitted throughl the air fiom vibrating bodices, whic, thoughl they strtike upon the car in regular recurrence are incompetent to excite thoe sensation of a musical note. Prob>ably, sounds are heard by insects, which entirely escape our 1)ercclptions; and, indeed, as regards human beilgs, thte self-same note may be of piercing shrillness to ole person11, while it is ttbsolutely unheard by another. Both as regards light and sound, our or0gans of sight and hearing embrace a certftin practical range, beyond which, on bot;h s5idcs, though t e objective cause exists, our nerves cease to be influencc d by it. (319) Wheu1n, therefore, I plnace this red-hot copper ball before your, an1d watchl the cwaningt of its light., you will have a perfectly clear conception of whatt is occurring here. iThe atoms of thle ball oscillate, blut they oscillate in a resisting mediulm, on whi fl tlheir motion is expended, and( which tran,,smlnits it o1 all sides withl inconceivable velocity. The oscillat.ions comal)etrent to produlce light are n1ow exhausted; the ball is (quite darlk, still its atoms oscillate, and still their oscillations are taken tup and transmitted on all sides by the ether. The ball, cools as it thtus loses its,molecular motion, but no cooling to which it can be practically subjected can entirely deprive it of its motion. That is to say, all bodies, whatever may be their ttemplerature, arCe radating rheat;. From t.he body of every individual herCe present;, waves are s8)eCding away; some of wh-ich strike upon tlis cooling ball, and restore a )orttion of its lost motionl. ]tlt: the motion thuls received b)y 228 JUlIAT AS A tMODE OF MOTION. the ball is far less thain that which it communicates, and the lifle1rncc letweenl themt expr)csscs the ball oss of l1motion. As longt as tf.his state of things continues, the ball weil contitlnu to show an ever-lowering temperature: its temperlature will sink until the qlualtity it emits is equal to tte quantity whichl it receives, and at this point its tCeml)erature becomes constant. 1ithus, thougfh you arce conscious of no receptionl of heat:, when you stand before a body of your o\tit temlt)crature an intteielhnigc of rays is passing between you. Elvery superficial atom of each mass is sending fortt its wav\s, whlich cross those that move in the opposite direction, every wave asserting its own individuality, amid tihe entanglement of its fellows. Whecn the suml of motion received is greater than t:hat given out, warmingl' is thle consequenco; when the stillum of:mot.ion given out is greater than tlhat received, chlilling tatkes place. This is Prevost's Theory of 4xchallngcs, expressed in the langtuage of thte Wfave Theory. (320) Let: us occupy ourselves now, ) for a short time, il illustrating expleritmentally the analogy bcetwcen light (land radiant ]leat, as reg'lrds reflection. You observed when thel thermo-lcctric pile wat s 1placed il front of 1my c1heek there wvas (attached to it 1an open cone 1which was not empl)oyed ill our former ex)perimentl.s. This cone is silvered inside, and it is ilntended to augment the action of feeble radiations) b)y converging them upon thle tice of the tlhermo-elcetric pile. It does this by reflection. Instead of shiooting widel of the pile, tas imany of them would do if the reflector wlere removed, the rays mcct the silvered surface, and glance from it again4 st; the pile. Theo augmentation of the ffectl; is thttus shownl. I place tihe pile at thlis end of the table without its reflector, and atR a distance of foilr or five feet. th)is colpel ball, ot......... — libut not re(t-hot; you observe scarcely any motion of the needlc of the galvanomcte'. ])isttulrbingl nothing;, I now attach the reflector to tthe pile; th;e need(le instanltly goes ut1) to 90~, declaring' the alg'llented action. (321) The law of this reflection is preciseiy tlhe satmile as T'IE1EORY OF EXXC(ANGES. 229 tiat of lilght. Let me devote a few minutes to the illustration of t:his stubject. Observe this aptarently solid luminous cylinder, issuing horizonftailly from our electric lamp, uantd naltkinlg its track thus vividly ui)on the iustt of our darkcencd oom.11 Permitting tihe beam to fall upI)on this mirrior, it is rflectced, and now st.rikes the ceiling. T'hel horlizontal beam ihe're is tlhe intcide&,t beam, thie vertical one is the r'qrlected beam, and the law of light, as many of you knlow, is, that tle ang'le of ilcidence is cqutal to tihe alngle of reflection. The incident anld reftlcted beamls now enclose a rip,)llt angle, and when this is the casge we may be sure that both b)etams fo:rm with a p)erl)peldicultall to tlhe surfce of the mirror, an angle of t50. (322) I placei the clectrico lamp at this corner, l,) of the ~~~~~~ I ~J Jail table (fig. 73); behind the table is fixed a lookilg-glass, -,, and on the table, you observe, is drawnl a large are, a b. Attached to the mirror is ta long straight lath, mt 2, so that the mirror, resting upon rollers, can be turned by the latlh, which is to serve as an index. Those in friont may sce that the lath itself and its reflection in tihe ntiirtor now form a.t straight line, which proves thflt the lath is perpendicular to thle mirror. ].ightl adl left of the central llinc, m W, thel arce is divided into ten equal parts; comlencinlg at tthe end tl, with 0~, the arc is grraduated lup to 20~. I now turn the lath-index, so that it shall l be in, thle line of tll belat emitted lby t(he laiml). The be imn falls up)on the mirror, stirikig it as a l)cr 230 1IEMAT AS A MOD)E OF MOTION. pendicular, and you see that it is refllcted back along tile Tline of incidence. I now move the indlex to I; tile reflected beam, as you oblserve, draws itself along the table, cutfting the figure 2,. I move thle index to 2, the lbeam is now at 4-; I mlove the inldex to 3, the b1eal is low at 6; move it to 5, the beam is now at 10; I move it to 0:tO, the beam is nlow (at 20. Standing midway betwcceen tlle incident and reflected beams, and strctclhingll out lmy arms, lmy finger.tips touch each of tlhem. One lies as much to the left of the perpcndictlar as the other does to the right. Thl anle of incidlence is equal to the angle of reflection. But we have also demonstrated that tthe betam moves twice as fast as the index; and this is usually cxI)rCssed in thll statement, that the angular velocity of a rflccctcd beam is twice that of the mirror which reflects it. (323) You rtave altcady seen that these incandescent coalpoints emit anl abundlance of osecure ra ys -....rays of puine heat, whiclh have no illuminatilng p)oxwel. WAre Itave now to learn thallt those rays of heat mtllitted by the llamp have obeyed lecisely thlle same laws as tlhe rays of lighlt. Here is a piece of black glass, so black that whlen you look through it at the electrio light, or even at the noonday sunll you s nothing. You observe the disappearance of thle beam, whe\l the glass is placcd in front of thell lamll. It cuts off cvcry rfay of light; but, strange as it may apptear to you, it is, to some cxtent, t;ranspare t to the obscure rays of tihe lamp. I extinguish the light by interrupting the current, and lay mly therllo-celect.tri pile on the table at the number 20, where the luminous beam fell a moment ago. The pile is conlcoted with the galvhanometer, and the needle of the instrumcnt is now at zero. Onl igniting thc lamp, no light ma'kes its ap)lcaranc c, but the needle of the galvanomleter has tlready swung to 900, througl the action of the non-luminous rays upon the pile. lWhen the pile is moved right;or left from its pi)esent position, the nendfle inediately. sinks; the calorific rays have puirsued the precise track of the luminouls rays; and for them, also, the angle of incidence is equal to the angle of reflection. Repeating the lth'ICTL1'TION FROM P'LANET SURFACES. 231 sexperiments already executedl with lighlt —b.riging g the index ill succession to 1, 2, 3, 5, etc., it may be proved that, in the case of radian:t heat also, tile angular velocity of th}e reflected beanm is twice tha;llt of thle mirrol: (324) The heat of a fire obeys the same law. This sheet of tin is a homely reflectorI, but it w\ill answer my purpose. At this end of tlhe table is placed t}he thermo-clectri pile, l(and att the other end the fin reflector. Thle needle of the galvanomeeter is 1now at, zero. I turn thle reflctorl, so as to cause t.he heat striking it to rebound toward the pile; it now mects trhe inlstrument, alnd the nccdle at once declares its arrival. O)bserve tCe positions of the fire, of the reflector, and of the pile; you see that tthey are such as make the angle of incidence equal to that of reflection. (3,25) llit in these experi'ments the hleat is, or has been, associated vith light.. Let mle now show that the law holds good for rays emanating from a trtly obscure body. HIere * IG It. F... 4.. is a coplper ball, heated to dull redness. Plunging it into water for a, momentr its hlight totally disaplpears, but it is still warm. It still gives out radiant healt. I set it on a candle 232 1lIEAT AS A MtODJE OF MOTION. stick c (fig, 71), as a support, anld now I place my lpile, 1l, turning its conical reflector away fr'om, so tat no direct ray froml the ball in position can reach the pile. The needle remains att zero. I now introduce the tin reflector, 3 80, that a line drawn to it from tle ball shall make thle nsamie angle with a. perpendlicular to the reflector, as a line drawn from tleo pile. The axis of thie conical reflector lies in thris latter line. Trtuc to the law\, tile h(eat-rays emanating from the ball rebound from it, anld strike thle pile, and you observe tihe consequent 1rom0l)t motion of the needle. (326) l'ikce the rays of light, thle rays of heat emanating fiom our ball ptoceed iin straight lines through space, diminishiilng in intensity, exactly as light dinlishes. Thus, this ball, which when close to thel pile causes tlle needle of tile galvvanom1 eter to fly up to 900, at a distance of 4 fcet 6 inches shows searely a sensible action. Its rays are squandercd oil sl sides, and compartatively few of them reach the pile. B13lt Inow introduce between the lpile and thle ball this tin tube, A!B (fig. 75), four feet long. It is polished wixitiln, and thereFla. 75. A 1 F;it fore capable of reflection. The calorific rays strike the interior surface obliquely, are reflected firlom side to side of the tule, and thus cnabled to reach the pile. You sce the result; the needle, which a momelnt ago showed no sensible action, movcs I)romptly to> its stops. (3'27) We have Bnow dwelt sufilciently long on the reflection of radiant lheat by pt&lai sasacwes; let: us turn for a mlomcnt, to reflection front curved surfaces. ThiS concave mirror, REFLEECTION FROM CURVYEI) SURFA1CES. 233 mt i (fig. 76), is formed of co)pper, but is coated with silver.'T1he warm copper ball, B, is placcd ati a distance of eightccnll inchcs fromt the pile, whose conical reflector is now removed you observe scarcely any motion of the nceedl. If tle reflector, Mt N, wCere )laced properly behind a candle, its rays would be collected, and sent back in a cylinder of liglt. The mirror thuls collects anld reflects the calorific rays emittcd b1y the ball si; you cannot, of course, see the track of these oblscure rays, as you can that of thel luminous ones; but thlo galvInometer reveals the action, the needle of the instrument going promlnptly ulp to 90O FY.'0.,...............:...........................::'.......::. * -.:'?). ~............................. ~~- t~~~~~~~..............................' 1 (328) A pair of much larger mirrors is now before you, o1ne of them being p)lacd e lat: upon thle tA bl'JT curvature of this mirror is so regulated, that if a litght b)c )laccd at its focus, tho rays which fall divcerent upon the curvcd surface arc reflcctetd upward friom it l)arallel. Let 1us make the cxpcrimcnit t: In the focuts I place our electric coal-points, bring them into contact-., and then ditraw them a little apart; the clectric lighit flashcs againlstt te mirror, a. splendid vcertical cylinder, marked by the shliinig dust of the room, bcing cast il)ward by the reflector. If wte reversed the cxperiment, and allowed a t)larallcl beam to fall uptoln tho mirror, the rays of 234 IIA1 A ST AS MOA)1DE OF MOTION. that beamln, after reflection, would be collected ill the foctus. 1'o.?*. AraWe can actually malke this experinent by intro-:: duciing a scond mirror, whliich, indeed, is already here suspendedl from the ceiling. i)rawing it up t a, heightt of 20 or 25 feet. the vertical beam,, ii gi:ii iwhliclh 1previously fell i uponI the ceiling'~ is n1ow1 rece ived by the reflector i above. In tle focus at the upper mirror is hung t: bit of oiled paper, to i cnable you to see tle i~~~~~;i ~s~; ~ a~:j collection of tile rays at i!the focus. xrou observe i how itn tensely tihat piece ill~~~l~~er~-~ iof paper is now illuni-.....:;; t i i; ntcd, not;1)y t;he direct light fihortn below, but by % the reflectc d light con-? verged ulil it by the noir 2above. IV?: j iX::~ii- -i:,(320) Some of yvo h ave p obably witnessced i!;:i:-11,~!:the extriordinnry action of ligh t ulon a mixttre o, f hyldogenc and chloi.. inelc, antd I will now exhilit i this action i'n a lovlcl way. T}his traili- i;;;00;0:::;;f;; parenlt collodion 1ball0oo0 is filled with the mixed.' ga'ses; I lower thlle up, CONJUGATIE MIRIRORS. 235'per reflector, and suspend thte. balloon from a hlook attachdl to it, so tlat thie little globe sthall swing' inl tlhe focus. \e \will nomw drawt the mirror quite upl to the ceiling (fig. 77). Placinlg, as before, the coal-points in the focus of ti;t lowcvr minr'or, the moment tlhey arc drawn apart, thle gases cxplode. And, remember, this? is the action of tIlhe hlight; you, know that collodion is an inflalmable substance, an1ld )lhence mighlt; supt)osc it to be tte h eat of tte coal-points Ywhich ignites it., communicating its combustion to the gases. But you see the flakcs of the batlloon desending to the table, plroving that the luminous rtys went harmlessly throughl it;, caused the gascs to explode, the hydrochloric, acid, formed by their combustion, having actually prescrved the inflammable envelop. (330) I again lower the mirror, and hlang in its focus a second blalloon, containinin a mixture of oxygen alnd hydrogen, on Xwhich light has no sensible effect; I raise tbhe mirror, and il the focus of the lower one place this red-hot copper balll. Tlic calorifie rays are now reflected an(l converged, as the luiminous ones were reflected and converged inl the last cxpei'imcnitt; but they act upon tihe enveulop, whicleh ha1s zbeen urposely blackecnced to enable it to intercept thle eat-rays; t;le action is, not so sudden as in thle last atse, but therie is tlhe expllosion, and you now see no trace of tlhe ballool; tile inflammable substance is elntirely dissip)attd. (331) Let us lower tle ul)per mirror once mole, and susp)end in its focus a flask of hot water. The thermo-clcctrie pile is now pllaccd at thle focus of ihc lower mirror; its face being turned upwa'ad, and cxplosed to the direct radiation of tle -warm flask, thcere is no sensible actio n produceld by the direct rays. Blut whcen the face of the pile is turned dowynwardl, if liglht and ]heat behave alike, the rays fi'om the flask whlich strike thl reflector will be collectced at its focus. You see tlhat this is tile case; the ncedlc, which wa\s not senlsibly affected by the direct rays, goes up to its stops. The direction 230 JI:!AT AS A MODE O 01 MOTION. of that deflection is to be noted; the red end of the needle moves toward you. (332) Tln the place of thle flask of ]hot water, ] now sus)end aL second one containinig at freezing mixture. Placing, as ill the form'er ase, thfe pile ill the focus of the lower mirror; cwhen turned directly toward the upper flask, there is no action. Turned downward, the needle moves: observe the direction of the motion-.- tlhe red end comes toward mlfe. (333) Does it not appear as if this body in th.le upper focus wiere now emitting rays of coldl, whichl are converged by tlhe lower mirror, like the rays of hleat ill our former experiment? The facts are exactly complcmelentary, and it would seem that we have precisely tlle same right to infer from the experiments the existence and convergence of cold-rays, as we have to infer the existence and convergence of heat-rays. Manylf of you, no doubt;, have already perceivcd thel real state of thld case. The pile is a, warm body, but, in tle last expleriment, the heat which it lost by radiation was more than mnade good by that received fromn the hot flask above,.iAbw the case is reversed; the quantity whlich the pile radiates is inl excess of the quallttity A'whicll it rcccices, and hence the pile is chilled- —.the exchangcs are against it, its loss of lheat is only partially colipensated —and tl e deflection due to cold is the necessary consequonce. SINGUING FJAMES. 237 APl"I? Ni)IX TO (I5IAPTAIt'VItI. ON T'Jl'E BSOUND)S PROD)UCOtI) BY T'1li1' COlMBUSTION 0? (GASE$1 IiN1 TIUBES.* ]N tho first volrume of Nicholsolls Journal, published ini 1802, tho soundls produced by the combustion of hydrogen ill tnles are referred to as having been " made in Italy; " Dr. liggtins, in the samle place, shows thalnt hoe had discovered tthem in the year.17'7, whilo observing the water formed in a glass vessel, by the slow comblustion of a slender st tream of hlydrogen. Olndll, in h; 11d A.Alu.sti;k,) publisletd in 1802, ). t/4, speaks, of their being mcltioncd, and inGcorrcmtly x — platned, 1,y D) hLlue. in hlis "lNow Ideas on MlNeteorology:" I do not know tihe (lo te of the volmune. O1hIadnl himself showed that tlhe tones plroduced were the same a s those of an open 1 ipe, of the sameo lengtlh a tis tube whichl encoml-passed tiel flane. leo also sttcceded in obtaining a tone and its octave firom the same thube, and in one case obtained the fifth of the octave. In a p)aper published in the "Journal de Physique " i1n 1802, G(. D)o 1a IRive endeavored to account bfor the so1lunds by rloferring thl1o0m to the alterInato contraction and expansion of aqueous vapor; basing his opinion upon a series of oxperiments of groat beauty and ingeumity, made with thoe )bulbs of thermometers, TIn 1818 Mr. Faraday took up) trhe sultjet, i and showed that the tonTes were produced when the glass tube was enveloped by ani atmosphere higher in temlperature thanl 2120~ Fa'n. iThat tlhey were ot due to aqueeous vapor was furtller shlown 1y the fact that they could be produced b1y the combulstion of carbonic oxide. lie referredl thle sounds to successive explosions, produced by thle periodic combitlnation of tte atlmospheric oxygen with the issuinllg jet of hydrogenol gas. I am not aware that tle dep lendncee of t1he pitch of the note oin F1 romI the Philoso4phlical Magazinle for July, 185'7. ]ly John Tyndall8 F. R. S f n t ts, vol v f Journal orf Science and t, e Arts, vol. v. p. 274. 238 IiAT' AS' A MO DEo 01] MOTION. the size of the flaome as yet been noticed. To this point I will, inl the first place, briefly direct attention. A tube 25 inches long was placed over an irgnited jet of hydrogen; tlht sound produced'was the fundamental note of the tube. A tube 12 inl6hes lontg was brought over the sme fllame, but no sound vwas ol)tailcl. The flame was lowered inl order to rmake it as small as possible, and the tube last mentioned was again brouglht over it it gave a cleatr melodious note, the octave of that obtained with the 25-inch tube. The 25-ilch tube was now l)rouglht over the sanme flame; it no longer gtave its fulndamental note, but exactly the same as that obtailledl fromil t.lhe tube of half its length. Thus wo see, that although the speed wit.h wvhich the explosions succeed each other depends utpon the le ngth of the tuble, the fla"me hbas also a voice in tae matter; thfat, to produce a musical solnd, its size mlust be suclt as to enable it to explode in lison:, Oither withl tthe ftlndamtentt putlse. of the tube, or with the pulses of its harmonic divisions. With a tube 6 feet 9 inches longl, by varySing the, size of tlhe flame, and adjusting the deptll to wldich it reached witin the tublo, I hatve obtained a series of notes in the ratio of the munbemirs 1, 2, 3, These experiments expllain tihe capricious n aturo of the sounlds sometiime's obtained by lecturers npon this sulject. It is, however, always possible to render the sounds clear and sweet, by suitably anjusting tilhe ize, of the flameT to thte length of tile tuble.* Since the oxperilments of Mr. Faraday, nothing, that I am1 aware of, has been a(dded to this slbject, untb l quite recently. il a recentt numlber of Poggendorlftfs "Annalon," an interesting exp)eriment is described by 5[. von Schaffigotseh, and m,,ade tihe sutlbjct of some renmarks by Prof. Ploggendoritf himsell f A rmusical note was obtained with a jet of ordinaryt coal-gas, and it wvas found that, when the voice vwas pitched to thie same note, thfe flIame assumed a lively motion, wthich could be augmented until the tlame was actually Cxt'inguislhed i. von 8ehafftotsech does not describe tlhe conditions necessary to the succoss of his oxperiment; and it was whlile endeavoring to find * Wx ith a tube 141- inclics in length and aln exceetdingly minutlto jet of gas, t obtained, without nltering the quantity of gas, a note andI its octave: trhe flame pocsscssel the power of changing its own dimlenmsions to suit both notes. SINGING FLAMES. 239 out these conditionls that I alighted upon the facts which form the principal -subject of this brief notice. I )-may rerark, that AM. von Mc}haflgotscht's result 1Smay be )produtced, ith certainty, if thIo gas be caluse(l to issuo, }under suflicie t pressure, through a very small orilice. int the first cxperimlents I matde use of a tapering brass burner, )10, inches longt, 1havinlfg al superi or orifice abo t, jot1t of all ilnchl in diailmeter. The shaking of tflo singing Ilame withill tile glass tube, whllen tihe voice was lroporly pitlced, was so manifest as to 1)0 seen 3by several hundred p)coplo at. once. 1 placed a syren within a few fect of tlhe singing flame, and gradually heightened the note produced by tihe instrunment. As tlhe sounds of the flanme and syrcn approached perfect, unison, thle flame shook, jlumping ull) and down within the tube. The interval bet'weeCt thle julplt s b:ec"aml greater, until the unllison was perfect, when tihe 1motioln casel fori anl inst}ant; the syr1ol still increasing inl pitlch, tlhe motion of the flame again apleared, the jtinpling becam quicker and quicker, unitil finally it ceased to bo discernible. This experivment skhowed that the jumpingt of tle lftlame, observed by A, yon Shaffgotst h, isl tiho optical exlpr-ession of the beaCts which occur at each side of thso perfect unison; tle bheats could be ]heart' iln exact accordance ewithl te shortelling anld lcngtlheling of the flame. Beyond the region of these bCats, il both directiolns, t-he sound of the syrcan t)rodtcctd no visible motion of the flame. What is true of tlhe syren is tl'le of the voice. While repeating and varying these experiments, I once had a silenlt flamle withill a tulbe, and oln pitching tlly voice to th1e note of the tilube the flal), to mny great surprise, instantly started into song. P)lacing thle finger on the oend of tXhe tube, and silencing the tmelody, on0 repeating the experiment tile samel result wvas o)btained. I placed the syren near thle filme, as before. Th. lhatter was burning trantquilly within its tube. Ascending gradually from the lovwest, notes of tloe instrumentl, at the moment when the sound of the syren reachled the pitch of the tube which surrounded the gasflalme, the latter suddenly streotched itself and counmencet its song, which continued indefinitely after the syren h}ad ceased to sound. With tle burtner which I have described, and a glass tube 152 inclhes long, land frolm I- to Af of anl inch in interaitl diameter, thli result can be obtained with ease and certainlty. If the voice be tlhrown a little higher or lower than tile note due to the tube, no visille effect is 24~0 I tIEA AS A MOD1E 01F MOTION. produced upon tlhe flame; tihe pitch of tlhe voice must lie within thl region of tihe audlliblo beats. lBy varying tho length of the tube, wvo vary the note )rodttced, and thle voice must be Imodifietl accordingly. That t;le shaking of the flamre, to which I have already referred, l)proceds in exact accordance with the hboats, is beautifufly shown by a tuning-fork whlichl gives tXhe s$ame note as the flame. Loading the fork, so as to tirow\ it sli;ghtly out of unison with th el fla1e, swhenV the former is solmlded and brought near the flame, the jumpings are seen at exactly the sa&me intervals as thos ill which thte b)eat are1 heard. Whe\rn th\e tuning-fork is brought over a resolnant jar or betttic the bcats may bo heard land theo jiunpings seen, bty a thousand ipeople at once. y changling the lead ulponl ti ttuning-fork, or by slightly altering tile size of the flame, trhe qtuicknless Nwith which tih beats succeed each other may blo changed, b\ut in all cases thle jmIpings addregs the eye at the samer moment that tile heats address the ear.l. Withl the tuning-fork, I hlave obtained thle samet results as with thie voice and syren. Ifoldiwg a fork over a tube whichl responlds to it, and which contains within it a,silent flame of gas, thle latter immediately starts into song. I lhavo obtained this result with a series of tubes varying firom 10i to 29 inchles inl length1..Theo following oxperimenht could be made: A series of tubes, capable of producing tile notes of tlhe gamut, might le l)laced over suita)le jets of gas; all being silent, let tile gatmut )be run over by a musician, with all instrument sufliciently powertfil, placet at a dist-ance of twenty or thirty yards. At thel sound of each particular lote, the gas-jet contained ill tile corresponding tube would instantly start into song. I must remark, how\ever, tlhat, withl the jet \whiceh I havo lused, tilh experiment is most easily made withl a tube albout 1I or 12 inches long' with longer tutbes, it is more difficult to l)reovent the flame tfrom singinig spontaneously,fl that is, without external excitation. The )princip)al point to l)o attended to is this: With a tulle, say of 12 inches i lenllgth, the flame requires to occtupy a certain fposition nll tihe tulbe, in order that it shall sing with a maximum intensity. Let the tube 1)o raised, so that tile ilflan may l)clletl'at it to a less exteolt; thel enlergy of tle sound will be thereby diminiselld, and a lpoint (A) will at lenggtl be attained, where it Nwill cease altogcthel. Above this )point~ for a certain (distance, the flame may t)o caused to b)turn tIranquilly and silently for any length of time, b)ut ulwheln excited by th;e voiNe it wtill sing, $INCGING fAtJ\MES118. 241 Whent the flame is too nearl th\e poilit (A), oin being cxcited by thef voice or by a tuning-fork, it will resptond for a short timet, and then cense. A littleo abo-ve te )point whelre this cessation occurs, tlhe flame burns tranquilly, if unexcited; but, if once caused to sing, it w\,ill contiue to (1do so. WithI sluclth a flame, whichl is not too sensitive to extert al impressions, I have been lable to iceersc the the ct Aiitreto desc':ibed, and to st:o tile song at pleasure by thle sound of my voice, or by a tuning-tforkl, without quenching the flame itself. Sucll a Mlame, I fi(nd, nmaly be made to obey thle word of command, and to sing or clease to sing, as th1e ex)perimenter lpleases. T1'e more clalpping of the lhands, prodlucing an explosion, sfhouting at an incorrect pitch, shaking of the tube surrounding tihe flname, are, whnch the arrangeme nts are p)rolerly lmalde ineltkectual.?.Each of t~hese modes of disturbance doubtless affects the flalme but thbe impuilses do not accumulate, as inl thle case where the n1ote of tlhe tube itself is struck. It appears as if the flame f were de-(f to a single imnpulse, as the tymlpalmtl would probhably be, and, like thle latter, needls the accumulation of impulses to give it sufficient motion. A dillffbrnco of half a tone, be\teen two tuning-forks, is sufficient to cause one of these to set tihe flame singing, while the other is )powerless to produce this effect. I haveo said tftat tlhe voice must le pitchled to tJhe note of the tube whichb surrounds the flniame; it would be miore correct to say, the note produced by the flame wthen singig. In all cases, this note is,smensibly highler than that due to tlhe open tube which) surrounds thel flalme; thlis ought to )e the case, beca1use of thle high temp)erature of the, vibrating column. An open tutbe for example, whliell, \w llen a tuning-fork is held over its tlend, gives a maximium re0nforccment, plrodulces, when surrounding a singing flame, a note higher thian that of t he fork. To obtain t}he latter note) the tube must be lensibly longer. What is the constitution of the faieme of gas, while it produces these, musifcal soundlts This is the next question to wlticht I w\%ill briefly call attention. Looked at with the nakced eye, tfhe sounding flameo aplpears constant; but is tile constancy real? Supposing each iulse to be accomplanied by a physical change of the ftlame, such a change would not b1e i)ercel)tible to the naked eye, on account of trhe velocity with whiclh tlhe l)ulses succeed each other. The light of tflhe lamtio n ould appear continuous, on tihe same princil)le that the trouhled portion of a descending liquid jet appears continous, althtough, Hl 242 lIEA1T AS A 1MODE, OF MOTION. by properl means, this portion of a jet can be shown to be com)osed of isolated drols). if we cause thto image of the ftlame to pass speedily ove\r diile'Cent portions of thle retina, thle chllanges accompanying the periodic impulses will 1manllifest themselves, iln the character of the image thus traced. I took a glass tube, 3 feet 2 inches long, and about an inch and a half inl internal diameter, and, placingl it ovel a very sn1mall flamre of 01fiulant gas (co)limtontt l gas will also tinswer), obtained thle fundamtlntlal note of the tube: on moving the head to and fro, the image of thlo soundling flame was separated into a series of distinct ilnages; tlhe distance between the iimages depended upon the velocity with whvlich tlhe head Awas moved. This experiment is suitedC to a darkened lectu'c-r0oolm. It wa1s still easier to oltain the separation of the imageos in this way, when a tube 6 feet 9 inclhes in length, }and a larger flame, were 1lmatde us ot The same result is obtained, when ali opera-glass is mloved to and fro before the eye. But theo most convenient mode of observing the fllame is swith a mirror; anld it can be seen eitlher directly ill the mlirror, oir by projectionl upoll a screen. A lens of 33 centimetres focus was p)laced in fiont of a flame of coml1monl gas, nitpward of an ineh long, andl a paper screen was hunlg at about 6 or 8 feet distance 1behind the flame. In front of the lens, a small looking-glass was held, lwhich received the light thalt; ad p)assedl througll the le1s, and reflected it back nupon the screen placed behind the latter. By adjusting tle position of the lenls, a well-de"fined inverted image of the tlame was obtained upon tlhe scree'n. Olt Imlovilng thle mirror tlhe inuago was displaced, and, owing to thIe retelition of thel imp)ression of the retinla, 1whenl thoe novnement was sufliciently specdy, the image describedt a continuous luminlous track. l[olding thie milrror mlotionless, the 6-foot 9-inch tube was pllnaced over tlhe flame: thte atter changfed its shaele, the moment it commencedt soundingl, remaining, howver, hwell defined(t pon the sreen't. O() 1now nmoving the mirror, a totally different effect was plroducced: instead of a continuous track of light, a series of distinct images of tlho sounding flame was obslrved.'ahe distance of these imagest apairt varie(d w itt the motion of tile mirror; and., of course, could be ntade, by suitably turning t1he reflector, to form a ring of images. Thle Cx1)erilmlent is beautifil, and in a (lark room may beo made visible to a large audience. SINX ING FLAME S. 243 The experiment was also varied in the following mantlner: A. triangular prisml of wood ha d its sides coated with rectangular pieces of looking-glass; it was suspended by a thread, wtith its axis vertical; torsion was imntparted to tile thlread, and the prism, acted ulpon by this torsion, caused to rotate. It wva; so placed, that its throee ftaces received int sucession the boeam of light, sent firom thec lame throughtll the lenlls in tfri'ont of it), and threw the images upon the screen. On coimmlencing its motion, thle images were but slighltly separated, but became 1more o and more so, as th11e motion al)roachcd its maximum. This once passed, the imtlages drew closer together again, unttil they ended in a kind of lumttino1 us ripple. Allowing the required torsion to react, thle same series of effects could be Iproduced, tihe motion boing in an opposite dircction. In these experiments, that half of the tube 1f01which was turned toward the screen wvas conted with lamp-black, so as to cut off tihe direct light of tlhe jet firom thmo selen.* liut what is tlhe state of the flaime, in tihe interval between two images? The flame of common gas, or olefiant gas, owes its brightntess to tlhe solid particles of carbon discharged into it., If iwe blow,against a luminlous gas-flame, a souId is heard, a small explosion, in fact, and by such a putf the lightt tmaly be caused to lisalppear. l)urilg a windy night., tlhe exposed gas-jets in tio shops are often decprived of their light, and burn blue. In like nmanner, tile common blowpipe-jet deprives burninlg coal-gas of its brilliant light. I henolc concluded that tihe explosions, the repeotition of which ptroduteels the musical sound, rendered, at the moment thtey occurred, the combulstion so itprfeet as to extinguish the solid carbon particles; but I imatgincd that the images oin thte srceen would, oil closer examiniation, 1be lounLd lunited by spaces of bluo, wlhiot, owitll to their dilnless, were not seen by theo method of projection. Tbhis in many instances wa;s found to be, the case, I was not, howevel, p)rep)ared for tile followintg result A fldante of olefiant gas, relidered ahlnost as small as it could be, was p)rocutred. The S-foot 2-inch tube was nlaced over it; the flame, on singing, bex Since these experimlents were mtade, Mr. Wheat-stone has drawmn tly attention to thie following passage, which proves that lte hnad alreatly ltade use of tie rotatit(ng mirror ill examining l a singitg flamei:' A fllle of l'hydrogen gas blurntlitg in ti e open air presenlts a coltiltutous circle ila tile mirror; bilt, w\tile producing a sotmid w\ithin a glass tube, regular interrtissionis of intensity are observed, which presenlt a lchaimn-like appealratnc, anld imldicate alterlnato colltraetiotts and dilatations of the fluame correspontdinlg with the sonorwous vibrations of tlhe column of air." (1i1il. Tr man. 1831, p. 586.) 244 Itl4lIEAT AS A MODEl OF MOTION. oanlo elongated, and lost socehl of its light, still it was bright at its to1); looked at in the mnoring nirrlot, a beaded line of great beauty was observed; in fi'ont of each head was a little lluminous star, after it., and continluou1s witht it, a spot of rich blue, ligrht, which terminated, and( left, tas iar as I could judge, a lerfectly dark spitCe, between it, and tihe next following liltmious star. I shall extamine this tfurther when time permits me, but, as ftr as I Ican at present judlge, the flalme was actually extinlguished and relighted, in accordalnce with the sonorous 1pl)lsation, W\telc1 a silent flame, capable, however, of being excited by the voice in the manner already described, is I)laced within a tutlbe, and tlhe continuous line of light produced by it inl the moving rmirror is observed, I know no expterimenl t more pretty than the resolution of this line into a string of rictly-lluminous pearls, at the instalnt t1he voice. is pitched to the proper note. This may be (done at a consilderable distance firom the jet., and with the back turned towarcd it, The change lrotucecd in the line of beads, when a tuning-fork, capable of giving beats with the flanme, is broughlt over the tube, or over a resonant jar near it, is also extremely interesting to observe. I will not at present enter into a more minute descri)ption of these results. Suflicient, I trust, has beenll l said, to induee experimentersl to reproduce tle effects for thiesellyes; the sight of thenm will give more plelasure than alny, descriltion of mine could possibly do, T'RlAN$IXATION OF A PAPE3R ONN ACOU$TIO( ]E;X;PEIMi;'NT$.* A (:i-AS tlbe op)en at )oth Clds, when simply blown up)Oll by the -moutlth, gives its ftindamental tone., i. C., the (deeplest tonte belonglling to it, as an oplen organ1-pipe, leebly but distinctly. On p)lacing the open tlanld upoln one of thle openings, and rapidly witlhdrawing it, the tube yields two notes, one after the. other; first the fundamental note of tlhe closed pipe, and then the note of the open pipe, already mentionedl, which is a.n octaw Ifig/er. ly the application of b]eat, thlese fildamental tones, of which only tlhe higher one will bo taken into consideration Ihere, arle raised, as is well klfnownl; this is observed ilmmediately on blowing upon a tlube heated externadly, or by a gasflame burni ng in its interiorl. For exampnle, a tulbe 242 millims. in length, and 20 mrillims. in diameter, heated thlroughout its whole B By Count Sctaft'gotscl: Phil, Mag., i)ctelmbel), 18t. SINGINC FLJAMXFS, 245 length, wthen blown upon even before it reaches.a 1red heat, gives a tone raised a major third, namely, the second (- sharp in the treble clef, instea(d of the corresponding E. If a gas-flame 14 Illims. in lengtll, and 1 millim. in lreadth at the bottom, is burnilg in tfl-e tube; the! tone rises to the second treble F sllarp. Thel same gas-flame raises the totne of a tube 273 millims. in length, and 21 millims. in widtht, friom the secondt treble 1) to tthe corresponding E. These two tul)es, which for brevity vwill hereafter be retferred to as the E, tube and the i) tube, served for all t}he following experiments, the object for whichl was to show a vwell-kIace at 7 metres, \lwhein t el flame was1 only 10 millims. in lleight, and thlo first treble I) sharip wans Sung. 4. The last-lmentioned flame is also extinguished by thle note ( sharp, sounded close to it. Noises:, steoh aS thlel clap)ping of hands, pushing a chair, or shutting a 1)ook, (1o not produce this effect. 246 hEAT ASl A MOD)E' OPF MOTION. 5. A burner with all aperttre of 0'5 milliml., I)rojecting fi'om below 60 millims into the 1) tube, yielded a globular gas-flame 3 to 8'3 nillins. in diaimeter. By gradually closing a stop-cock, the passage of gas was mlore andt more liited. Tihe tlamc suddenly tbecamno mutch longer, but at the same time nar'rower, and nearly c ylindrical, acquirting a bluish color tlhroughout, and froml the tube a piercring second treble D) was sounded; this is the )phtnornenon of the socalled chemical harmnonica, whiclh as been known for eiglhty years. AW\lcn thle stolcock is still fihtnther closed, the tone becomes stronger, thie fllame longer, tnarl'row'er, annd spindle-shalled; at last it disappears.An effect, exactly similar to tltat caused by cutting off tthe gas, is p)rodtccel upon th.e small gas-flame by a 1), or the firs.t treble 1) slung or sounded from instrumentsl; and, in this casC it is to b0 observed that thle flame generally becomes the more senlsitive, the smaller it is, and. thet firther the burnter projects into the glass tube. (3. The flamte in the J1) tuble was 2 or 3 millims. in length; at a distance of 16'3 metres (more than 51 fet) fi'om it, tlhe fist treblo 1) was sounded. The tlame immell iately acquited thoe unusual tform, and the second treble 1) sounded and contilnued to sound from the tublle. T.,tWhile the secoond treble,t ) of the preceding ext)erlment was soundin'g, the first treble ) wars soundedt loudly, close to the tube, when the flame became excessively ClongQatced attd then disappeared. 8. The flame being only' t milil. in length, the first treble 1) was sounded, Thle flame gave out the second treble 1) (and pcrhatps solmetimes also a higher l)) only for a Imoment, anll disal)lpearcd.'ito flame is also affectedt by varliouts D)'s of an adjust-able labial pipe, Iby thet contrta, 1),I), the first treble l), and tle secodnd treble D) of a harmollnium but by no single U sharp or 1) sharp of this l)owerfdll insltruiment. It is also affected by the third trebleo D) of a clarinet, although only when quite close. The sung note also acts, \when produceld by inspiration (itn this case tlhe second trlble), or wlen ttle mouth is turned from the flarme. 9. In immediate proximity, thoe nloteo C slunfr is oftoctive. Somee influence is exertedt by loises, but not )by all, and ofteln not.by the strongest antd nearest, evidently becauseo tihe excifting, tone is not containled il ttthem. 10. The flanme burning quietly in the interior of tlhe I) tube was about'25 millims. inl length. In the next room, thle door of which SINGUING FLAMES. 247 was open,; the four legs of a lchair wore stampeod simultanetllously uponl the woodeln floor The pllenomenon of the chelmical harmionica imlmediately occurred. A very small flame is, of courise, extitguislhcd, after soundling for lan instant., by tthe noise of a chair. A tambourinll, whlen struck, acts som01etimes, but not in gnceral. 11. The flami burning, in the excited singing condition, in the interior of the 1) tulbe, the latter was slowly raised as high as po.ssible, without causing the return of the flame to the ordinary condition. The note, the first treble I), was sung strongly and brokA'ct qio s(ddZcrat, at a distaneo of 1'5 metre. The harmonic tone ceased, and the flame fell into a state of repose, without being ex-:12. The same remstlt wars produced by acting upon tlhe draught of air in the tube by a fanning motion of the open hland, close above the Iupper aperture of tlhe tube. 13. In the 1) tube there wvere two burners close together; noln of theml, 0'5 millim. in aperture, opened five millims. below the ot)her, the diameter of which was 1 milliem. or more. Currents of gas, independent of each otheir, Jlowed out of both; thlat flowing from tho nalt1rrower\ burner being very feeble, and bu)rninig, when ignitetl, with a ilame albout 1'5 millim-. in length, nearly ivisible in the day; the first treble D was s1ung at a distaltco of three metres. The strong current of gas vwas immediately inliame(t, because tlhe little flame situated below it, becorming elongatetl, larned up into it.:i1y a stronger action of the tone, thle small flame itself is extinguislcdt, so that an actual transfer of tle flamo friom onle lburner to the other takes place. Soon afterward, thie feeble current of gas is usually again inftlamed by the large ltiare, and, if theh latter eb3 agailn extinguished alone, every thing is ready for a repetition of tlhe experiment.'14. The same resultt is fulrnished by staral)ing witt the chair, etc. It is evident tflatt in this -way, gas-flames of any desired size, and any mechanical action, may be produced by musical tones andl noises, if a wire stretceld by a w ightt 1)e passed thtrough tle tglass tube, in such a way that theo laring gas-flamte mutst burn upon it. 15. If the flamne of thle cllemical harmnionica be looked at steadfastly, arld if, at the same time, the lthead be im-oved rapidly to the right and left alternately, an. uninlterrupted str'eak of light is not seen, such as is given llby.ery other luminout.s body, but at series of 248 11IlAT AS A MODE OF1 MOTION. closely approximated flames, and often dentated and undulated figures, li espeially when tutbes of a lmetrt, and flames of a. centimetre', ilt length, aroe employed. This experimlent; also succeeds very easily without moving tho eyes, whot thle flamtt i looked at t hrough ani opera-gIlass, the objectglass of which is moved rapidly to and fro, or in a circle; anld also when the picture of tlhe flame is observed in a hand-mirror shakcnl about. It is, however, only a variation of tihe experimlcnt long sincoe described and exptained by'Wheatstone, for which a mirror turned by watchwork was employed. [I:t is, perhaps, but right, that I should drnaw attention to the relation of the foregoing paper to one that I have published ol the same subject. Ont May tth, alnd the days immediately following, the principal facts described in mIy paper were discoveredt; but, on April 30th, the foregoing results had bceen comlunlituicatctd by 1Prof. Poggendolff to the Academly of Sciences it iBerlin.'ttrolgh the kinduesS of M. yon Schafflotsch hlimiself, I received his )paper at Chamoumfi, rmany weeks after the publication of my own, and mutil thent I was Inot awaret of his having continued his experiments upon the subject. We thus worked independently of each other; but, as far as the described phienomena are conmmon to both, all thle merit of priority rests with Count Schafotstch. —..J. T.] DIMINUTION WITH[ )I)XSTANIONl. 249 (At l Atl' t I X.:,AW OP ll.lNU'IJTfIN WI-:T1'TIi T,- SAC: S -ht D:R..o: W.AVY} OP SOUND L.OCNGUDIISNAL;':TosTO: O1' lMt(BIlr'TRANSYVFI:EAIA.-'-AV5IIN. tf lEt' OSClATIE;T1, T'tl MOi,ECUfiES. Or IlXEtI}'}:RENT Il1)O3L COMMUNICA'TE DIITT'lFtENT &MOt'itS OF MOIOt'ON TO'll't'tE'I..' ADIATION'T11tl; COMM',NIT'IAION 01' MOITON 0'TilE "tEIMR; IE ABSORPT'ION 7S flfT ACCEPT'ANCE OF MOTION. IRON.IIti}: };"t:fIER —4 IXtOS SUP}I'tACE$ W IIIC(l }RAD}IAT' VEI IAE BILSORtBl WV-ElI,-. A C IO' SE: WvOOlLEN CO~VERING,'AClLITA'tE$ COOL ING.I':IrESRvAT''IV, INrLUENC.t Or (1G Olt0, —I'AT-' —'IlE AT'ONMS oF IIODILt[S~ INS INCUB'TIT CE};RTA'IN, A'V S. ANTI,,10 O'th}IR$'TO T'88'.I ——.-3,NAANSPALWNYt ANID) I)'IITE}}MAN~~ -. DIAt, I E-MI1E BODIE)1S BtAD RADhllAiTOI.S ]/}Wt'INI'ION OP ltIl'IERfM'qUAlITYI AI A P'II}.tt'l) i BADIAN', t)lAT.A'CN - IEA —'ItI'I 8 RAT[ A AWI'C'PASt'S WTlIOUT AIBtSOIIP-'TION DO NOT f 1EA'IlA ME IDIUM NI.PRTOt'ORTIION OP lUMINNOUS AND OI3.SOURgH RAYS IN YAItlU 1B' AM ES.3 (334) r -aripil intenlsity of radiajn t heat; diminishes witih the distance, in the Sate mannel as thalmt of lghlt. AVhlat; is the tlaw of di(ninution for light? lach side of this squalre sheet of paper measures tvwo feet; folded thus, it formi s a smaller square, each side of which is a foot; in length. 1The electric lamp now stands at a distance of sixteen fee; frilom the screen; )iand at at distance of eight feet, tlhat is, exactly midway between the screen and the lamp, I holdt this square of pap1er. The lamp is naked, unsurrounded by its cametra, and the rays, uninfluh enced by any lens, are emitted in straight; lines on all sides. Yrout see tfhe shaidow of the squ1are of 1,rl)C onl the screenl; let us lnmeasure the boundary of that shadow, tand then1 unfold the sheet of paper, so as to obtain tihe original largeo square. You see, y thel creases, that it; is exactly four times the area of the smaller one. This Jairge sltcet;, when 1)licedl agailst the screenl, exactly covers the space formerly occupticd )by thel slhadowl\ of the stnaill square. (335) On the smlall square, therefore, when it stood mid 9250 sIIFAT S A AM tODl OF MOTION. wvay between the lamp and screen, a quantity of light fell which, W\vhen the small square is removed, is diffused over four times tlhe area. upon 0thle screen. ]ullt, if the same quantity of light is diffused over four times the area, it must; be diluted to one-fourtll of its original intensity. Hlcnce, 1y doubilng tlhe distance from the source of light, we dimlinish thie intensity to onoefourth}. By a prcecisely similar mode of experiment, we could prove that,, by trebling the distance, e diminishli the intensity to onlleninth l; and by quadrupling tle distance we reduce the intensity to one-sixteenth: ill short, we thuts d(emonstrate the law that the iJnte nsity of light diminishesC as the' l'I. 78. V I of inverse Squares, as appliedl to li~ghlt. (33~) iBut; it has been stated lthat heat di minislhes, accordinlg to the same law. Observe the expteriment, which I am now aboutl to t)erform1 before you. Here is ita narrow till vessel, Mt Nx (fig. 78), )presentitng a side, coated Awith lamp-blaclk, a squlare yard in arttea, The vessel is filled wvith hot watern-, \lwhich converts thlis larg'e surface into a source of radianlt heat. I now p)laee tlhe conical reflector on t; thltetrmo-elctric pile, l, butt, instecad of permittting it to remain a reflector, I. push inlt lMEITLON 18 DIJMONSTIRAION. 251 the hollow cone this linillng of black paper, which lits exactly, and which, instead of reflecting anyl he-at that may fall obliquely on it), effectually cuts off the oblique radiation. The pile is now connected with the galvanometer, and its reflector is close to the radiating surftcc, the face of the pile itself being about S.ix inches distant from the surft'ce. (337).'lihe needle of tlhe galvanometer moves; it, now points steadily to 60~, and there it will remain as long as the temperature of the radiating slrfatce 1remains sellsibly constant. I will now gradually witlhdraw the pile friom the surface, and ask you to observe the effect upon the galvanometer. rYou will naturally expect that, as tlhe pile is withldraln, the initensity of the licat will diminish, anid that the deflection of the galvanometer w\ill fill, in a corresponding degree. Tlhe pile is now at double tei distance, but the needle does rnot move; at treble the distance, the needle is still stationalry; we may successively quadrulle, quintulle.......go to tien times the distance, but the needle is rigid ill its adlherence to the deflcetion of 60~. There is, to all appearance, no diminution at all of intensity with the increase of distance. (338) From this experiment, which might at flrst sight appear fatal to the law of inverse sO quares, as anplicd to lheat, MAellonli, in the most ingenious mannetr, l)i'ovcd the law. I will hiere follow his reasotSningl. Illmaginel the hollow cone in friont of the pile prolongted; it wvould cut the radiating surfatce in a circle, land this circle is the only portion of that surfaice whose rays canl reach the pile. All the other rays are cut off by the nol-reflecting li1ningtc of tlhe colic. VWhenl thle pile is moved to dlouble the distance, the section of the cone prolonged encloses a circle exactly four tilmes the area of the former one; at treble tthe distance, the radliating s-urface is aIgmentedt nine times; at ten times tihe distance, the rafdiating surfatce is augmented 100 times. Now, the constlancy of the deflection proves that the auginmentation of the surface must be exactly neutralized by the diminution of the intensity. Blht the radiating surface aucgmcnts as the square of the distance, '2562 hEMPWII,, A'S A MODE) OF MOTION. hence thle intensity of thie heat must diinlishl as the square of the distance; and thus the experimcntl, wNhiclh miglht a4t first sight appear fatal to the law, demonstrates that lLaw in the most simnl)c alld conclusive matt erll(t'. (33Sa ) I have spoken of the dilution suffered lby light whenl it is diffused over a1 large surface. Thlis, however, is but a vague way of expressilng tlhe real fact. The diminution of intensity in the case both of light and radiant heat is, in reality, a, diminution of motion. ]Every ctlher-p)artlicle, as a mwave pases it, makes a, complete oscillation to and fro. At the two limits of its excurs1ion it is roughit momentarily to rest; midway between those limits its vcelocity is a maximlnun. Now. the intensity of thte light is proportional to the sqetrt'e of this tmaxismunum velocity. lThe range of the vibration of an ether-particle is technlically call-1ed its amplitude; 41and tlle illn tensity of the light is also pr1ol)ortional to tte square of the.lampllitude. It can b1e proved that both thle maximum velocitt and the amplitude vary inversely as thle distance from thle radianlt point; lence the intensity of the lighft and heat cmittec 1by thati point must vary inversely as the square of the distance. The plroblleml is one of pure mchclanics. (339) Jet us now revert for a momlent to our fundameltal coneeptions rcgarding radiant heat,. Its origiln is an oseillatory motion of the ultimate pIarticles of mattcr........a motion taken up by the other, aind propagated tlnhough it in Awaves. The particles of ethler in these waves do not oscillate in the same manner as the pa)R'ticles of air, in the c1ase of sotlud." The air particles move to and fro, in t]he direction in whicel the sound travels; the cthcr-particles move to and fi'ro, across the line in whlich the light travels. Tl'ihe undulations of thelic air are lonp;itludlinal, those of thle other transvcrsal.'The ether-waves resemble more thile ri>ppl)es of water thltll they 1do t-1he aerial pullses whieh l)Ioduce sound; that tfhis is tihe ctase has been inlferred frtom optical ptllnomena, But; it is manifcest that the disturb- flThe i}ntensity in bo)0th cses varies in accordance witlh tle samle law.,'e "'Tyntdall on 8ound(" p. 11. I0ongman., T'lElA, MTATS AS IA8 RtDIATORS. 25= ante produced in the ether must depend upon the character of the oscillating molecule; one atom mIay be more unwieldy than another, and a single atom could not be expected to produce so great a disturbance as a group of atoms oscillatingl as a system. Thus, when diffrent; bodies lare heated, we may fhtirly expect that their atoms will not all create the same amroulnlt of disturbance in ether. it is probable tlat solme will communicate at g'reater allttoutl; of motion tflhanll others; inl other words, that some will radiate more copiously thlan ot.hers; for radiation, strictly defilted, is the commeuntication of motion from the particles of a hcated bod'y, to the etheir int /which these particles are izmmerIsed, and through which the,.motion, i's p2ropagated. (340) L,et us now test; this idea I1y experliment. This et. bical vessel, i; (i'g 7), is called a "I eslie's cube," bealuse vessels of this shape were used by Sir Johnl Leslie in his beautiful researchels oin radiantt heat. Thle cube is of pxc'ter, b1ut onoe of its sides is coatedl itd h a la.yer of gold, lanother withl a layer of silverX, a third wtithl a latyer of copper, while the fourthl Vsa I'1 is coated -with a vflarnish of isinglass. Let tus fill time ct.ube with hot water, rand, keeping it at a constant distance from the thlermo-electric pile, I, allow its four faces to radiate, in 254 )111VAT AS A [TODJ)E, OF 110TION. successionl against thell pile.'lh hot gol( surfacc, yrO see, produces scarcely antly deflectionl; tile hot silver is equally itnopierative; the salme is the casoe wit.h tfhe COp)l)er; but, whnle1 this \varnishelCd sIfe is turlned to-wardi the )ile, the gush of lheat becomes suddenly so great that thle needle moves up to its stops. Illence we infer that, through someC )hysical cause or other, the molecules of the varnishll, wlhen agitated by the hot water withinl the cube, communi cate more motion to the ether than do the atoms of thle metatls; in other mwords, tlte varnish is a better radiator ithan tihe metals are. A similar resutilt is obtaind when this silver teapot is complared wit.h this earthen-ware one,; both being filled with boiling Awater, tile silver produces bbut little effect, while the radiation f'-or the eartlhen-walre is so copious 1as to d(rive the needle up to 90~. Thus, also, if tlhis pewter pot be coml)aricd wit h tItis glass beaker, when both arec filled with lhot watter, tlle radiation from the glass p)iroves to be much more powerful tthan thalt; from the pewtenr (341) Yotu ihave often heard of thle etkect of colors on radi'ation, and heard a good deal, as shall tafterward be shownl, that is unwarranted by experiment;. One of the sides of t.his cube is coated with whiting, another wvith carmine, a third with lamp)-black, wlile tile fourth is left uncoated. On presenting the black surface to the pile, tle cube being filled witih boiling water, the needle moves up, and now\ points steadily to 65.% Tlhe cube rests upon a little turn-table, and, )by turning the support, the white face is presented to t}he pile; the needle remains stationary, proving the radiation from the white sutrface to be julst as copious as that from the black. When the red surface is turned toyalrd the pile there is no change in tihe:positionl of tlhe needle. I nlow turn the uncoated side; thel needle instantly falls, proving the inferiority of the metallic surface as a radiator. Prceisely the same experiments may be repeatcld with this clublc, the sides of w\hich are covered with velvet; one face with back, another with white, d a third with red. The three velvet surfaces r-adilate alike, while tlhe 1NFTLUENEU - OF COLORS ON RAD)IATiION. 255 nakeI surtface radiates less tlanu any of them. These experimenlts show tthat the radiation firom the clothes which cover the lhuman body is indeptendent of thleir color; that of tan animalt's fur beig equally incoll)Cetent to influenc til he radiation. These are the conclusions 1arrived at by Mclloni jbr obscuroe heat. iWe shaltl subsequently pushl the invcstigation of this subject. much beyond the ploint at whic h' Melloni left it. (342) Now, if the coated surface in the fbregoing experiments communicates more mottion to the etherl than fthe uncoated one, it necessarily follows thal the coated vessel Nwill cool mtore quickly than tlhe uncoated one, H'cre are t~wo cubcs, one of wlhich is covered with lamp-lack, wvhile the other is brighit.'l hre- qullrters of an hour ago bloiling water wtas pIouIred into these vessels, a thermometer being tlpaced in each of tlhem. 13oth tfermometers then showed e te sae templerature, but low one of thtcm is two dcgrees below the ot1her, the vCssel which htas cooled most rapidly being the coated one.:Here are two vessels, one of which is brighlt, and the other closely coated %with flannel. ][alf an hour ago t{w'o thermometers, plu1gled in these vessels, showed tllhe same temleratre, but the covered vessel lIas lnow a temtlerature two or three degrees lower tlhan thle naked one..[t is not unusual to preserve thle heat of teapots by a, woollen coverinig, but the "cosey " lltust fit very closely. A closely-fitting cosey, wvidchl has the heatt of the teapot: fre'ely imparted to it by contact, would, as ve hatve seen, prlompote thle loss which it is intended to diminish, and thus 1do more harm than good. (34'3) One of tle most iteresting poilltS connected wvith this subject is the reci)rocityr which exists between the lpower of a body to cotmmunicate motion to tihe ctlher or to radiate; and its power to accel)t motion from the other, or to absorb. As reogarls radiation, we have already coml)ared lamp-])lack and wviting with metallic surfaces we will now compare the salme slubstances witlh refernce to their plowCrs of absorpti.l Of tlhese tw\o slheets of till, M N, O (fig. 80), one, o 1', is coated with whtiting, and the other, MAt x, left uncoated; I 256 IIJlI-ATJ AS A MOI)DE OF MOTION. p)lec tfiln thlus I)arallol to each other, and at a a distlnce of about two feet asunder. To the cdge of ecach shct is soldered a scrcw, and from one steet to thel other is stretched a copper Via. FO. wire, a b. At the back of each s}lcct is soldered ole end of a little b)ar of bismluth, to the otl ecr end, e, of wlhich a w\ire iS attacled, and terminating by a binding screw. XWith these two binlding scrows are connected the two ends of the wi're, coming f'ron tltc galvanometer beyond (c, and you observe that weo havte now 0 an unbroken circuit:, in whichl the gatvanlometer is included. You know already \what the bis, mutll bars tare intentded for. When the Nwarm finger is placed on this left-hand one, a currenttl; is immdiatCly devloped, which passes fiom tlhe bismuth to the thin, thence tlhroughl tlhe \ire conecctint the two sheets, thence round the galvanometer, and back to thle point frlom which it started. The needle of tlec gablvanometer moves through a larIIge arc; the red cld going tow REC1PROC ITY OF RAD)IAtION AND) AB3SORPTION. 25 ard you. I now place my finger upon the bisnitth at; the back of the other pIlate; a large dlellectio in thle opposite direction is the consequence.,When th1e inger is withdrawn, tlec junction cools, and once more the nccdle sinks to zecro. (344) Exactly midway betwecn the two shlccts of tiln is set a stand, on which is placed a hcatcd copiper ball; the ball radiates against both sheets: on the right, however, thle rays strike upon a, coated surface, while on the left they strike upon a naked metallic o1n. If bothl surfaces absorbed equally;the radiant at...... — if both; aceCpteCd %with equal freccdolm the motion of tlhc ethereal waves-....the bismnuth junctions at tell backs would be equally warmcd, and one of tflheml would netutlralizc tile other. 3tl-, if one surface be a. more powerful absorber fan tllhe other, a deflection of the galvanometer nccdle will )be thle consequelnce, and the direetion of the deflection will tell us whicl is the belst absorber. The ball is now upoln the stand, and the 1)romptt and energetic detlection of the nccdle inlforms us that the coated surfatc is thlle most powerfiul absorber. In tlhe same way compl)arl' lamp-black anld varnish with tiln, and find tihe two former to be b)y far the, best absorbers. (345).The thinnestl metallic coating furnishes a powerful dcfcnce against thie absorption of radiant hlcat. The baclk of this shleet of "gold-paper ". —-.the cg old being merely colpper reduced to great tcruityt. —-is coated with the red iodide of mcrcury. This iodide, as many of you know, hlas its red color d(iscllrgcd by hecat, thle powderi becoming a lpale ycllow. I lay the l)a.pcr flat on a board, w\ith the colored surface downwlardt' on its upper metallic surfaice are pasted picccs of paper so as to form a compllicated pattern. I now p)ass a red-hot slatula several times over thle sheet; the sp)atula radiates str1ongly against tlhe sheiet, butl; its rays arc absorbed in very dinclrent det'rces. Tlhe metallic surfac absorbs but; little; the paper strfaces absorb greedily; land, on tturning ull ) the sheet, you see that the iodide underneathl1 the metallic portion is p)erfectly uncIlanged, while under every lit of )pap)er the color is discharged. Aln exact copy of the figures pastcd on the opposite 258 hEIE1AT r S A 3MODE OF M[OTION. surface of the sheet is thus formled, Fi.or another example of the same kind, I am indtebted to AMr. Hill. A fire sent its rays 1against this painted piece of -wood (fig. 81), on which the numb er 338 was printed in gold-leaf lettelrs; the p)aint is blistered and, charredtall round the letters, butt ldn dlnctth thle letters the wood and ~~pd;:'..anlt are quit unafIfected. This, ~ *~( ~ thin filmti of gold thats been qullite jutihcicntt to prevent the absorpIAJ >.-l f ionil, to whriel thcte (clest'ucetion of th e saumroanding surface is due. (34G) The luminiiferlolus other fills stellar space; it makes the unlivelrs a w'wlole, Mtd rlndlc's possible the intercommunicttioll of lighdt and energy bctwcen star' and star. nBut the subtle substance penetrates fartherll; it surrlounds the very atomls of solid land liquid substances. Transparent bodic s are sluch, because thle ether and the atoms of such bodies are so related to eachl other, thatlthe waves which excite light can p1ass through them witlhout transferring their motion to the atoms. In colored blodics, certaill waves are absolrbed; but those which give the body its color pass without absorption. Through this solution of sulphate of col)p)er, for cxamptle, the blue'waves sp)eetd uillmpdccd, while the red waves 1are destroyed. A brilliant spectrum is now formed upon the screen; when the beaml is sent throwugh this solution, the red end of the spectrill is cuIt away, This piece of red glass, on the contrary, owes its colol t to the fact tlat its substance can be traversced ficely by the lohnger undulations of red, -while the shtorter waves are tabsorbed. P.laced ill thle path of the lightll, it leaves merely a. vivid red anld upon the screen. This blue liqid, then, cuts off the rays translmitted b)y the red glass; and thel red glass cuts off those transmitte(l by the liquid; by the unionl of both we oulgiht to tlave perfect opacity,,and so we have. ernlclt both t r )laccd il the )atht of the beam, the cltire E',LECTIV, ], A1SOJRPTION. 259 s)cctrum disappears; the union of thle two partially) ti ransparen{t bocldics producing an opiacity) cqual to that of pitch or metIl,. (347) A solution of the pcrilmlnganate of potash p)laced inl thfl path of the beam pcrmits the two ends of thle spccitrum to pass frectly through; you have the rcd and tte bluc, but betwccln both a space of intense b)lackness. The yellow of the spectrum is pitilessly dcstroyedl by) this liquid; among its atoms thcsc yellow rays Cannott pass, while thet red and the blue get throtugh] the intllratomic sp)aces, wtithout sensil)le hinderance. And hence thle gorgeous color'of tils liquid. Tu'ning the lamp round, and. projecting a dlislk of light; two fcct in diameter upon1 the screeln, 11. introduce this liquid. Can any thing b)c more splendid hanl the color of thlat disk? 1j turn thCe lanp obliqutly, land introducc a prism; thle violet compl)olCent of that beauttiful color lhals slidden away from the red. You see two defirite disks of these two colors which overlap in the centre, and exhibit there the tint of the composite light which passces through the liquid. (34t8) Thrs, as regards thle rwarves of light., bodies exercisc as it werel, an elecctive power, singlig out certain waves for dlestruction, and permittting others to paiss.'J.Transplarency to waves of one length does not a~t all imply t'ransparency to wavecs of another lcngth, and from this wve might reasonably infer tI hat transplarcncy to light does not necccssarily imply tlransparency to radiant; heat. This conclusion is cntirely verified by experiment. This tin screen, -m Y (fig. 82), is pierced 3by an al'rture', b:ehind whiclt is soldercdt a smatll stanlldt s, I llace this copper )ball, it heated to dull redness, on a pr'ope stand, at one side of the screen. At thle other sidc is J)lpaced the thellrmo-clectric p)ilc v; the rays from the ball nIow pass through'l the aplcrturC in tlhe screen and fall upon the pl)ilc —tlhe needle moves, and finally comes' to rest with 1a stetady deflection of 80~. I pllac t;his gltass cell, a, qiuarter of ant inch wide, filled with distilled \\water, oil the stand s, so tal;t -all rays reaching thle pile mlst patss tllrough the water. WhaVlt takes place? The needlc stetadily sink.s to zero; scarcely a ray 260 I 14T ASf A 1MODE O01F MOTION. fr'om te ball can cross te water; to the unttdulations issuilg fiom the bhall the water is practically opaque, though so extremely transplarent to tle rays of light. Before remllovill' tlhe FI. $2. till i i cell of sater, [ 1place behild it a similar cell, containing' trans)arclnt l)isutl)hide of carbon; so that now,'whcn t;hle watcr-ccll is removed, the aperturc is still barred by the new liquid. W"~hat occurs? The ncICdl promptly moves upward, and d(cscribcs a large arc; so that the self-same rays which found tlce water impcnetrable, find easy access thlrough the bisulphide of carbon. In t he same tway:, wYhen alcohol is compared withl chloridc of phosphorus, we find the former almost opaqulc to thle rays emitted ly our warm ball, while the latter permits themln to pass freely. (349) So, also, as rcgards solid bodies.'.'This plate of very pure glass is now pltaced onl the st(t1and, betwccl thte pile and this cube of ]hot water. No movement of tlle needle is l'erceptible. I now displace the plate of glass by a l)latc of rock-salt of ten times the thickness; you scc how promptly tle needle moves, until arrested by its stops. To tlhese rays, then, rock-stalt is cminel ntly transparcnt, while glass is practically opaque to them. RADIATION TUIROUGW[ SOLIDS. 261 (350) For these, and Inunl)erless sililar results, we aro indebted to Melloni, w\l may be l almost regarded as the creator of this branch of our subject. To express the )owcer of tralnsmitting rad iant lheatt, he p1roposed the word dilthermantcy. D)iatflhrmancy bears the same relation to radianltt ]teat that transparency does to light. Instead of giilng yout, at this st-age of outi inquiries, dtcermilatiolns of my O\wn of the diathermancy of solids and liquids, I vill makeli a slcection from the fables of the emlinent Italian philosol)her just referred to. In these dctellnilnt;lols, Mtelloni uses fotur diferent somlces of heat: tie ille of f a foceatelli lamp; a spiral Iplatinum wtire, kep)t incandescc nt, by the flame of anl alcolhol-lamp; a pl)ate of co)l)er heated to 4000 Cent., and t plate of coppler lheated to 1000 Cent;., tle last-mentioned sourcel beincg the Surfacc of at col)l)er tube, containing boiling water. The eCl)eriments were mnade in the following manner: First, thle radiation of the solurce:, tiht is to say the galvanometric deflection p)roduced by it, was determined, wh'i en nothing but air intervnc d bctceen t;he source of lheat and the pile. This deflection expressed the total radiation. Then the substance Iwh-ose dlathermaliney wN'as to be examined was introducled, and the consequent deflection noted; this deflection expressed the (quantiity of heat fransmlittcd y tflec substancc. Calling the total radiation 100, the p)rol)ortionate quanttities transmitted by twecnty-five diflerent; substances are givcn in the table on the following page. (351) This tablle showls, in the first ptlaee,v wvhat very different transmissive powers (different solid bodies possess.:it show16s {us also tthat, w\itht a sinlgle xcel)tion, the diatbermaney of the bodies mentioned var'ies with the qluality of thle lheat. lock-salt, only, is equally transptarent to Iheat from the four sources. It must, here e borne in mindll tlhat the luminous ralyis are also calorific rays; tllat the self-same ray, falling upon the nerve of \vision, produces the impression of light; while, implinging upon other nerves of the body, it produces tlhe iimpression of hleat. The luminotus calorific rays 1have, however, a shlOter wave-length than the obscure calorific rays; and, 262 111X1AT A A sAMODE OF MOTION. Tlana'TnSltssio: p~rceotate of the total radiation..Nanlws of Snbhstances.-.... re,.c~ed to a. Co1n1,01, thclktelt:ss of ~othl of an Intchl ('2'l6 ini}ttltinl I,) 1escenyt Co)per at!Copper at lamp. dei"Colt 400' 0. IOU, C. platinmi),0. 1 Itock-sa t..,,c........a 92'3 92-3 92'3 92%3 2 Sicilian sulphml.....7... 60 t 5 3 l no-spa.................. 2 69 42 33 4 tiral.tt.,. t..', t3 13 0 5 Iceland spar.. 39 28 6 0 6 C;lass.. 39 2i4 6 0 7 Roekcrkysttl (le a )..... 38 28 t 3 8 Smoky quartz.......... 28 6 3 9 Chronamte of potas h......... 4 2 15 0 10 White topaz............3 8 2 4 4 0 1I Cartbonate of?leadt. 32 23, 0 12 Sulphate of bav ytt.. 224 18 3 O 13 s'tlspar.., 23 19 6 0 14 Amethyst (violet)... 21. 9 2 0 15ii Artificil aber......... 21 6 0 f16 13oates o tf' soda,. 18 12 8 0O 17 Tourmatlinllle (deeop green). 18 16 3 0 18 CoImmont gn...........3....., 0 0 19 Selellte............ I. 1 0 0 20 Citric it,, ad..........1..., 2 0o 0 21 lPartrate of lpotashl.. 11 3 I. 0 O0 22 Natura11t lm.t..... 1,1 6 0 0 23 Alm.................., 1 ) 24 Sugatr-eand......... 8 1 o0 { 26 e......... 6.............0 0 25 Iee,...................... knowing, as we do, how differently waves, of difTlrent lenogths are absorbcd( by bodies, we are in a measure l preparld for thle results of the foregoing table. Thllls, while glass of the tllickness specified permits 39 per cent. of the rays of Liocatelli's lamp, and 24 per cent. of the rays from thte incanlldescnt pltinutl to )ass, it transmits only 6 per cent. of the rays from a soIure of 4000~., while it is absolutely opaque to all rays emlit.ted from a source of 100 C0. %e lalso see that liquid ice, so highly t, ransparent to light, tranlsmits only 6 per cent. of the rays of the lamp, and 0.5 per cent., of the rays of the inl candescent platiinum, while it cuts off all rays issuingl from th1e RADIATION THROUGH I IQUIJDS. 263 other two sourcCs. We have cere an intimation, that by far the greatcr portion of the rays cmitted by tihe lanl) of Locatelli must be1 ob)scure. tLumilous rays pass t lhrough ice, of the thickncss here given, without sensible abl)sorptio, and tetc fact;, that 94 per cent. of the rtays issuing from Locatelli's flame are destroyd )by the ice, proves that this propo)'rtionl of thelse rays has no light-g'iving' power. As regards the influnce of transl p)tiarcy, cleani and smoky quartz'l arl very inst'ructive. HIler are the two substances, olle pcerfectly pellucid, the othcr a dark br)own; still, for the luminous rays only do these tw\o specimcns shlow a difference of transmission. The clear qcuartz. transmits 38 per cenrt., and the smoky qlualrtz 37 per cent. of the rays from thc lamp, while, for the othellcr thrce sourcecs, tle transmissions of both1 sutbstanccs are identical. (3552) Melloni suptl)oscd rock-salt, to be perfectly t-ransparent to all kinds of calorific rays, the 7'7 per cent. less than 1a lumdred which the foregoing table exhibits being due, not; to absorption, but to reflection at the two surtfaces of the plate of salt.. B3ut the accurate experiments of TMMr. de la Provosttye and l)csains p)rove that this substance is permeable iln different dcgrccs to heat of diflfernt kinds; while Mr. 1Balfour Stewart hals established the important fact, thalt rock-salt is particularll y opaique to rays issuing' from a heatcd picce of the samce sul)stancc. Wre shall retlurn to this il)ortant; subject. (353) In tie following table, which is also b)orrowed from MAIctlloni, the calorific transmissions of nincteen diflecrcnt liquids'lans-nlsstonw: perleCntat)o of Namles of ltqutds; thickness, 0'8 In. total radiation,. Blsullphtidt of Cear)on.... 63 2 Bich11loride of sulphr'.... 63 3 Protohlltoride of phosphorl s ls., 62 4 fssencc of turpentine.., 31 5 Olive-oil,... 30 6 Naphtha.... 28 1 1,assenee of lavender.... 20 8 Sultphuric ether.. 21 9 Sulphulric acid,... 1, 26,0t IIAT AS A MiODE o00t 0 MOTION.'fl'auslliis-ioo!: perceutntae of Names of Liquids; thicknelkss, 03'86 tn. total radiation, 10 Hfydrate of tammonia.... 16 11 Nitric acid.. 15 12 Absolute aleohol... 16 13 Hlydrate of potash.... 13 14 Acetic acid. 12 156 Pyroligneous acid.. 12 10 Concentrated solution of sugar.. 12!II Solution of rock-salt.... 12 18 White of egg.. 11 19 D)istilled water...... 1 are given.'lhc source of lheat was an Argand lamp,) furnished w\ithl a glass chimney, and the liquidIs were enclosed in I cell witih glass sides, t.he thickel ss of the liquid layer being 9'21 millimetres, or 0'3G of an inch, Liquids are Ihere shown to be as1 diverse i tlheir powers of transmission as soli1ds; and it is also w\orthy of rem rk, that;v lter maintains its position as regnards opatity, notwithstanlding the chltange in its state of aggregation. (354) The reciprocity, which we have already demonsitrated between ra(liation and absorption, in the case, of metals, varnishes, etc., may now be extended to the blodies contained inl MTelloni's tabtles. One or two illustrations, borrowed from an extremely suggest\ive memlloir by:Mr. Balfour Stewart, will be suffilcienlt. Inl this copper vessel water is kept in a state of gentle ebullition. On the flat copper lid of the vessel are laid pllates of glass and of rock-salt, until they assume the temperature of the lid. When the plate of heated rockl-salt is fixed uponll a stalnd, in front of the thcrmo-electric pile, the delecction lprodced(l is so small as to be scarcely sensible. Rlecmoving the rock-salt, I put in its p)lace a1 )late of heated glass; the needlee mov s through ra large are, thusl conclusively sh8owing that the glass, which is the more lowcrful absorber of obscure heat, is also the more powerful radiator, Alum, utlnfortunately, melts at a. temperature lower than that here made use of; but, though its templerature is'lot so high as that of INFLUENrO OP TLIClKNESS. 265 the glass, you can see that it transcends the glass as a radiator; the action on the galvanometer is still mor elnergetic thanl inl tile last experiment. (355) Absorption takes l)lace withim the absorb)ing body; a certain thickness being requisite to effect the absor>tioln. This is true of both light and radiant heat. A very thin stratum of pale ale is almost as colorless as a striatium of water, the absorption being too inconsiderablle to produce the decided tintl which larger masses of the ale cxhibit. iWheln distilled wvater is poutred into a drinkintg-glass, it exhibits no trace of color; but an exl)criment is here arranged which will slow you that this pellucid liquid, in sufficient thiclkness, has a very decided color. This tube A n (fig. 83), fifteen feet long, r I.,83, At A. lt; t; - 0-^0. 80.0 - — o is placed horizontally, its ends being stopped by pices of plate-glass. At; one end of the tube stanlds an electric lamp, xl from which a cylinder of light vwill be sent through the tube. It is now half filled witt water, tile upp)l)r surface of which cuts the tube in two equal parts horizontally. Thus, half of the beaml is sent through air, and hbalf tl)rough water, and wit.h this lens, c, a magnified image of the adjacent end of the ttube is projected on the screen. You now see the image, o P, composed of two semicir'cles, one of which is formed by the light which has passed through the water, the other the light which has passed tllhroughl the air. Placed thuls, side by side, you can compare them, and you notice that, while the air semicircle is a pure white, thfe water semicircle 12 266 HIEAM1 AS A 0M 1 E OF M 1OTION. is a bright and delicate bluce-green. Thus, by augmenting thle thicknellss through which tlhe light; has to pass, we deepen the color; this )roves tLhat the destruction of the light-rays takes place woithibt the absorbingl body, and that it is not ain effect of surface merely. (350) [Melloni shows the same to be true of radiant heat. In his experilment:s, already recorded at page 262, the thickness of the plates used was 2'6 miillimnetres, ut, l)y rendering the plate thinner, we enable a greater quantity of hceat to get throllugh it, and, by rendering a very opaque substance sufficiently thin, we imay almost reach tile transmt ission of rocksalt:. The following table shows the influence of thickness on the transmissive power of a plate of glass: TransmiSsio lIs bAY (lIas of ditierent thikncnessea: plrcnttago th'ickneiss oof tho total Rladiation. lPlates iN m1iliicatellt Minip. 1tiu Copper at4' C.Coppri nt 100o. 206 39 24< G O 0o 41 31 12 1 0o07 77 7 3- 4 12 (357) Thusl, e see that, by diminishing the t hickness of the plate from 26' to 0'07 millimetres, thie quantity of heant t.ransmitted rises, in the case of thle lahmp of I 4ocatelli, from 39 to 77 p1er cent,; inl the case of the incandescent platinun, it'nstrlissions by Sclenite of dfit'ereilt tltckntsses: pereentago of the total ladliation. Thickniess of PtIates inl til: -0.4 )ttll JT)tl. i8l Copierant0 0 - C. Copperat 1000 0. 211 1t 0 0 O04i as 18 7/ O 0o01 6o.1 2 21:.............................. SIFTING OF IALORItFIMBI, f EMS. 26 7 from 21 to 57 pcer cent.; ill thle case of copper at, 4:000 C., from 6 to 34 per cenlt,; and in the case of copper at 1000 C., from absolute opacity to a tansmission of 12 per' cent. (358)'Thlle influence of thle thiccness of a plate of selenite on tl quanlltity of lheat which it transmits is exhibited ill the foregoing table. Theses experiments prove conclusively that; the albsorption of hleat takes place wvithinl the body, and is not a surtl ce action. (359) The decomposition of the solar beam produces the solar spectrtum; luminous ill the ccnt're, calorific at onle ccndl 111and chemic; l at the other. The stilu is, thereforebr a source of }ltcerogencous rays, and there canll scarcly be a doubt that; most; other sources of heat,, luminous and obscure, part1ake of this hetcrogelleity. In gCeneral', when such mixed rays enter a, diatleimelic substance, somle are ilntercl)tcd, others permitted to pass. Supposing, then, that \we take a slhcaf of calorific ralys;, lwhich have already passet through a dinthermlic plate, and permit them to fall upon a scconll plate of the same mallterial, the transparency of this second plate to the heat incident upon it -,must be greater than thle transparency of the first plate to to te heat incident on it. The first plate, if sufficicntly thick, hads alreadly extinguishlcd, in great part, the rays which the substance is captable of abso)rbing; and the recsidual rays, as a matter of course, pass freely throigh a scconld )latc of the se saic substance. The original beam is si'ted by thle first plate, anid tfhe lplurified beam possesses, for the tsame substance, a higher pncltrat ive power than tite original beam. (360) Thrlis power of plenetration has usually been taken as a test of the qawlity of lheat; the hceat of the pXurified beam is said to be diflferent in quality from tflit of the unpurified beam. It is not, however, tlhat any individual rtay or watve has changed its character, b)ut that from the beam, as a whole, ccrtain components lvye been withdrawln; and tlhat; this withdratwal has altered tlc pl)roportiol of the incident heat tanllsmittcd 1by a second substance. This is thi truLe icaninltp of the term "' quality," as app)llied to rad(iant heat. In tile patht 268 elIAT AS A MODIE.- OF MOTION. of the bcam from a lamp, let plates of rock-salt, alum, bichromate of p)ot as, and sclenite, be successivecly placed), ach plate 21' millimetxres in thtickness: let the hcat emerglcnt from these reslcctive plates fall upon a second serics of the satlme thlicktncss; oult of cvtry lhundred units of this heat the following l)roportion1s are transmitted: Rock-salt..,,.. 92'3 Atun... 00 Clhromatc of pIotash... l Selenite...... 91 (361) lReferring to the ttable, p. 262, we find that, of the %,whole heat emitted by the Locatelli lamp, only 34 per cent. is transmnitted by the chromate of potash; here we find the pcsvccntagc 7.1. Of the entire radiltion, selcnite transmits only 14 per cent,, but of the beam vwhich has been purified by a pllate of its own substance, it transmits 91 per cent. The same remark applies to tlle alum, wlhich transmits only 9 pec cent. of thle ul)urific d bclam, and 90 pec cent. of th;e p)urified beam. In rock-salt, oil tlte contrary, tlhe t. ransmissions of the sifted an1d unsifted }beam are the stame, because the substance is sensibly equally transparent to rays of all thle qualities hotr employed. In these ases I l have sup)osed the beam emergent from rock-salt to pass through rock-salt; the beam emergent from atlum to pass t.lhrough alum, and so of tile otlhers; but., as might be expected, ilthe sifting of the bcam by any substance will alter the proportion i~n whiceh it wNill be transmitted by almlost any other second substance, (362) 11 will conclude these observations wvith an experimtelont, which will show you the influcnce of siftilng, ill a very strilking mlanner. -Here is a scn11sitifve differential air-thelrmolmcter with a clean glass bulb. The slightest; touh of my lhand causes a de'Cression of t.Ih thermometric column. Let us now converge the powerful beam of our elcctric lamp on the bulb of that t:lcrmomcter. The, focus falls directly on the bulb, and the air withlin it is traversed by a beam of intense powe'r; ACTION OF1 GASS FRlliF.-SCREENS. 269 but not tile sl'glltest deXpression of thle thermometric columnll is disccrnible.'When this experiment Ntwas first shown to a person hlcre ptresent, hie almost doubtcd the evidence of his senses; but thle explanation is simplle.'The beam, beftore it reaches the bul)b, is adready sifted by the glass lenls used to concentrate it; anld, having passed through 12g or 14: feet of air, it contains no constituenjt, which can be sensibly (absorbed by the air within the bulb. HIence, thle hot beam passes through both air and glass, without warming either.:t is competelnt, however, to warmn the thermo-electric pile whose exposure to it, for a single instant., sufftlics to drive thle needlle violently aside. I now coat with laml-black the portion of: thle glass bulb struck by tilhe beam; you see the effect: tle heat is now absorbed, the air exp)ands, and the thermiometric column is forcib)ly depresscel. (363) We use glass fire-screens, which allow t.e l)leasanlt light of the fire to pass, while they cut off tihe ]heat; tlhe reason is, that by far the greater part of the h]eat emittcd 13by a fire is obscure, and to this the glass is op)aque. But in no case is there anlly loss. The hceat absorbed by the glass warms it; the motion of the ctlereal waves is here transferred to the molecules of the solid body. Bu3t you may be inclined to i-rge that, under these circumstanlees, thle glass itself oulght to becotoe a sourcc of heat, and that, therefore, we otught to de rive no boneftlt from) the absorption. The ftit is so, but the conclusion is unwarrantced, The p1hilosophy of thel screen is this: Let r (fig. 81) be a point of a fire, from whichl the rays proceed in straight litnes, towar\ d a person at 1r. Before thie screenl is introduced, eacl ray lpursues its collse direct to:; but now let a screen be l)laced at s. The screen interceptbs thCl heat, and becomes warmed; )u;,t ist lad of sendingl on tho rays in t1heir original direction only, it, as a warm body, emits thel it alt (Hir'ectibOns. Hence, it camlot transmit to the person at p all the fheat intercet)ted. A portion of the bheat is restored, butt 1by far the greater p)art is diverted froml r, and distibhuted in other directions. 270 H!EXAT AS A M1OD F Of MOTION. (364) MWVhCec tlhe vwaves pur)sut their rway unabsorbed, to1 motion of heatt is ilmp:arted, as we have seen ill tlee ase of the air-tlermometer. A joint of nmeat mighit be roasted before a.......................................................... fire the ai naround the joint bceing cold'as ice.'he ai r onI hig]h moutltains may b)e intcnsely cold, \ hil a l burning stlun is ovelrhead; tlhe solar rtays which, striking on the human skin, are dhllnost intolerable, ar) incomptctcnt to }heat the air scmsib)ly, anld wrc have only to withdraw into perfcct shlllde to fel the chill of the atmosphlere. ]I never, on any occasion, suiffered so muttch from solar heat as in descending from 0the " (ori'dor " to the Grand Plateau of Mont Blanc on August 13, 1857; tholgh my comt panionl and myself were at the time liip-dec1 ) in snow, the sun b)lazed against us in unendurable powtvcr. lmlncrsion in the shadow of the i])ome dutl Goft6 at once chlanged my feetlings; fi:or hre tlc air was at a freezing teCmpelature. it was not, however, sensibly colder tan tan l air tlrouglh which the sunbelams l)asse; anltd we suffered, not from tlte contacL of hot air, but from radiant heat, vwhlich hitad reached us through an icy cold mcdium. (365) T'lc beams of the stn l llpnctrate glass without sensibly heating, it; the leason is, that, having passcd througl our atlmoslphere, te ihetat a1s been in a great; nmeasure del)rived of thlose constituents li;able to be absorbed b)y glass. An experimelt twas lmade on a formert occasion, whcich you will now completely understand. A betam was senlt from t1he RADIATION TIfROUItt OPAQUIE BODIEiS. 2r11 electric lamp through a plate of ice without melfting it. Thlo beam had been p)reviously sifted by sending it through a. vessel of watert in which the heat, capable of )eing; absorbed by the ice, was lodgedlc, nd lodged so copiously that the water was raised almost to t het boiling-point; duringl the experiment. It is here worthy of remarl-k, that thle liquid water and the solid ice appbe ar to be )cervious and impervious to the same nrays; the one may be used as a sieve for the otlher: a result which indicantes that tihe quality of the absorlption is not influenced. in this ease) by the difllernce of ag'-'rcgattion. It is eas-y to prove that thc beam which bhas t, raversed ice witholut melting it is really a calorific beam. Allowing it to fill upon our1 ther'll-eletl'ric )ile, it causes thle needle to move with energty to its stops. (366)'When the calorific wavees are intercepted, they, as a general rule, raise the templerature of tfi body by which they are absorbed; but, when the absorbingll' body is ice, at a temperatt'le of 32~ I'thlr., it is ilmpossible to raise its temnlperature. I:[0ow then, does the hemat absorbed by the ice emplloy itself? It produces internal liquefaction, it tlkes down the crwystallilne atoms, and thus forms those lovely liquid flowers shown to you on a former occasion, (367)3 Vc ehave seen that transpallrency is not at all a test of diathermanlcy; tlat a body, highly transl)arentl to the tltuinou1s lundulations, may be hiiglhly opaque to tile non-luminous ones. A body may, as w\e have scc, be absolutely ol)atque to ligbt, and still, in a considerable degree, transpatrenlt to heatit Iticlre is anotiher example of the salme ind, The convlergent bleamn of the electric lamnp now marks its course tlroughl tlhe (ltst of the room: you see the point: of convergence of the beam, at a distance of fifteen feet fioml th-e lamp.,et uls ma'rk that. point accurately. This )late of rock-salt is coated so thicikly with soot that fthe light, not only of every gas-lampn in this room, but the electric light itself, is cut: off by it. WNrihen this plate of smolced salt is )l(aced in the pathl of the beam, the liglht is intereep)ted, but the mark enacbles me to filld the 2l2 lEAT AS A MODfE OF MOTION. )lace whter the foolls fell. I place tile pile at this focus: you see no beam falling onl it, but the violent action of thfe needlo instantly revcals to the mind's eye a fcts of leat, at tile point from which the light has been withdrawn. (368) You might;, perhaps, be (lisposed to think that the heat falling on the pile has been first absorbed by the soot., and then radiated from it, as from an independent source. 5etlloni ihas removed every objcction of this kind; but none of his experilntlts, I tlhink, are mIorle conclusivc, as a refutation of thle objection, than that now performed before you. Fqr, if the smoked salt we\re the source, the rays could not converge here to a focus, thoe salt being at this side of the converging lens. You also see that, when the pile is displhaced a little, laterally, but still turned toward the smoked salt, the needle sinks to zero. The heatt moreover, flling onil tile lile, is, as shown Iby I-Melloni, l)ractieally indeptendent of tile position of the plate of rock-satlt; you may cut off tile beam, at a dlistatnce of fifteen feet from the pile, or a distance of one foot: the resuilt is sensibly the scame, -which could not be tle case, if the smoked salt; itself were the souree of iheat. (369) WAhlen the experiment is repeated withl this black glass, the result, as you see, is tle same. Now, the glass'efleets a considerable portion of the li)ghlt and ]heatt from tlhe lamp; when held a little obliquelyt to thle beam, you call scee the reflected portion. WAhile the g;lass is in this position, I will coat; it with an opaque layer of lamp-black, thereby ceausilng it to absorb, lnot only all the luminous ray's which are nlow entering it, but also the portion vwhich it formerly retlected. Whlat is thle result? Though the glass 1 )late has become thie seat of augmented. absorp)tion, it has ceased to affect, tite pile, and thte nee'dle desends to zero, thus fuirnishing' additional proof tchat the heat which, in the fitrst place, acted ulpol tlhe pile, came direct from the lampl, and traversed the black glass, as ligp'ht traverses at trallnsl)arent substance. (370) {ock-salt transmits all rays, luminous and obscure; alum, of the thiockness already given, tlransmits, according to PROPOOtTION OF V1SIBLEt TO INVISIBLE RDAYS. 273 AIlloni, only the luminous rays; *' hence, the difletrenc between alum and rock-salt will give t;t v tle of te obsc ure radiation. Tcst(l in this way, M'cloli finds the following proportions of luminous to obscure rays, for the three sources mentioncd: Sourcewo. Lumiinou.ls. Obscuro. Flaltme of oil., 90 Incandescent platinum.., 2 98 Flame of alcohol...', 1 99 Thus, of the l ihet radiated from the flame, of oil, 90 per cent,. is d(uc to obscul;re rays; of thle heat rladiated fromn incandescent platinum, 98 per cent. is due to obscure rays; \while, of the heat radiated friom the flame of alcohol, fully 99 per cent. is duc to obscure radjiation. * Wo shall stlbsqcjently learn that this is an error. 2'/4 1INA'T AS A A MODE; 01F MOTION, (1:'Ii'tA'.t'I:t X, AIISORIItlON OP IF IEAT NY GASEOUS MA'TI. -'}: -..APPAR'A'IUS E%, );J'IOYEl-....EAY }: ) TICUIA:IE,"3 — L-AhEltIEIMANGYtl O} AI)i AND OF'SHltE'lRANSlPAI}ENT E}IE.SENTARY G.ASES-W -AtI')MEEMANOY OY OLEFIANT GA(S ANGND OIl OTll}}ER CiMPOtUNt GASES-.AlS:tORP't ION OF l'ADIANT 1IEf:' BY VAI'ORIS -I}'itR. DlON' tIAT }Y (}ASES.. E.I.l'lEECtI'SY F tlADIAtl'ON ANDt A:SOI'PtION -N- I'fIUENCE- OF TOLECUIAR CONStI'UItON ON E11I PA'SSAGE}' OF RADIAN:T MLEAT.r —TRAINSMtISfSION 01 ItEAT Itl1SOUOil OPAtQtlE ]1O)lE,. -- I1'AT'S-S1:GU, DEfTACfIIE) PRO0M.1101:T-.P'-'SPRUM YT AN OPAQUTF: PREIS. (351) " VI\ IOr have now cxamined thl diathcrmany,y, or t.ansparcney c to heat, of solid ad liquid bodies; andll learned that, closely as the atoms of such bodies are packed togrither, tihe interstitial spaces betwen the atoms afford ft'ce play and passage to tit ethereal undulations, whiich arc in many case s transmitted without sensible hinderlancl amolng the atoms. in other ceses, howcver, we found thalt the molecules stoppled thle waves of heat; whiclh impinged upon them; but thlat, in so doing', tlhey t hemselves became cenltrc s of motiolt. Thus we leartled that, lwhile p)trfectly diatltlermic b)odies allowed the heat-undulations to pass through them, without suffer-ing any change of templerature, tt(hose bodies -which stopped the calorific ftlux became heated by the absorption., Through ice itself we sent a powverful calorific beam; but, because tOie beam was of such a quality as not to be interceipted by thel ice, it 1)assed thlrough this highly-sensitive substance without Itnclting it.'We hlave now to deal xtith gyaseous bodies; and heretihe iunteratomic spaces are so vastly augm-ented, the moleca.les are so completely released from all mutual catangllement,f thlat we should be almtost justified in conclt ding that gases and FIllST IEXPERIMENTS AVITHI GAISES. 2'/5 alpors furnish a perfectly open door for the passage of tlhe calorifitc waves.'This, indeed, until quite recently, was tlhe universal belief; and the conclusion was vcrified by such cxperiments as had been made oil atmosp)hric air which wwas found to give no evidence of absorption. (372) Bhnt1 each succeeding year augments our cxp)erimental power; the invention of implroved imethodts enabling ius to renew our inquiries withl increased chlances of success. i].ct us, t, hen, test once mlole the diathermanc y of atmosplecric air. -We may make a i)ltreliminay essy s tlhe followingl way W Ae (ave here:a hollow tin cylinder, tA t (fig. 85), 4 feet long, a11nd nearly 3 inches in diameter, through which we may send our F.o, 85. I c'1 calorific beam. We must, lhowever, b)c ale to compare tlhe passage of the heat throughl the air withl its passage tfirough a vacuum, and helice we must have some means of stopping the ends of our cylinder, so as to be ab)le to exhaust it. l:ere wye encounter our first experimnetal difficulty. As a general lrule, obscure heat; is more greedily absorbed than luminous hcat, and, as our object is to make the ablsorption of a hlighllydiathllrmic body sensible, -w\e are most likely to cfflct this object by emplloyintg,; te radiation from an ol)scurC source. (373) Our tube, therefore, must; be- stolpc)d 1by a substance which permits of the free passage of suceh heat. Shall we use 2'ItI JIMAUT AS A M3OD)f OF MO'TION. glass for the purpose? An inspection of thce table at page 262 shows us, thait for such heat plates of glass would be perfectly opaque; w\e might -as wcYll stop our tuibes vith plates of metal. Observte her how one invcstigator's resultl s arett turned to account by another; 10oAw science g;rows by the conltinuilt degradation of ends to means, Ilad not; Melloni disov'rcd(l the diathermic properties of rock-salt, -we shhould now be utterly at af loss. For a time, howeverl t;lhe difficlty of obtaining )lattes of salt sufficiently large and pure to stop the ends of my tube was extremel, ]h3t a s cicetititl wvorker, if his wants are made known, does not long nlack ]hellp; and, thanl(s to such friendly aid, I lhave CherIe plat;es of this preciolus sulstanlce, which, by 1t-nclls of t.he caps A antid n1, ctan be screwed air-tight on to the ctads of my cylilinder.*. otl observe two stopcocks attached x. At a time when I was greatly int need of a sulpp)ly of rock-ssalt, I stated my wants in thte t Philosophical Magazinl,e' and met with anll immediate, response fr'om Sir John l:Iersehel. [le sent me a block of salt, accomrnpaied by a note, frol which, as it reftrs to the purpose for which the salt Nwas originally designed, I will mtake ant extract. I am also greatly indebted to )Dr. Szabo, the ltfnarianl Commllissioner to tle International E1xhibition of 180'2, by whom I have been raised to comparative olulence, as regards the possession of rock-salt.. T'o the l[essrs. Flletcher, of North\ wieh, and to'Mr. Colbett, of iBromsgro ve, my best thanks are also due for their ready kildness, To these acknowledgments I hlave now to add rmy respect ful tlhanlks to the Governmlent of Wfirtemberg, for the splendid block of salt placecd iln their depart.ment inl tihe lato Paris Elxhibition. HIere follows the extract l'rom Sir J. Hllrschel's note: " After thle publication of mly p)aper in thie Phil. Trans. 18t0, I was very desirous to dlisengagoe myself fr'om the influence of glass prisms and lenses, land nascertain, if pos.sible, whether in reality my inrsulated heat-spots p 8 ec inl the speetrum wteret of solar or terrestrial origin. RIock-salt was the obvious i'esourceO and after manytly and firuitless endeavors to obtain suffllicently large and pure specimelens, the late ])r. 0orterville was so good as to send nle (as I understood fi'om a friend in Cleshire) the very fine block which I nowN foirward. It is, however, nttucl cralked, but I have lno doubt pieces large enough for lenses and prisms (especially if cemented together) mnight be got from it. " ]ut I was not preplar ed for the working of it..evidently a very delicate and dilfieult process (I proposed to dissolve. off the corners, etc., and, as it were, lick it into shape), and tlough I have never quite lost sight of tlhe matter, I have inot yet been able to do any thing withl it; mealwv ile,.1 put it by. On looking at it a year or two after, I was dismayed to find it had lost luch by deliquilscnce. Accorldingly, I potted it up in salt in n earthen (lish, D)tECTS O0F METIOD. 27M1 to the cylinder. One of thlce> c, is connected with an air-)llml, t)y w\lhich the thtube cal be exhsausted; wh]ile throhtgh1 the other one, C,> air, or any other gas, can be allowed to enter the ttubc. (374) At one end of the cylinder is placed a Letslic's cube C containing 1)oiling wtater, and co-ated with laml-bl)lack to tug'mellnt its p)ocer of radiation. At thc other cend of tie cylinder stands our thermo-Celct, ric pile, from which wiresC lead to the galvanometer. ]Betwcen tihe end n of the cyliltder and tilh cube C, is int,'roduced a tin sereen'C, \lil, when w'ithdralwn, ill allow t.he calortfic rays to Itss from c through the tube to the pile. Wlre first exhaust thle cylindelr, then draw tihe screen a little aside, and now the rays are traversing ar valltum anld falling upon) the pile. Thle tin sCreen, )you observe, is only partially withdrawn, and the stetady deflection, produced by thle heat at present transmitted, is 30 degrees. (375) Let us now admit dry air; w\e can (1o so by means of the cock c', from whlich a piece of flexible tubing leads to the bent tutles u, u', the luse of'whlich shall be now cxp)laincld. The taube U is filled wvith fragments of pumice-stolle moistened witdh a solution of caustic potash; it is employed to Nwittl(hdraw whatever catlrbonic acid may be contained in the air. The b tel:)C u' is fillcd with fragment.s of ptulice-stolle, Imoistenled vwith sutlltllc acid; it is intended to absorb the aquceous vaporl of t he air. Th's, the air reaches the cylinder deprived both of its aqueous vapor and its carbonic acid. Itt is nomw cllteringf -...htl mercurytl-gauge of the dplli)p is desccnding, and, as it elnte'rs, you iwill observe'the needle. If tihe entrance of the' air dimin,1ishl the radliation through the cylindel --— if aifr )be a substanlce which is competent to interceet the waves of ether in any sensible delgree...thris will be declared by the dimlishlcd deflection of tile galvanometer. The tube is now full, but you sec no change in the position of lthe needle, nor could you see with iron rim, and placced it on an upper shelf in a room wNitfl an Arnott stove, ylwhere it bas rcmained ever since. " It you sholld find it of' any tue, I wouldt ask you, it' possiblc, to repeat miy experiment as desceribed, and settle that point, ywhich has always strtck Ito nas a very important oneo. 27s8 i PtET' AS A M0ODE) OF MOTION. lany chfanCge, evenl if;you were close to tile i'stlrumcltt. T'l'h aitr tlthus examined seems as transparent to radiant het as the vautull itself, (376) B]y changing the scrccn we can alter the amount of helat fallinlig )upon the Pile; thus, by graduatlly Awitlhdrawing it, the needle can be ctaused to stand at 40O, 500~, 600, 70~, and 80, in succession; tand, while it occupies each position, the cXl)erimcnt just t)erformced before you can be repeated. In no ilnstance could you recognize the slightest novement of tlhe needle.'T'he same is the case if the screen be puslhed forward, so as to reduce the deftlcction to 20 or 10 degrees. (377) Thie cxperiment just made is a qucstion addresssed to Nature, and her silence might 1) conlstrued into a negative reply. )3it ual natural philosoplher nust not lightly accept a neglative, and I am not sure that w ee ave lput our question in tilec best possible language. Let us analyze' what we lhav done, and first consider the ease of our sma1lllcst dtcflcction of 10 dcegrees. Sult)posing tlhlt tlhe air is not perfctly di:athermic;i that it reatlly intereep)ts a small lportlion l —---- say the thoulsanldth part of tlhe heat )assing through the tub.t....-.... that out of every tlhousland r1ays it intercepted one; should we be able to detect this action? Such aabsorption, if it took p)lace, would lower the deftlection ft}c thousandth part of ten degrees, or the hutlldredthll part of one degrcc, a diminultion whfich it -would be impossible for you to see, even if you were close to the gal\,anometer.* IhI the case hlere suplposed, the total quantity q/f heat fa!lling 0upo the pile is so ihconsid'rable, that a small jfraction of it, evn, if absorbed, might well escape detection. (378) B:ut we have not confined ourselves to a smlall quaintity of heat; the result was the samre when tfhe deflection was 800, as when it vwas 10~. Ifere I must ats you to sharpen your attentlion and accompany me, ftor a timnc, over rather ditllicult griound. I want now to mnake clearly intelligible to you an imlportant Pc culiarityr of the galvanometer. It w ill be borne in mind that I allm here lcakinsg of ytralaomt rk/i, not of tems md~ea4}rw dcrees. REtlLATION OF DI)PFJECTION TO AIBSORPTION. 2'/o9 (379)'t.h'he needle being at zero, let us suptpose a quantity of iheat to fall upon the pile, sufilcient to 1)oduce a deflection of onc dleglce. Suppose the quantitiy of lceat to be aftevrward augllentedl, so as to produce doflectionls of t.wo degrees, tihree detgrees, four dt'r:ccs, fiv detrccs; tlt the quanltities of hlcat whliclt poduce I) thse deC lections stand to each othler inl the ratios of 1: 2: 3: 45: 5. th q(uantity of heat which produtces l decflct(ion of 5~ being exactly fitve tillcs thatl; whiclh prloduces a dceflection of 1%. ]But this lopolortionallity exists only so longl as the deflections do not; excccd a certain magnitulde.:For, as the needle is drawn more and more aside fiom zero,: the currellt acts upon: it at an everl-auglenting disadlvantage.' Iih case is illust'ttcd )rby a, sailor workinll2g a capstlan: lhe always applies his strclng;th at right anglles to the lever, for, if lie a(tplied it obliquely, only a portion of that strength would ble efkctive in turninig' thlie cal)stan round. And in the case of our electric currlllllt, whell the nccedle is very oblique to the ciltrent's direction, only a )polrtion of its force is cffective in moving the needle. Thus it hap)lens thlat;, though tlhe quantitry of heat may be, and, in our case, is, act. curately expressed by the strength of the current whliCh it excits,' still the larger deflectiollns inasmuch as they do not, give us t he action of the whole current,, 1but only a part of it, callot be a trule measure of the amount of heat falling upon th}e pile. (380)'The g;alvanometer fnow before you is so constructed, that the angles of deflection, up to 30~ or thlereabouts, are i:ioporitional to the qtluantitics of hcat; t etth L quantity necessary to move thle needle from 29~ to 30~ is sensibly the same as that required to remove it fiLom 00 to 10. 1But beyond 30~ tlhe proportionalit; y ceases. The Cqu(antity of iheat required to move the neccdle fiosm 400 to 410 is three times that necessary to move it froml 00 to (:; to d(eflect it from 500 to 51.0 reuiries five times thle lcat necess(ary to mnove it from 0~ to 10; to deflet it fr'om 60 to 610 requtlires tabout seve\ ttimlcs tihcl heat necessary to move it fi1om 00 to 10; to defllect it from 700 to 710 requires eleven thimecs, while to move it from 800 to 810 re 280 IIElT.1' AS A nMODE1 0O]? MOTION. quires more tlan fifty timcs tle liheat; ecessary to 1m1ove it froml 00 to 1'. TIus, tbhe higher wre Icgo, thle greater is tho q(uantiity of heat recpresented by at degree of detflection; tl2 reason beingcl, th;at the force \which then moves thle needle is only a fraction of thoe force of the currenlt really circullatinng inl thoe wire, and hence represents only a fraction of the Iheat falling upon tihe 1ile. (381) 1 y a. process, to be afterward d(escribed,) tthe higher degrees of the galvanometer can be expressed in termns of the lower ones.'rVc thus learl!, that while (leflections of 10, 200, 300, rcsspectively express qllluantitics of hefatt rep)resented(l by thi e numbers ]10, 20, 30, a (leflection of 400 replresents a (tuanti.ity of heat etpressed by tle iXnuber 47; a detflection of 50 expiesses a (ulantity of ltcat expressed by the nmItbcr 80; while the deflcctions 60~, 700, 800, express (quantities of liheat which increase in a much more rapid ratio than tlhe dencfctions thelmselves. (382) What is thle upshot of this Analysis? It Awill lead us to a better method of qulestioning Nature. It. suggests thie reflection that, when ve lmake our angles saill, the quantitly of h]eat falling on thce pile is so inconlsiderable that, even if a fraction of it were absorbed, it might, escape detection; while, if w e make our deflectlions large, by employing a powerful flux of ]heat, t.ht nccdle is ini a position frioml whvichi it would require t considerabll aldditlion or abstl'raction of heatt to m1love it.'The 1,000tlh part of the whole radiation, in the one clase, would )be too small), ablsolutely, to be measulred: the.,000tth part in tlhe othetcr case might be very considerable, Nwithout., ho1wever, being considerable enough to afict the needle ill any sen-sible degrcc. Whcn, for example, tlhe deflection is over 800, an augmentation or diminution of hieat., cquivalent to 15 or 20 of the lower degrees of the galvalnometer, would be scarcely sensible. (383) Vec are now face to face Nwitlh our pro1bllcm it is this, to work Awitlh a1I flux of heant so large that a small fracX *SC& Appendix to this Chapter. NEAW MWPT[OI0). 281 tional partt of it will not be infinitesimal, and still to keep our needle in its mos{t sensitive position. If wve can accomlplishi thlis, we shall. augment indefinitely our Iex)teriment al power. Tf laccd on its stand at the cndl of thI experimental cylinder, and fur'-'li~shcd w\ith its two conical reflector.s. c' is the comIenso(.ti:gy cutbe, usedl to nlutralize the radiation froml o; It is thel( at/.tstbAy sciest, lwhlich is capable of ain exceewding'ly finlle motion to land fro. N N is a delicate galvanomoeter colnncted wvith the pile IMPROVEDJ) APPAILA'TUS. 283 l, )by the wires w w'.'helC graduated tube o o (to the right of the jplate), andl the appendLage a ( (attached to the centre of the experimental tulc), slhall be referred to more )alrticularly by-and-by. (389) It; would hardly sustain your interest, were 1. to state the diiiicultics which at first beset tlhe invest, iga'tion conlucted with this apparatus, or the numberlcss precautions which the exact balancing of the tiwo powerful sources of heat, here resorted to, 1 rendered Inecessary. I believe the experiments, made with atmospl)cric air alon, might be numbered by tenlls of tholsands. Sotithes for a wleek or cvcll fol a fortnight, coincident and satisfactory results would be obtainel the strict conditions of accurate experimenting'would a)lppe'ar to be found, wheln al additiontal day's experience Vwould destroy tte superstructurl of hope, and necessitate a recotmmencCment, under changed conditions, of the \whIole incquiry. ].t is t.his whicl daunts tle cexl)erimenter; it is thllis Irelilminary fight with the entanglements of a. sublject, so (ldark, so doubtful, so u1ncleheringl w.. ithlout lany knowledge whYcether the conflict is to Iead to any thing worth po sscss-i n..... whbich cenders discovery difilcult and rare. iut the experimenter, estpecially the, yolug exl)rlimenter, ouglht, to knoow that., as regards his own moral imanlood, hIe cannot but win, if hIe only contend aright;. 14ven with a negatiNve result;, t he con'sciousnless thatl he has gone fiirly to the bottom of hlis subject, as fatr as his means allowed —— the feeling that lhe has not shunned labor, though that labor maI have resulted in taying' bare the nakcednesls of his case.. ——.reacts upon his own mindl, and gives it fitrnmellss for future work. (390) B3ut to return: I first; neglected atmosp)lcrli vapor and carbonic Iacild altogether; concluding, as others afterwlard did, that, the quantities of these stubstances being so small, the;ir effect upon radiant hbeat mst t Ibe quite inal)preciable; aftetr a time, however, this assumption was found to be leading; me quite astray. Chloride of calcium xwas first used a;s at drying agcnt, but I }ad to (abandon it,. Pumice-stonel tmoistenedt 284 1lEAT AS A MOD1E OF MOTION. with sulphurll ic acid, vwas next, used, but it also proved utsuiLt able. I finally resorted to purie glass broken intb small frag^ meits, wetted wit;h sulplhuric acid, andl inscrtedl b1 means of at ftnnel into at U tube. This arrangemenlt was found to'boe thle best, but even here the greatest care vwas needed. It was necessary to cover each column with a layer of dry glass fragments, for the smallcst particle of dust; frol t.he cork, or a quantity of sealing-wax not more than thie twentieth l)art of a pin's-head in size, was quite suflicient, if it raehched the acid, to vit-iate the results..The dryiing-trtbes, morcover, had to be frequently changed, as the organici matter of the atmosphere, infinitesimal though it was, after a time introduced disturblance. (391.)'to remove the carbonic acid, plure Carrara marble wavs broken into fragments, wetted with caustic potash, and introduced into a U tube. 1These, theln, are the agents now cmployed for drying tile gas and removing the carbonic acid; but-, previous to their final adoption, tlhe arrang1ement shown ill Plate I. was made use of. lle glass tubes markl d Yv, each thrce feet long, were filled with chloride of calcium,'after flelm w\ere )ltcet two t tube s, t'x, filled with ptumicestllne and sul tthuric acid. I Icncc, tile air, in tle first place, hlad to pass over 18 feet of chloride of calcium, and afterward through tfle siulpluric-acid. tublcs, before entering the expcerimental tube C s'. A glasholder, (1 G', was emp)loyed for otlher gases than atlmosptlicric air. In1 tile investf;igatioll on which I alm at lrcsent engaged, this arrangement,, as already stated, is abandoncd, a simpllcr one being founllld more effectual. (392) IBothl tile front chlamber, v, and thle experimentall tube s s' being exhausted, the rays pass froml the source( oe thl'Otltl11 tLle front chlamber; acloss the plate of rock-satlt at s, through tle experimental tubt, across tihe plate at s', afterward impinging, upon t he anterior surface of the pile i,. Thiis radiation is neutralized 1/y that from the compelnsating cube'. The cneedle, you will observe, i.s at zero. \e will commence ourl experiments by )appllying this severe test to dtry air. It is now entering the experimenital cylinder; but, at youlr lisa ACTION 01 OEVJIANT GAiS. 285 tlance, you see no motion of thte needle, and thus our more 1powerfiul mode of Cxperiment fails to detect any absorption onl the part of tlhe air. Its (atoms, al))arentlyt,,re incomp)etent to stop a single calorific wave; it is a pEtactical vauceum, as r'egar-ds the -rays qf heat. Oxygent, lhyltrogelb, ant.td nitroglln, wtitcn carefully purified, exhibit the action of atmlosp)heric air; they are sensibly neutral. (393) T[his is tthe deportfment which, prior to the researcles now to be described, wavcs ascribed to gases generally. Let us see whether rightly or not. T'lhis gatsholder conltlins olelanlit gas.. -colo. coatl-gas would also answer my purplose. lThe perfect, transparenycy of this gas to light is demonstrated by discharging'it into the aiir; you see not.hingi, tlhe gas is not to b)e distinguished from the air. Thhe experimentall tube isl now exhausted, and tlhe needle points to zero. OObserve the cijtet wln thle olefiant gas is tl)ertitted to enter. The ncedle moves in a momelint; thle transpl)arecnt gas intercepts thle h}eat, like atn opaque body —-lthe final and permanent deflection, when the tube is full of gas, amounting to 70~. (394) Let us fnow interpose a metal screen betweel tile pile i and thoe end s' of the eX)primental tube, tltls entirely cuittingt off the radiation tihrough t lhe tube. The face of the ipile turned toward thie metal screen wastes its heat; speedily by radiation; it is now a t th;e temilperature of this room, and the radiation from the compensating cubl) alone acts on the pile, producing a deflection of 75'. lBut, at the commencelInmnt of the experiment, t;he radiations from both cubes wero cequtal; hence, thle deflcction 750 corresl)onds to thle total radiation t;lrough the expsertcintal tube, when the letter is cx-, hausted. (395) Tatking8 as unit the quantity of hcnat ncccssary to move th.e ncedlc friom 00 to 10, thle number of units exlprssed b3y a deflection of 75~ is The numbet of unitsl expressed by a dcflection of 70 is'211. 286 IIEAT AS A MolD}] OF MOTION. Out of a tottal, therefore, of 276, oleiantl gas has interce)pted 21; that is, about seven-nilttis of tlth whole, or about 80 per cent-. (396) D)ocs it not sccm to you as if all opaque layer had been suddenly precipitated on our plates of salt, when the gas entered?'ic subistance, however, deposits no ssuch ayclr. hANh1en a current of the dried gas is discharged against a polished plate of salt, you do not perceive the slightest dimnless. The roCk-salt plates, moreover, thotugh necessary for exact measlreCmllCnlts, are not nlecessaryt to show the destructive power of thils gas. Htere is an open t.il cylinder, interposed between the pile and our radiating source; when olefiant gas is forced gently into the cylinder fromtt this gasholder, you see thle needle fly up to its stops. Observe the smallness of the quantity of gas now to be eml)loyed. First cleanlsing tthe open tulml,) by forcing a current of air thlrough it., and bringing the needle to zero, I turn this cock on and off as speedily as possible. A mere bubble of tile gas enters the tube in this brief inter. val; still you see that its presence caiuses the needle to swing to 700. Let us tow abolish tfle open tube, and leave nothing but the free air between the pile and source; from the g;asometer I: dischalrge olefiantt gas into this op)Ce space. You see nothinig' in the air; but the swing of thie needle throl gh anl are of 60~ declares thoe presence of this invisible barrier to the calorific rays. (397)'T'lhus, it is shown that the ethereal undulations which g-lide anmong the atoms of oxygen, nitro'ellu, and hydrogern, without hinderance, are powerfully intercepted by the molecules of olefiant gas. ire shall find other tranlsparent gases,,also, almost immeasurably superior to air. eVo can limit at pleasure the number of the gaseous atoms, and thus vary t;e amlount of dlestruetion of the ethereal waves. Attached to the air-p)umt1 is a barometric tube, by means of \vwhich measured portions of the gas can be admitted.'The experimen0tal cylinder is now exhausted, turning this cocl slowly on and observing the mercury-gauge, olefiant gas enters, till thlt ACTI'JON O' OLtEFfANT (GAS. 28'? mercurial column has been depressed an inch. I observe the galvanolceter, and read the dcflection. D)ctcrmining thus the absorption produced by one inchl, another inch is added, and the absorption cffected by two inches of the gas is determined. P'rocccding thus, w\e obtain forl tensions from I to 10 inchcs the following absorption Olelant G(as. it inches. Absorption. 1........ Q90 2... 123 3.*........ 1412 4. 157?......,,... 182 8.......... i80 9,....., 1. 9(I 10.......... 193 (398) Trlc untit here uaised is the amount of lheat absorbed when a whole atmospqhere of dried air is allowed to enter the tube. fli'c table, for cxamplce, shows that one-thirt;icetl of an atmosphere of olefiant gas exercises ninety times the absorption of fa whole at'mosphlere of air. The deflection lroduced l)y tli tubeftil of dry air is here taken to be one degree: it is i)robably less even than this infinitesimal amouit., (399) TIhe table also inform:s uns tihat eachl additional inch of oleflant gas produces less effect than the plreceding olne. A single inch, at the commencement., intercepts 90 rays, lbut a second inch absorl)s only 33, while the adfdition of anl inch, when nine inches are already in the tul)c, eflfects the destruction of only 3 rays. T}is is bwhat might reasonably be expecteed. T.lc number of rays emitted( is finite, and the disclarge of the first inch of olefiant gas among themn haas so thlimll their ranlks that the exeition produlcecd by thle second inlch is naturally less than that of the first'T'}is executlion must diminishf, as tlthe number of rays capable of being' dc 288 li1EAT AS A MODlE OF [MOTION. stroyed by the gas bcecomes less; until, finally, all absorbable rays being removedl the residual heat p)asses through the gas unimpecded.* (400) But supposing the quantity of gas first, int.roduced to be so inconsiderable thalt thle hcat interceptcd by it is a vanishing quaintity, compared with the total almount, we niglht tlhen reasonably expect that, for some time at least, the quantity of hleat intercepted wvould be proportional to the qmantitity of gas present. Tha1t a double quanltity of gas wotld producc a doulle effect, a treble quantitr a treble cffect;; or, in gencral termns, tlhat the tabsorption wvouldl, for a timei, be found proport ionatl to the density. (401) 1To test thlis idea, wve will make use of a portion of the aplaratus omitted in tlhe general description. o o (PllatC 1.) is a graduated glass tube, the end of which dips into the basin of water, x. The tube is closed albove ty means of tlhe 5stop)cock r; d d is a tube containingl fragmlrents of chloride of calcium which dries the gas. The tube o o is first filled Nwith Nwater up to the cock', and t he water is afterward cariefilly displalcc by olefiant gas, introduced in bubbles from below.''The gas is admitted into the explerimental cylinder by the cock 1. ald,,as it enters, thle water rises in o o, each 0f the divisions of which represents,a volume of 6i6.th of a cubic inch. Successive measures of tfhis capacity are pl)ermitted to enter the tube, and the labsorlption in each particular case is determined. (4:02) In the following table, the first coltumn contains the quantity of gas admtitted into the tube; the second contains the corresponding a bsorptioll; the third column contains the * A wavo of ether startinl froe a radiant point in all directions, i n a tuifbor medill, constitutes a spherical shell, which expands with tle velocity of light. or of radinut heat. A i'tt of light, or n i'ay of heat, is a line perpendicular to the wave, and(, in the caso,here stpplosce, the rays would be tho radii of the spherical shell. The word1 " ray," however, is used in thlo text, to lavoid circumllocuttion, as equivalet to thle term ait qf Atl. Thlus, calling the amount of heat iaterceptcd by a whol1e atmosphere of air 1, the amount intercepted by -'6tl of an atiospnhll re of olefiant gasn is 90. RE1LATION OF D1tENSITY TO ABSORPTION. 289 ab).sorltion, calculated on the supplosition t hat it is proportional to thie density: Oleflant Gas. Unit.necasturc, 610th of a cubic inc1. Absorption, A.... x... ~.................. W.................................................... _ Mtcasutvis of tGas Observed. Calculated. 1.22..... 22 2.... 4'. 4., It.. 6.... 6, 4. S. 8'8.... 8'S:6,.....110.....110 4..... 12'0... 1382 4.....14'.8.....158.... 1.... 9.....198.....198 10.....22. 224) 11.....24'0.....24'2 12..... 2,.'4 17w. 1.. 2. 2'., 26'4 13..... 290. 28 14.... 302.298 16.......... 33'0 (403) This tab)le proves the correctness of the surmise that, when very small quantities of t he gas arc Cml)loyed, t he absorption is sensibly proportionatl to the density. Butt consider for a momentt the tenuity of the gas writl which wre have 1her1e operated. The volume of our experimental t:ube is 220 cubic inclels; imagine -60th of a cubic inch of gas diffused in this space, and you have t he atmosphere ththrough which thle calorific rays paI.ssed in our firMst ex)erilent. L This atmosphere posscsses a pressure not exceeding.1-0ith of that of of ordinary air. It would depress the mcrcurial columln colnnected with lhe ailr-pur1p not more than -3.-th of anl ]Jlglisll inchll. Its action, however, upon the calorlific rays is p)erfectly mccasurable, being more than twice thart of a, whole atmosphere of dry air. (4t04) But the absorptive energy of olcfiant gas, extraordiniary ns it is shown to be by the foregoing experiments, is 13 290 IE1ATl AS A MODE 0F MOTION. exceeded by that of various vapors, thlo action of whlich on radiant iheat is now to ob illustrated,. Thlis glass flask, o (fig. 86), is provided Nwit.h a brass cap, into which a stop-cock can be screwed air-tight. A small qtuantity of sulplhuric ether is pourcd into the flask, and by means of an air-pump the airl which fills the flask above the liquid is completely removetd. I attach the flask to thle experimental tube, hichl lo. S6. is now CxhIausted-the needle pointing to zero —. and l)ermit the vapor from thle flask to enter it.. The mercury of the ga uge sinks, andt, when it is ~:7.jii: depressed one inch, the further supply of vapor will be stopped.''The moment the vapor entered, the needle moved, and it now points to 65; T canr add anotherl1 ilncl, and agSa1in determine the absorption; a third inch, and do the same. The absorptions effectcd by four inches, introduced in this way, are given in the following table. For the sake of comparison, the corrcsponding ablsorptions of olcfiant gas are placed in the third column: Pressuro in inches Correispondlig absorption of lstrclury. Absorption, of oleflalt gas. 1.....211..... 2.282 123 38...831.....14*2 4.. 330. 164 (4t05) lFor tlcse pressures the absorption of radiant heat )y tihe val)or of sulphuric ethelr is about two and two-t.hird times the absorption by olefiant gas. There is, moreover, lno proportionality between tle clquantity of vapor and the absorption. (406) But; rcflections similar to those which we have already applied to olefiant gas are also apl)licable to sulphuric ether. Supposing we make our unit-measulre small elnoulh), thle number of rays first destroyed will vanish in complarison A13SORPTION 3Y RTHEER VAPOR. 291 with the total imnbcer, and probably, for a time, the absorption will be directly proportional to the density. To examine whethler t.his is the case, thte other portion of the apparatus, omitted in the general description, was made iuse of. x< (Plate 1.) is one of the small flasks already described, with a brass cap, which is closely screvwed on to tlhe stopcock c'. Between t.he cocks c' and c, the latter of which is connected with the experlimental tube, is the chamber m, the capacity of whlich is accurately determined. The flask K is )artfially filled with ether, the air above the liquid and tlhat dissolved in it; being removed. The tube s s' and the chlamber m being exhausted, the cock c is shut off; land,' being turndc on, the clamber m is filled with puwe ether-vapor. By turning c' off anid C on, this quantity of vapor is ng rcserved for each liquid, so as to rendcr the admixture of thle vapors impossible. Flrom each flask the air is carefdlly removed. —not only the air above t.he liquid, but the; air dissolved in it, this latter bubbling freely away when the flask is exhausted. I. now attach my flaskl to the exhausted experimental tube, and allow the vapor to enter, wfitlout Cermittilng any ebullition to occur. 1t11 mIrcury-c olultllmn of the i)um) sinis, and, whcn the required depression has been obtailned, the supply of va)por is clut off. hi this wfay, the vapors of the substanllces mentioned in the next table lthave bccn cxaltnined, at pressures of 0'1, 0', and 1. inch, respectivtely. Absorptiol of Vntapors at the paressures 01 05B 1i0 Bisutlphlidc of carbon. 16 411 62 Iodide of tthyl.,. 35 14,' 2,12 3Benzol......66 182 260 (hIoroforl..... 86 182 236 M[ethylic alcohol. 109 390 690 Anlwyleon......, 4 182 635 823 Stp1hutric acid.....300'tO 8710 Alcohol.., 325 622 Formxic ethe'.....480 8O1) 1075i Acttic ether...... 90 980 1 196 I'ropionate of ethyl,.59 9 70 Boracic acid....... 620 (4:3,1) These numbers refer to the absorption of ta whole atmospthere of drly air as t;heir luit; thtat is to say, -04th of 1an inch of bisulphidet of carbon vapor does fiftceen t imes t he cxccution of 30 inches of atmos)pheric air; while -11-0ti of an inch of boracic-thler vapor (I0es 6G20 times the execution of a wh1ole at;mosl)hler of atmosphellri air. C(omparing nit a; a )pressutre of 0'01 wiit btoraeio oether at thei samne p)r'ssure, the absorption 306 III:iAT AS A MODE OF MOTION. of the latter is probably more tlhan 180,000 times that of tihe former. (4,34a) It is easy to show in a general way the absorption of radiant heat l)y vapor. An open tuf)e will answer the purpose when quantitative results are not soughit,. The tube even may be dispensed xwitlh, and the vapor discharged from a slit into the open air betwee the pile and the source. A few spelcimen results obtained in this rough way will suffice for illustration. Twro cubes of boiling w\ater are employed, antld in the 8usual imanner the needle was broug'lht to zero. D)ry air vwas then urgcd from a gas-bag (a common bcllows would anlswer the same p1urpose) through a U-tube containing fragments of,glass, moistcned with the liquid whose vapor was to be cxallined. The mixed air and vapor were discharged in the open air in front of the pile, and the extreme limit of the swing of tlhe galvanometer needle was noted. Yapor discharged Limlt of swing tl openl air. of needle, 8tlltic ether.... 118 Formic cther..... 11l Acetic cther.. 92 Amylne.... 1 Bistlphidc of carbon G... 01 Yaletcic ether. 32 IBenzol..,.. 31 Alcohol,,,.. 31 The influence of volatility hlere forces itself upon the attention. The action, of course, depcnds on the amounl;t of rapor discharged, a quantity dircectly dependent on the volatility of t-he liquid. T; is in conlsequlence of its g'rcater volatility that bisulplhide of carbon is here able to transcend the far more cncrgetic alcohol. C(ALBRATION OF t'i GALVANOMEiTERt. 30 APPENDI)X T1') O CIAP'R. X. I (},I hero thlo method of calibrating t he galvanometer whichl Mellon!i recommend.s, as leaving not-hinhg to bo desiredt as regards facility, promptness, and precision. lis own statement of tllo m-eCthod, translated fi'om " La Thermol brose," p. 9, is as follows: Tw\o small vessels, v v, are half filled mwith moreury, and connected separately, F1 0. by two short wttires, with tho extremities / (i c of the galvanol eter. lthe vessels and wires tihus disposed make no change in thle action of the instrument; tlhe thermo - electrio cmirrent being freely \ - f-f transmitted, as before, from tlhe pile to.: — the ganlvanomleterl. But, if, b)y mlean1s of a i; wile r, a com0lmuaic iation b1 establishled between the twvo vessels, part of thle cur- ):x rent wvill pass thronugh this wire and return to tIle pile. The quantity of electricity cirnculating in the galvanometer will be thus diminished, and witht it the deflection of the needled Suppose, then, that by this artifice wve havo reduced the galvancmetrio deviation to its fourth or fifth p)art; in other wor(ds, sup11posing that the nccdle, being at 10 or 12 degrccs under the action of a constant source of heat, placed at a fixed distance froml the pile, descendls to 2. or 3 degrees, when a portion of the current is diverted by the external wire; I say, that by c.ausing the source to act frt om varlious distances, and observing in each case the total deflection, anil thle ed rn eed deflection, we have a11 the data necessary to determnile tlhe ratio of the deflections of thle needle to thle forces which produce these defectiols. To render thi expositioni clearer, and to fulrnis l, at tle same timel, an example of thle mode of oleration, I Nwill take tihe mnlbers relating to tho application of theo method to one of mly thermo-i-nmltipliers. 30Q8 11EAT AS A MODE OF MOTION. The external circuit beitng interrupted, and the source of heat hbe ing sufficiently diStanllt r'Dio the )ile to give a deflection not exceeding5 detgrees of the galvlanometer, let the Awire be placed from v to v; the needle falls to 1'5~. The connection between the two vessels being again interrupteld, let trhe source be brought near enough to obtain stccessively the deflections: 60, 100, 160, 200, 26~, 300, 350, 400, 40. interposing after each the same wire between v and v, we. obtain thfe followinlg nuatlmllbers: 1,50, 30, 4.50 06.3~, 8.4~, 11.23, 1630~, 224" 2099.0 Assuming tlho fbcoe ncccssary to cause tle neecdlo to describe each of the first degrees of the galvanoll eter to be equal to unity, we have tlho inumber 5 as thlo exlpression of the force corresplondling to the first observation. The other forces are eansily obtained by the proportions: 1'5 5::::::::5. a.:8: 3'333. whoere (t relrsents the dericection when tlhe exterior circuit is closed. WYe tthus obtainL W5 10, 16'2, 21, 283, 37'3 fobr the forces, corresponding to the deflections, 6O 10~, 1o0, 200, 250, 300. Xin this instrument., therltobrc, the forces aro sensibly l)roportionall to the arcs, up to nearly 15 degrees. Beyond this, the plroportionality ceases, and the divergence autgments ans the aarcs increas ill size. The forces belonging to thl intermedliate degrees are oibtained withl great ease, either by calculation or by graphlical construction, w+hictit Iatter is sufficiently accurat for these determlinations. BJy these means we findl D)cgrces. 130,'14, 165, 16~, 1l~, 18~, 190, 200, 21~. Forces.. 13, 1-'1, 15'2, 1603, 17'4, 18'-, 1908, 21, 22'3. ])iffirclees.. 1'1, 11, P 1, 1'1, 1'2, 1P2, 12, 1'3. I)Degrees.. 220, 23~, 24t0, 260, 20, 2970, 280, 290, 300. Forces.. 235, 240, 2 26' 28, 2907/, 316'5, 33'4, 35'3, 37'3. 1)iftrelcnccs., 1'i, 1), 17, 1'8, 10', 1'9 2.'l That is to say, one reduced currentt is to the total currtent to wMhielh it corresponds, as any other reduced current is to its corresponding totIal urtrent. CALIBRATION 0OF TIlEA (ATArANOMETEIIR. 309 Il this table we do not take into account anlly of thel degrees lreocedi'ng, the 13th, becallust the force co1rres1pon1dingll to each of theml possseseS thOe samle v1alute as the deflectiolln. The forces corresiponding to the first 30 degrees being known, notillng is easier thanl to detormine thie values of the forces corresp)onding to 35, 40, 45 degreeos, and upjward. The reduced deflections of thetso three arcs arel...s... 1563", 22.,~ 29,~0. Lot tus collsider them soparately; commencing With the first. In1 the first place, then, 15 degrees, according to our calculatioll, are equal to l15'; we obtain thlo value of th]o decimal 0'3 lby multiplying this frac'tion by the difbrncet 1'1 which exists between the 165t and 6th degrees; for weo havo evidently the 1proportion 1: 11 ^. 03: *... 0'3. The valtue of th}e reduced deflection corresponding to the 35th dtgree vwill not, therefore, be 15.3~, but 1520~ -1- 03~:::::: 10. By iillitar considerations \we find 2,30~ +.- 0':::: 24t~ instead of 22.4' and 361'7 instead of 2049~ for the reduced deflcctions of 40 and 45 degrees. it now only remains to calculato thle forches belonltging to these three deflections, 15'5, 24'1~, and 30'7~, by means of tle expression 3'333 a; this gives us.. — thle foreqs, 51", 80'3, 122'3; for the degrees, 35, 40, 4t5. Comp)aring tlhese numbers with those of the l)rceceding table, we sco that the senlsitiveness of our galvatometer diminishes considetrably when we uset deileotions greater tlhan 30 degrees. 10 El1,~AT AS A MODE 0OF MOTION. CI0.'l APTlRtl{ XI. AO1ION Or ODOROUS SUlBSTrAN.CES UPON' RADIAN lIEAT- -OTxON o' oOZONR UPON tBAl.IAN' }REAT —IDERMIfNAT'ION OIF'tlIE }IA)IAT'TION AN'D ABSORPtION OF OASES AND VAPORB.ltOUT. ANNY fOURR O! EAT X'Tr:!INA, 0''tlO I OASEOtUS BODY ---—..DYNAMIS R}ADIAVION AND'lABSOR'II:N.IN —— A ilA'ItAIOSN T'IROUGII rTltl TIRE RARI' AT'MOSPII''E- —. ]N-,'l.f;SC orP T}x AQUEOU8'VAPOR OF TI'El A'MNOSPtRI'1ER ON ABRDJANT lIERAT. —.CONNEI'Ol'ON OP. TIRft RADIAANT AND ABSOURDENT Poi't OF AQUEOUS VAIPOR WIRfitT!It'EOROLODIOAT. PIIENOME NA, APPNs).s:-.R.IUTEIliR DET.X'AIlS 0r T71R AOTION OF IltUMID AR, (435) 0JCENTS and enfluvia generally have long ocul)icd the attention of observant men, and they ha-ve formed favorite illustrations of the "'divisibility of matter." No chemlist ever wcighed the perfume of a rose; but in radiant hcat w he have a test mlore refined tlan the chcmist's balance.'l'he results brought before you inl our last chapter would enable you to hiear the assertion with;lout surprise that tlhe quantity of volatile matter rcmoved from a. hartshorn-bottte by aniy person in this room, l)y a single act of inlhalation, would exorcise a more potnt action on radiant hceat than the whole body of oxygen and nitrogen whichl the room contains.,et us apply this test to other odors, and sec whlther tllcy also, notwithlstanding their almost infiite attenuation, (10o not exercise a 1measurable influence on radiant teat. (436) We will operate in a very simple way. A number of small and equal squarcs of bibulous 1aper are rolled up so as to form little cylinders, each about two inclecs in length. Elach Pai)1Cp cylinder is tlhen moistened by dipping one end of ACTION OF PERFUtME, 311 it into aln aromatic oil; the oil creeps l)y cal)ialla ry attraction through t1lhe0 paper, until the whlole of the roll bccoImes moist. The roll is introduced thus into a glass tube, of a diameter which enables the paper cylinder to fill it without being squeezed, and betwccnt the drying apparatus and the experimental tube is placed the tube containing the scented paper. The experimental tuble is now cxhausted, and the nccdle at zero. Turning tfhis cock on, dr1y air passes gently through the fold of thle saturated paper. The air takes up the )perfiule of the aromatic oil, and carries it forward into the cxperimlental tube. The absorlption of one atmosphere of dry:air vwe asslume to b1 unity; auld any additional absorption which these experiments rcvceal must be) duc to the scent which accompanies the air. (437) 1'The following table will give a condensed view of the absorpttion of the substances mentioned in it, witl reference to the unit jlust mentioned: Namlo of perfimol. Absorption. Patchoulli...... 30 anlal-wood..... 32 Geranium.... *. * 33 Oil of cloves... 34 Otto of roses...... B3ergamot...... 44 Neroli... t41 Lavender...... 60 e, on.... 6. Portugal... 601 Thyme...... 68 Rosemary..,.. 114 Oil of laurel...... 80 Camomile flowcers.. 8 Cassia. 10)9 Spikenard.... 35 Aniseed...... 2 (438) The numbelcr of atoms of air here in the tube must be regarded as a{lmost infinite, in comparison withll those of 8 12 1E1AT AS A AMODE OF MOTION. the odors; still the latter, thinly seattered as they are, intercept, in the case of patchouli, 30 times tile quantity absorbed by the air; otto of roses does upwlard of 36 times the execution of the air; t, hyme, 74 times; spikenlard, 355 times; an5 d aniseed 372 times. Itwould be idle to speculate on the quantities of mattcr implicated in these results. 1Probably they would,have to be multiplied by milliols to bring them up to the pressure of ordinary air. T,mus. —-- " T'he sweet southl Thant Lbreathes, upon a balk of violets, Stealing and giving odor," owes its sweetness to an agenlt, wlrticl, thomugh almost illnfinitely attenuated, may be more potent., as an intercepter of terrestrial traliattion, thlan the entire atmosphere from " ballnk " to skIy. (439) 1Jn addition to thiese experiments onl the essential oils, others were made on aromatiC herbL. A number of such were obtained from Covent-Garden Market; they wcere dry, il tile common acceptlationt of thle terml; that is to say, they were not g'reen, blut withered. Still, I fear the results obtained Awith theml cannot be rcgarded as plure, on account of the probable admixtture of aqueous vapor. Thle aromaltic p)arts of the planlts wevcre stuff'd into a. glass tube eighteen inches lolng and a quarter of {lan inch in diameter. Previous to connccting them %wNith tihe experimental tube, tlhey were attached to at second air-pump, and dry air was carried over them for soime minultes. They were theln connected with the excerimlelttal cylinder, and treated as the essential oils; t{he only difltrence beingl that; a lengt;hd of eighteen illnces, instead of two, was occupied by the herbs. Thyme, tlhus examined, gpave an action tbhirty-threce timCes that of the air whichl passed over it. Peppermint exercised tirty-four tilmes the action of the air. Spearmlint exercised thirtty-eight times the aotion of the ACTION 0F OOZONE 313 Lamvender exercised thirty-two times the action of tile air. \Vorninwood exercised fort,-one times the action of tlhe an'. Cinnamon exercised fifty-three times thle action of tie air. As already hinted, thlese rcsults may be complicated witil thlle action of aqueous vapor: its quantity, however, must lhave been infinitesimal. (4410) T'htlere is another substance of great interest to the chemist, to'whllich we may apply the test of raldiant leati, but the attainable (quantities of it are so mirllte as almost to 0elude measurement. I mean that extraordinary substance ozone. This body is knowin to be liberated at thie oxygen electrode, when wvater is decomposed by an clectric current. To investigate its action, three different decomposing cells were co1nstructed. In the first), No. 1, tie l)latinlum plates used as electrodes had about four square inches of surface; the p)lates of the seconld (No. 2) had two square inchles of surface; Nlwhile the plates of the third (No. 3) lhad only one square inch of surface, each. (441) My reason for using electrodes of diffcrent sizes was thlis: On first applying radiant heat to the examilation of ozolne, T consttructed a dccomposing cell, in \which, to diminish thle resistance of the current, very large platinumm l)lates were used. The oxygen thus obtained, which ought to have embraced thle ozone, slhowed scarcely any of the reactions of this substance. It hardly discolored iodide of potassium, andl wtas almost without action on radiant iheat. A sccond dccomposing al)paratu ith smaller plates, t saller plaes as tried, and here tlhe action, both oil iodide of p)otassium and on radilant heat,, was found very decided. ]eing unable to refer tlhese diflrences to any other cause than tile difilrent magnitudes of the plates, I formally) attacked the subject, by operating with the thrcee cells att.bove described. Calling' the iction of tile main body of tihe electrolytic oxygent unity, that of the ozone which accompallicd it, in the respective cases, is given in the following table: 14 314 J1, AT AS A MODE Ot t MOT1ION. Numbher of Ce1. Absorptioln. No. 1 f.. 20 No. 2...... 34 No. 8. -..... 41 (44t2) Thus, tile 11modicutm of ozole which acconl)anicd the oxygen, fand in comparison to which it is a vanishling quantity, exerted, in the case of the first pair of plates, an action twenty times that of the oxygen itself, while with the hllird pair of plates the ozone was fo4ky-soevll tillis more energetic tithall the oxygsen.'Tlh influence of tihe size of tle plates, or, in otlher words, of thle dctasi2ty of thle current'whre it cntet' the liquid, on tihe production of ozone, is rcndercd strilfingly manifest, b3y these experimcnts. (4:43) PIortions of the plates of cell No. 2 3cwere then cut away, so as to make t}hem smaller than those of.No. 3. The relduction of the plates was accompanied b)y anl increlase of the action upon radiant heat; the absorption rose at once from 34 to 65. The reduced plates of No. 2 here transcend those of No. 3, whviclh, ill the first experillmenXts, g;'ave thle tlargest action.'1'he plates of No. 3 were next reduced, so as to make them;n smallest of all. The ozone now gen crated l)y No. 3 cfl'eted an absorption of 85.'T.htls, we see that the acttion upon radiant heabt advances as thle size of the electrodes is diminlishcld. Heat is known to be very destructtive of ozonec; and, sitspectinzg the development of heat at tlhe snmall clectroles of the cell last mlade use of, I surrounded the cell wvitalh a mixture of poundted ice and salt. Kept thlus cool, the absorption of the ozone generated rose to 136. (4,44) There is a perfect correspondence bctwccn tlheso results, and those of MAM. de la iRive, So-ret, and Meidinglr, CONSTITUTION OF OZONE0%0. 31 though there is no rcscmblalnce etwceeln our respcctive m,,odes of experilment. Sucti an agreement is calculated to augmelnt ourl1 co,0fidence in radiat t }heat, as an invosti,ator of molecular condition.* (445)'The quantities of ozone involved in the foregoing experiments must 1be I)erfectly unmll asurable by Ordinary lmeans, Still, its action ulpon radiant heat is so enlrgetic as to p)lace it beside oleiant gas, or boracic ctlher, as an ablsorb' ent-..1blulk for bulk, it might transcend either. No elementa'ry gas that i have examined behaves at. all like ozone. In its swing throulghi the cther it must l)\owrfutlly distlurbl thle dimn. it' it: be oxygen, it must1 b) oxygen-atoms, plackct into groups. I soug ht to decide the question whether it is oxy* -M. te[idingcl commenl ces his paper by showing thet a)bsnce of agreemlent between theory and expcrimlent inl the decolmposition of water, the diffi'rnce showing itselt very decidedly ill a deficiency of oxygen whken th(e ct-''ret. was st'ong. On) heating his clcctrolyte, lie found that this ldiftirence disappeared, the proper luantity of oxygen being then liberated. lie at. oneo surmiscd that the defect of oxygen migtht be due to the formation of ozone; but htow did tlhe slubstance act to produce the diminution of thet osxygen ll the defect ere \due to the great density of tlhe ozone, thte destrntion of this substance, by heat, would restore thet oxygen to its true volume. Strong heatintg, howev'r, which dlestroyed te ozone, produced no alteration of voliimne, llence M. 5etidliner concluded that the effect which ihe observed was n lot dulo to the ozone which remained minxed with the oxygen itself. ie finally concluded, and justified hi*s conclusion by satisfitatory experiments, that tlhe loss of oxygeln was duet to the fotrmation,l in the water, of peroxide of hydrog in by tlhe ozone.; the oxygen being thus withdrawns ftiom the tube to whichl it belonged. Ho also, as MX. de lIa Rive had previouItly done, expeiimlnted with electrodes of diferefnt sizes, amid found tie loss of oxygen muchll inoro consideralble when a small electrode vwas used than with a large one, swbhene, lie inferedd thatn the formation of ozone was filitatted by gmttenztfi if dttetASii qft' the curraet et the p1.tec withere eletrode, ad! e!ectrolyte met..The sa-noe conclusion is deduced ftrom tIe above expleriments on radiant heat. No tswo thing;s could be more diverse than tihe two modes of proceeding. M. Meiditge soughlt for tlhe oxygen whicth had disappentredt, and foundl it in tlte liquid I examined the oxygen actually liberated, and fottd that tIte ozone milxed with it augments in quantity, as the electtodes dim inislh Int size. It m1lay be added that, since the perusal of MN1. M5cidilger's paper, I hlave repeated his experitnents with mly own decomposition-cells, and found that thlose whict gave nme thle greatest absorption also showed the greatest defiliteny in the amlount of oxygeu liberated, 316 IEAfT AS A MODE1,l OP MiOTJON. gen, or a compound of hydrogen, in the following way: elat destroys olzone. If it were oxygen only, lheat W\ould convertc it into the common gas; if it were the hydrogen complound, which some chemists considert it to be, lheat would convert it; into oxygen, plus aqueous val)or.'I'lhe gas alone, admitted into the experimelntal tubc, would give tile neutlral act1ion of oxygn, but; the gas, 1)1us t}he aqueous vapor, would probably give a greater action. The dried elect.rolytic gas was first; caulsed to l)ass throul It gla ss tube heated to redness, and, thelnce, witout drying, direcct into the experimental tube. Secondly, after iheatilng, it wa\s dried before enterinlg the exp)erilmental tube. iitherto, I hve not beelln able to establish, Nwith certainty, a dilbrcence between the dried and undtried gas. If, therefore,, tile act of lheting develop aqueous vap)or, the experimental means emllployed have not yet cnallcd me to detect it. Folr the present, t.erefore, eI hold the belief that ozone is )tproduccd by the paeclking, of the atoms of elcmentary oxygen into oscillating groups; and that heat ing dissolves the bol)nd of union aind allows the atoms to switng singly, t-hbus disqtualifyingl them for cither intercepting' or generatsing tih motion, which), as systems, they -are competentt to interccpte and generate.* (41,6) Your attention is now to be dilrected to a series of facts which surprised and l)Cerplexed me, when they lwere ob-,served. il permitted, on one occasion, a quantity of alcohol val)or, sullicient to tdepress the merculry-g'laugc 0'5 of an inchl to enter the experimental tube; it prodtlccd a defleetion of 720. While the needle pointed to thi;s high figure, and before )ulpil)ing out thle vapor, I allowed dry air to stlream into the tu)be, anld happened, s it entered, to Ice)p my eye upon tfhe galvanometer. * The foregoing e coclunsion regnrdina thle constitution of ozoneo was describe d at a time whllet the~ m1lost eminllent authorities regarlded ozonle as collsisting of singlle atoms, and ordinary oxygent of Frolups of atolms. Chtmlicall investig'ations have since ittdependently established tho view suggested by the above experimentts on radiant heat. EIXTERNAl SOURCEI 01 OF AT OMITTED). 31 (44I7) The necedie to my astonislllment, sank speedily to zer0o, and welnt to 25~ onl the oppzosite side. l-The entry of thle ineffective air not only neutralized thbe absorption p)reviously o)served, but left a considerable balance in faivor of the face of the p)ile turncd toward tlhe sourcc. A repetition of the experiment broughlt the needle down f'rom 70~ to zero, and sent it to 380 on the olpposite side. In like, mainnel, a very small q(uantity of the vapor of sull)huric cthlr plroducc d at dellection of 300; on allo\wing dry air to fill the tube, the needle dcscinded sp)eedily to zero, and swung to 60~ at the oplposite side. AMy first thfouight, on observsing thlese extraordinary cffects, was. tlhat thle vapors had deposited themselves in opaque films on the plates of rockl-salt, and thatt the dry air, on entering, had clearcd these fillms a way, anld allowed the heat from tlle source freie transMi ssionl. (448) B]ti; a moment:'s reflection dissipatte this sutpl)osition. The clearing away of such a film could, at best, butt restore the state of thingis existing prior to the entrance of the vlapor. It mighltt be conceivc(d able to bring the needlc atgain to 00, but it could not po0sibly )' produce the neg'ative dcflecttion. Nevert;heless, I dismounted t. he tube, and sublj{cted the l)lates of salt to a searching examtiat, ion. No such deposit as that surmised was observcd.'The salt remained l)erlfctlr tl'ransi)ar1et while in contlact with the valpor. f ow, then, acre the ffect-s to bo accolunted for? (44:9)'c h\eave already made ourselves acquainted Awiteh the thermal effects p)rod(uced when air is )ermnittced to stream into a vacuum. We k cnow that; the air is vwarmed by its collision against the sides of the receiverl. Can it be thalt the ]heat thtus generatfed, implarted by the air to the alcohol and ether vat-pors, and radiated by them against the 1)ile, was more than sufflicient to make amends for the absorption? The e)pei'mewtzuta, erutogs at once sluggests itself here. If the effects observed be( due to the heating of tile air onl entering the partial vacuum in which tlhe vap)or was diflused, wre ought to obtain the same offects, when the sources of hmeat hitherto 318 hlEiAT AS A tMOI)ED OF MOTION. made use of are entirely abolished. We are thus led( to the consilderationl of tle novel, and, at first sitght;, utterly jptrodoxical Irolbleml.-to determine the radiation and absorption of a gas or vapor withtout any source of heat extei'nal to the gaseous body itsef (450) Let us, then, erect our apparatus, and abandon our two sources of heat. I-Here is our glass ttube, stopped at one el0(1 by a pllate of glass-.-.....for we do not now need the passage of the heat through this nd...... -and at the other cnd b1y a )late of rock-salt. IX friont of the salt is l)laccd the pile, connectcd with its galvanometer. Though therec is now no special source of fheat (acting upon the jpile, t;he needle does not come quite to zero; ind(ed, the walls of this room, and the teop)le who sit around, are so many sources of heat, to neutralize Nwhicl, and thus to blringt the leedle accurately to zero, 1 nmust sligltly' warm the defective face of the pile. T.lhiss is done without any difficulty 5by a cube of lukewarml water, p)laced at a distance; the needlet is noxw at zero. (451) t'ht csl)elriment:al tube being exhausted, air is pormitted to enter, till the tube is filled. l'his air is at lpresent warm; every one of its latomis is oscillatingl; and, if the atoms possesseld attly sensible powe'r of communlicating their motion to the lumilniferous cther, wc should have, from eachl atom, a tlrain of waves impinging on the tice of the pile. ulit you observe scarcely atty motion of the galvanometer, and!hencoe may infer that the quantity of heat radiated by the air is exceedilly ei, srmall. The deflect.ion produced is.7~ (452) Biut these to tare not really due to the radiation of the air. To lwhlat tahenr? I open one of the Iends of the cxperimenltal tube, and place a bit of blacl pl)aper as a lining within it; the pa)per merely constitutes a ring, which covers tie interior surface of the tube for a leng'tht of 12 inctcs Let uts now close thle tube anld releat tle last ex)e riment. The air is now entering'; but marl< th, e needle. — it leas already flown throug'h ar arc of 700, You see here cxemplifiled the influence of thlis bit of papjer linting; it is warimed by the air, and it radi D)YNAM;t RAiD)IA10N O1F GASEtS. 319 ates atgainst the iCle in this copious way. Yl'he inte'rior sut'f/ice of the tbtte itself' must do the seti e, though in a less deg'ree, and to the radiation from this surface, and not from the air itself, thet dceflection of 7~ whichl we lhave just obtained is, I believe, to b)e ascribed. (453) Rtemoving the bit of lining friom the tube, instead of air wve will permit nitlIrous oxide to stream into it; the needle swings to 28, thus showing the superior radiaftive pow\er of this gas over tha:t of air. Oin workling the pump, the gais witfiin the experimiental tubc is chilled; and into it the pile pours its heat, a swing of 20~ in the0 ol)posite direction being the coscequncce. (45,1) Xnstead of nitrous oxide, I allow olefiant gas to enter the exhausted tube. We have already learned that; this gas is highly gifted with ftle power of absorption and radil;tion, Its atoms are now warlcmed, and every one of them asserts its power; the tneedle swi\ gs through an are of 670, Let; it waste its heat;, and let the needle come to zero. On pumping out, thie chilling of the gas within the tube l)roduces a deflcction of 40~ on the side of cold.'CWe hlave certainlly here a key to the solution of the enigmatical effects, observed with the tlcohol and ether val)or. (4155) For the stake of convenience we may call the heating of the Pas on enteringl the vtacuum dyartmic ite atwef; its radiation may be called dyam oic r'tl.diiation,, nd its absorpttion, when it is chilled by pumping out, dlytnta.mic absoyPtion.'Thl.cse terms being understood, the following table explains itseltf Inl cach case, the extreme limit; to which the needle swung, on thle eltry of the gas into the experimental tube, is recorded..Dyantmie.Radiation of Cases. Namll1e, tnlmAt of first linpulsion. Air.....C Oxygen. HEdt (ogl 7 n Nitrogcln.'. 320 lIEAT AS A MOIDE OF MOTION, Nalta. tLimit of flist Impulsion. Carbonic oxide...19, Carbonic acid... 21 1Nitrous oxide. 31 Olcilant gas.... 83 (456) W\re observe tthat tho order of the radieative powers, dcterminc d in th~is Inove l way, is thile same as lthat already obtamined from a totally ditfterent mode of cexpcriment. It: must be borne in mind tlat t he discovery of dypnalmil radiation is quite retent, and tllatL the conditions of perfect; accuracy have not yet been dcvceloped; it is, however, certain that the mo-lde of experiment is susceptible of the highlest degree of precision, (457) Let us now turn to our valpors, and 1while dealing with them Ih shall endeavor to unite two effects whicht, at first siglht, might; appear utterly incongpruous. We have already learned thlat at polished metal surface emits ranl extremely fccble radiatoionll lut that, wheit thet same surflac is coatedt with varnish, the radiation is col)ious. In tile communication of 1otion to the etlher of space,* thie atoms of the metal need a mediator, and this they finld in the varniish. Yio may varnsh a -etalclic seutee by Saib oi aO powei:til gas. The arrange1menrt before you enlables mc to clause a thin st-ream of olelant gas to pass from the gasholder (c (fig. 89) through a slit tutbe a b, over tle helated surface of t:he cube c. At present no gas issuecs, and t.he radiation from e is netralizced by tlhat from c'; bult now I pour the gas from (: over theC cube c; and though t;the surface is actually cooled by tihe passage of the, gas, for the gas has to be wtarmed by the metal, ithe effect is to augment considerably thel radiation. As soon as the gas begins to flow, the needle begins to move, and reaches an 1alnplitutde of 450 (458) We lavre here varnished a metal by a gas, but a more interestinglt and stubtle effect- is the varishing of ot071 If twe coul change either the name given to the interstellar mctdiuln or tlhat givte to certain volatile liquids by chemists, it would be an advantage. It is (ifficult to avoid confullsion in ttte use of tlo same terml tr objects so utterly diverse. GAS VARNISHIfD BY VA'POR. 321 gaseouts bodty by anothcr. This flaslk contains acctic ether, a volatile, and, as you ]lknow-, a highly-ab)sorbenllt substance. I attach thle flask to the experimentll tube, and )elirmit the Fio. 89. h.i vapor to enter it, luntil the mercury column has been depressed half an inch. There is now vapor, under half ae 305) exceeds thatt of aniy other stubstance hitl. DYNAMIC0 ERADIATION OF VAPOlRS, 323 crto examined; and its dyntamiic radintion may be )Lrecsntmcd to be commensurate. Let us exhaust the experimental tube as p}er'fctly as possible, and introduce into it a qlialntity of boracic ether varpor, sufficient to deprcss the mcrcury column -110-th of anll inch. The baroleter stands to-day at; 30 inclhes; hcnce, tie 1 plressure of the ether vapor now in our tube is -3.u.tth of an atmosl)herc. On sendinpg drSy air into the tube, the vapor is warmed) and tile dynamic radiation pro(luccs the deflection 560. By wpfrking the lpump, I reduce ihe residue of air within it to a pressurc of 02% of an inchl or i —10i-th of an atmlosphcre. A portiotl of the boracic ether vapor remains of course in the tubc, thle pressure s residue beino the part of ths e eig te at of the vapor, when it first entered the tulbe. When dDry air is p)ermittcd to enter, the dynamic radiation of the residual vapor )produces a deflection of 42~. WeVc will again workc the pump till the pressure of the air within it. is 0'2 of an inchl,; the quantity of ether vapor now in thle tube eCinl th of that plresent in the last expl)riment. The dynamic radiation of this residue gives a dellection of 200. Two additional experliments, conducted in the same way, gave deflections of 14~ and 1.0~ respectively. The question 11nowN is, Wh\\lat was the tc:nity of the bortacic cter vapor whxell this last defltection was obtained? The following table contains the answer to this question. Dynami c.icaiCdttion qf. Boraci'C.71ther. 1PresLurc in parts of atmosphere. Deflection. X' 1 6 000 42' t X.. X.6... 20 c x 6 x X "' Q,....... 14...... x X ~i6i& ( X'I' Ii X V- x 30 - -t-, -0 0i -:0.,. 14 (462) The air itself, watlin the ititerior of the tubc, producets, as we h11ave seenl, a detflectionl of 70; hencwe t.he entire (lefletion of 10 as nollt tile to the radiateioll of the vtaetpor 324 lIEuIAT AS A MOODJE OF MOTION. ])educting 7, it woidd leave a remainder of 3~, sti > supplosing wC entirely omlit thle last expclrimentt; we can then have no doubt that at least half the deflect. ion 14~ is due to tlhe residue of boracic ether vapor; this we find, by strict mllCaslrcment,i would have to be multiplied )by one thousand millions, to bring it upl to the lpressure of ordiltary atmospheric air. (463) \nother reflcction here presents itself, which is worthy of Otl' consideration. Were hNave measured thie dynlamic radiation of olefiant cras, by alloewing the gas to cetcr our tul)e until tile latter was quite filled. Let us consider the state of the warm raliating collln of olefiant gasi n this experimenlt. it is manifest, that those portions of tlhe column1 most distant; from the l)ile must radiate through the g#as mi' ti'ont of themt, and, in this forwoard portioln of the column of gas, a large quantity of tlhe rays emitted by its hinder portion wsNill be absorbedf, In fact, it is quite certain that, if wre %lmade our column suffliciently long, the frontal portions wtould act as a perfectly impenetrable screeccn to the radiation from the hinder oncs. Thus, by cutting off that l)art of tlhe gatscous column most distant from the pile, we might dimiinish only in a very sImall (egree thle amount of radiation whvichl reachcs the p)ile. (464) ]-Let us now colpare the dynamic rfadiation of a vap)or with that of olefiant gas. In case of our vtlapor, w USe\' l only 0'5 of an inch of pressure, hence the tadcliting2 molecules of the vapor are much wider aplart than thlose of the olefiant, gas, tunder 60 times theC preSurt'; and, consequently, the radi.ationl of the hinder portions of the column of vapor w\ill have a coml)aratively opl) door, through which to reach the l)ile. These considerations rendler it manifest that, ill the case of tlhe vnpor, ta greatCr length qf tube is available for radiation thlan in the case of olefiant:gas.'t'his leadrs filurthcr to thle coic'111sion tltat, if Nwe' shortetl tn the tube, wNe 1hall dimilnish the radiation, in th1e case of thle Nrvapor, more considerably than lin tthat of the gas. ILet us 10ow )ring our retsoning to the test of exp3erunXent.. ,ONG ANI) SItORt' RADIATING COLJUMNS. 325 (4d65) VWe have foundtt the dynamic radiation of the following four substances, when the radiating column was 2 feet 9 inches long, to be represente(d by the arnnexd deflections: Olefiant gas.......30. 6 Sulphuvic ether vapor...... 64 lFormic ether.... 69 Acetic ether.,t.... q olefiant gas giving herc the least dylnalmio radiation, (466) lxperincnt.ls made, in precisely tile sat-le manller, with a tube 3 inches long, or - of the former lengfth, gave the following deflections: Oletfant gas..... 390 Sulphrl'ic ether vapor 11 Formni ct1e'...,. 12, Acetic ther. 16 The vcrification of our reasoning is therefore complete. It is proved that in the long tube tlhe dynamic radiation of the vapor exceeds that of the gtas, while in a short one the dynamic radiation of the gas exceeds thatl of the vapor. The result 1)ovCes if proof welre needed, that., thloughll diffused in air, tile vapor molecules are really the ccntres of radiation. (467) Up to the present point, I have purposely omitted a reference to them most; imlportlant vapor of all, as faur as our world is colncrned. —.the vapor of water. This vapor, as you know, is always diffused throutgh the atmosp)here. Thrje clearest day is not exemplt fi'om it: indeed, in the Alps, the purest sl(ies are often the most treacherous, the blue deepening with the amount of aqueous vapor in the air, It is neeccdless, therefore, to remind you that, when aqueous vapor is spoken of, notJhing' visible is meant; it is not. fog; it is not cloud; it is not mist of any kindl. Th'lcse are formed of vapor which -has becen condensed to water; but the trule valor, with which twe have to deal, is ant impalpable tr'ansparent gas. It is difftsced everywhere throughout the atlosp}here, though in very differcnAt proportions. 326 HI.EAT AS A MODE OF MtOTION. (408) To prove the existence of aqueous vapor in the air b)y which Nwe tare now surrounded, a. coplteri vessel, filled all hour ago wvith a mixture of p)ounded ice and salt;, is placed in frolt of thle table. The surface of the vessel was then black, but it is nlow white —-fulirred all over with hIoar-frost- --— prodtece )y thle tcondlesation, and sutscqtucnt congelation t1)upon its surface, of thle aqueouls vapor. This white substance can be scral)ed off; as the frozen vapor is removed, the black surface of th.] vessel reappears; and now a sufficient quantity is collected to form a respectable snowb)all. Let uts go one step) furtherl.. place this snow in a mould, and squeeze it before you. into a cup of icc; and thus, without quitt ing this room, wve hlze experimentally illustrated the manufacture of glaciers, from beginning to end. On the plate of glass used to cover tfle vessel, the vapor is not congealed, blut it is condcnsed so copiously that, when the plate is Icld edgeways, tle wvater runs off it ill a, stream. (469) The qtuantity of this vapor is small. OxySgen and nitrogefn constitute about 994- per cent. of our atmos)phere; of the remaining 0'5, about 0 44- is aqueous wtl)or; * thi rest; is car)onice acid. Had we not bceen already acquainlted with the action of allmost infinitesimal quantitieis of lmatter on1 tadiant heat, we mighlt well desp"air of b)eing able to establish a Imncasurable action) oll the )al't of the aqueous vapor of our atmlosp)lere. Indeed, I quite neglected tile action of thlis substance for a time, and could hardly credit myS first result, whlich made the actioll of the aqueous vapor of our laboratory fifteen times that of tile ailr inl which it was difflsed,. Th;is, thowever, by no meanls expresses t}he true relation between'aqueous vapor and dry air,. (470)'To illustrate this point, our first arranllgement, shown The known tenuity of the aqueous vapor of the atmosphere. eaused Prof. Mtgntlrs, whenl h made hlis first expcrimttents oin the vapor of th-te air, to say, thlhat it was evident beforehand that such vapor could exert no sensible actioln." l[cad he approached tlhe stubject, ns we have done, thrllouglt the foircgoing experiments, so cautiots a philosoplert would not, I think, have mad.e this statmtienlt., A13SORI1UTION JBY AQUE OUS VAPOR. 327 itl Pate I., has been resume(l, a bra 1ss i )tue bcing cmployed, and t;wo SOulrces Of heat, acltinl on the opposite faces of the ile. The experiment with ldry air is repeated, 3by perfmitting it to enter the exp)rimental cy- lildcr.''he nleedle does not move senllsibly. f close to it, you would observe a motion through about1 one degree. Could we get our lair quite p)urc, its action would be even less tthan this. Let us again pumll) outt, and allow the air of t}is rooml to enter tile experlimental cylinder direct, withlout permitting it to pass thlrough the drying all)aratus. The neettle, you ob)serve, moves as the air enters, andc the deflection is 480. The ncedle will p)oint stcad — ily to thlls figure, as long as the sources of heat relainlll constant, and as long as the air coltinules inl tlhe tube. These 418 correspond to anl absorpltion of 72; that is to say, thl aqueous vtapor contained ill tlhe ittnosphter of this roorm to-day exerts anl action onl the radliant heat 72 times as powerful as that of the air itself. (47t) This result is obtiained wvith perfect case, still not without dule care, In comparing lli dry w'ith humid air, it is perfcctly essential thlat the substances be piire. You mltay work for months withl anl impl)rfcct drying ap)l)aratus, and fatil to obtain air which shows this almost total at)bsence of action on radiantl heat, Anl amount of organic itlluri!ty too simall to be seen by tile eye is suftlicient to augmlent fiftyfold tl;e actionl of fthe air. Knowing thie effect wlhich an almost iflinitesimal amounltt of mattert in ce'rtain cases, cat produce,. yo are better prteparcd for such facts than I was, when they first forced themselves oil my attention.'Thle Cexperimental result wh1ich weo have just obtained will, if true, liave so important anll influencce on the science of meteorology, that, before it is admitted, it ought to be subjccted to the closest scrutiny.'l'irst of all, tthel look at this piece of rock-salt, brought in. from t;he next room, \1)whlre it hlas stood for s)Iome time nCe(1ar a1t tank, tbu not in contact with visible moisture. T'Ihe salt, is wet; it is anl hygroscopic substance, and frccly condenses moisttre upon its stlritUcc.. lre, also, is,,' a p)olishcd Illate of thte 328 1I[F0ATl AS A,MODB[ OF MOTION. snbstacc, which is now qulite (try': I breathe upon it, and illstantly its aflilnity for' moisture causes the vapor of my breath to oversprea(d thlo surface, in at film which exhibitis beautifutlly thle colors of thlin plates.* Nowr we khnow, from a fortmer ftiable (plage 282), how opaque a solution of rock-salt is to thle calorific rays, an l hencC ariscs the qucstion whethr,; in thle above cxperimentl, with undried air, we may not., in reality, be imeasuring the action of a thin stiratum of sucht a solution, depositced on our plates of salt, instead of the pure action of the aqueous vapor of t;lhe air. (4Ut:T) If we operate incautiously, anll, more particularly, if it be our acltul intelntion to ret thle plates of salt,, w\e lmay readily obtain the ticposition of moisture.''This is a point on which anly COmpl)etent experimentlr will soonl instrutc himself; bult the cssence of good Cex)rimentilg consistAs in the xcliusion of circumst.ances which would render the )urle anll simltle questions, twhich wc intend to put to Nature, imlpure and conmposito ones. The first; way of rcplying to the doubtl; here raised is, to examine our plates of salt.; if the experiments have bccn properly conductcd, lno trhace of moisture is found upon tihle surfatcc.'P rcnder t;elt success of this experimlent more certain, wre w\ill slightly alter the arrangement, of our apl)ara'tls It ithel'rto we have had tlhe thermol-elcctric pil and its two rcflectors entircly 0outsid3 the exprimell tal cylindcr. Taking t)his righlt-hand rcflector ftoni tle pile, and removilng thlis plate of rock-s-alt,'l push the reflector into tcle cxperimental cylinder. Thle lollow rcflcctilng' conc is spltlg " at; its base a b (fig. 90), (this is our former arrangnelmlellm a ltce TI., witl thle single excgpt.ion t hat one of the reflectors of the p)ile I is now wit;hin the tubte) so that it is hlcld tighltly by its own plressure agatilnst the inner surface of thle cylinder. The * Receivinll the beam frilo tho electric hlap upon tho polished plate of salt, so isl to rClecCt: the lighft on to) a screen1, and placing a les in front of tIt salt, so as to produce aM ilmagt of its polished surface on the screent, oin lbrenthlinfg aaif'lnnst thle saltf throulh a glass tube, rilngs of vivid iridtescncee ilnsat -Itly 1l:ash irtLh, whicli may t e sctn by huadrcds at once., The orlder of tlhe colos iis that of Newton's rilngs. ACTION OF? AlTMOSPITERIO VAPOR. 329 space bletw\en the outer surface of tlhe reflectorx and thle inner surtalce of ithe tubc is filled with fragnllents of fuscd chloride of calcium, which Care prevInted from falling out by a little Fio. 90, 1i L' —'I< I [I..................... screen of wire gauze, and t.eln the plate of salt is reattachted. Ag'afinst thle inner surface of tlhe salt the narro~w cend of thle reflector now abluts. ]Brilu)ing the face of the pile close up to thle plate, tfhough not into actual contact witht it, our arrangc - mcnt is complete. (473) In the llrst place, it is to be remlarked, that the plates of salt nearest to the source of \eCat is never moistcnced, unless the experimcnts are of the roughest chilaracter. Its proximity to the source cnables the iheat to chase away every trlace of humidity from its surface. The distant plate is' the one in (langer', and now we have the circumferential portions of this plate kept perfectly dry by the chloridte of calcium. No moist air canl t atall reach the rim of thle plate; while upon its central port, ion, measuring about a square inch ill aMrca, we have cono0veraed our' entire radiatioun. OnT a priori grounds, we shotuld conclude thKat it is quite iimpossib)le forl a film of moisurc to collect therc; and this conclusion is justified by fact. Testing, as Iefore, the dried and tle tundried tair of this room, we find, as in tlhe former instance, that the lattcr lproduces seventy times tfle effect of the former. Tlhe needleo is now deflected, by the absorption of the undricd.air; allowing thlis 330 lIWAT AS A MODE OF MOTION, air to remain in the tubie, we will unsere-w the plate of salt, t1and examile its surface. AVc tmay eveln use a lens for thlis p3urpose, t-aking care, 1Ahowever, that the breath tdoes not strike the plate. It was care1fcully!polished when attached to the tube; it is perfectly polished now. Glass, or rock-crystal, could not show a sutlrtce more exempt from any appearance of moisture. When a dry handkerchief place d over my linger is drawn ilongl the surfatce, no trace is left behind,'llThere is not the slightest deposition of moisture, still, re see that absorption IhalS tfiaken place. This experiment is conclusive against fte hyp)othlcsis that tho offects observed are due to a filn- of brine, instead of to aqtemous vaplor. (471-) The dloubt ntmay, however, linger, that, althoughl we are unable to detect the film of moisture, it may still be the're. This doubt is answered in tlhe following way: I detach the experimental tube from the front chamber, and remove t.he two plahtes of rock-salt; the tube is now opew. at 1boUt ends, and ilmy aim will be to introduce dry ald moist air into this open tutl, and to compare their effects uploln the radiation, And here, as in all other cases, the praetical tact of the experimentcr must come into play. Tlhe source on the one hand, and tfle pile oni thle other, 1arl now fircely exposed to the- air; a very slalgtht; t:itation, acting upon either, would disturb, anid might, indeed, altogether maslk, the effect we seek. Th1te air, then, must be fie. 9t1. l)e introduced ilnto tle open ttl)c, -wtitlhout producing any commotion, cither near tile sourcet or near the pile. The length of t}C experimenltal tuibe is now 4 feet 3 inclhes; at c (fig. 91) is a coclk iontnc:te, with all TIndia-rubber bag containing AB1SOlPTION IN AN OPEJN TUBlE. 331 common air, and subjected by a weighlt to gentle )1'ressure; at i.) is a second cock, connected by a flesxille tutlbe t, with atl air-pump. B3etween the cock e and the tIndia-rubber bag, our drying tubes are introduced; and, when thlat cock is opened, the air is forced gently lthrough the drying tubes into the experimnental cylinder. I'The tair-pumplt is slowly wvolrked at t-e same timc, the dry air being' t;lhereby drawn toward l). The distance of o friom the source s is 18 inches, and thle disttance of J) from the pile P is 12 inclhes: the compelnsating cube c, awd the screen it, serve the same purposes as before. B]y thus isolating' thte central portion of the tube, we can displace dry air by moist, or moist air by dry, without permitting anly agitation to rcach cither tlte source or t;he pile. (475) At preselnt the tube is filled with the common air of thle l)aboratory, and the needle of the galvanometer points stca(dily to zero. I allow air to pass through the dryingl apparatus, and to enter the open tube at c, thle pump being worked as already described. Markl the cffect. When the dry air' enters, the neeCdll commences moving, and the direction of its motion shows tlhat more heat is now passing thalt before. The substitution of dry air for the air of the laboratory has rcn(leered t he tuble more transl)arentl to tfhe rays of heat. The final deflcction thrs obtained is 415~. H.ere the needle steadily remailns, and beyond lthis poinlt it cannot be loved by any firther drawing' ill of dry air. (476) Let us now slhut off thte supply of dry air, and cease wotrkinl tlhe pump; the needle sinks, but with great slowness, indicating a correspondingly slow difision of the aqueous vapor of the acljaccnt air inito the dry air of the tube. If the. pum1)p be worked, the remloval of the dry air is hastc ned, and the needle silnks more speedily —- it now p)oilnts to zero. The exlleriment; may be) made a hundred times in succession without any deviation from this result; ol the entrance of the dry air, tic ncedtle invariably gtoes up) to,450, showing' augmelnted transplarency3 on the entrance of the undried air, the nccdle silnks to 0~, show\ing laugmented absorption. 332 IhEAT AS A O101)E OF MOTION. (477) But the atmlos1plhere to-day is not saturatetd with moistlcre; hlnce, if saturatedc we might expect; to find a greater action. I remove the drying apt)aratus, and liput in its 1place at'U-tubo, filled with fragments of glass moistened'by distilled wNater.'Throutgh thlis tube air is forced f0roml t;he illdixa-rubber b)ag1 the p1ump bing w)orkcd as before. We are now d(isplacing tlhe humid air of the lat)oratory by still mnor humid air, and see the conseIquenlce. The needle moves inl a direction which indicates augmented opacity, the final dieflection being 15. (478) I tre, then, Nwe haxrve substwanitially the same result as that; obtained when our tube was stopped wsith plates of roek-salt; the action, therefore, cannot be referred to a filml of moisture deposited upon the surfatce of the plates. And be it remarked that there is not the slig'htest caprice or uncerttainty in theso experimentts when properly conducted. They hlave been cescutdc at difllrent times and seasons; thec tube has been d ismounted and remounted; the sugglestiolns of eminent men w\ho hlave seen the Cxperiments, 1and whose object it was to test; the results, have f)eel complied with; but no deviation firom the eftfects just recorded has been observed. The entrance of each kind of air is invariably accomplanied by its characteristic action; the nccele is under the most compl)leto control: inl short., no experiments hitherto mtade with solid and liquid bodies are more certain in their execution tihan1 the foregoing exp)erincents on dry and humid air (479) We canl easily estimate the preeccntage of the entire radiation absorbed by the common air, betwceen the points o aenld I). Intiroducing this tin screen bctween the cxp)erimcntal cylinder and the pile, one of the s ources of heat is cntirely shut off. The deflection pIrodIucc d b)y the other source indicates the total radation.. h'Ilis deflection corresponds to about 780 of the units Nwhichl have been hitherto tadopted; onlem unit being tte quantit.y of 7heat ncccssary to rmove the needle fiom 00 to 1.'. Th'l deflection of 45~ corrcspondlds to 6G units; out of 780, therefore, 6G, in thlis illnsctlnc, hlave beenl AOTION OF AIRt FlROM VARIOUS JLOCAXTITS. 333 absorIbed by thle moist air. The following statement gives us thle ab-sorption per' hundred: T780 1.00;( 62:'19 An absorption of nearly 8 per cent. was, therefore, effected by the atmospheric vapor wtelic occupied the tube betwcen C and D). Air i')e:ictity saturated gives a still greater ablsotrption. (480) This absorption toolk l)lace, notwithstanding the 1)artial sft/iZg of the heat, in its passage fr'om the source to c, and from I) to thle pile. Thle moist air, moreover, was, probably, only ill 1)trt (lisplaced by the dry. In other experillmenlts with a tube 4 feet long, and polished within, it wfas found t.hat the atmoslpheric vapor, on a day of average dryness, absorbed over 10 perc cent. of the radiation from our source. liegardilln tl earth'tl s a source of heat, no doubt at least 1.0 pe6? cent. oq its heat is intercep)ted owithin ten ffeet ofJ the siyface.*'T'Ihis single fact su;ggests the lenormouls influence Vwhic'h t his neIwlydeveloped plrope'rty of aqueous vapor must have in the l)lenomena of meteorology. (480cta) ]h1t wve thave not yet disl)osed of all objections. I1; has lbeen intimated to me, that the air of our laboratory might be imtpure; the suspended carbon-particles of tthe L:ondon air tlave also been referred to, as a possible cause of the absorption ascribed to aqueous vapor. The, results, however, were ol)taincd, twhen th;e al)iaratus was removed friom the laboratory-they are obtainable in this room. Air, moreover, was blrouglt friom the following localities in impervious bags::Iydte Park, Primrose Hill, Hamtll)stead Ieat h, Epsom l )owns (nlearl thle Grand Stand); a field near Newp)ort, Tsle of ~Wight:; St. Catharine's D)own, Isle of Wight; the sea-beach near Black Gang Chine. ThlYe aqueous vapor' of the air fronm all these localities, examined in the utsual way, exerted an. absorption seventy times tlhat of the air i*n whaich the vapor was (dflfilset. * Under somo ciretunstanccs, the absorption, I havo reason to boliovo, considerably exceeds tthis amount. 334 HIEAT AS A,lMODEl 0F MOTION. (4S:3l) Agailn, experimented thtus: The air of tlte laboratory vwas dried atnd purified, until its tabsorption fell below unity; this purified air was then led through at U-tube, filled with fragnitcits of perfectly clean glass moistened with distillcd Nwater. Its neutrality, when dry, showCed tlhat all prejudicial substances have beeCn rl1 covdl from it, and, in passing througlh the U-tube, it could taklte utp nothitg but the plure vapor of vwatecr. The vapor thus calrried into the experimental tubc p)roduced an action ninety times greater than that of the ailr which carried it. (482) ]hBut fairl' and philosophic criticism dcoes not end even here. Th'e tube with which these exlperiments were made is polished Nwithin, and it might be surmisced tfnt the vapor of the humlid air had, on cntering, deposited itself upton tihe ilnterior surlace of the tube,) thus dimini8s}hing its reflcetive )towe'r, and producing an ceffect apparently tlhe same as absorptio n. 1To this it may, in thle first place, be repliedl that the amount, of etat interceplted is p)roportiollntl to the qualtlity of air present. This is shown by the following table, which gives the absorption, by Ilumid air, at pressures varying from 5 to 30 inchles of morculry: HiJmid Air. Absorption Proessure............. in ches Observed, Calculated. 5.......16 16 10...... 32 32 1.... 49 48 20 64 6 4 2..82 80 30.. 98 96 (483) The third column on this table is catlculated on the assumpltionl t{hat the absorp itio tinal to tle quantit of vapor in tlhe tutbe, and tile greement, of the calculated( and observed results sht;tws this to be the case, within the limits of the explerimentl. t cannot; e supl)osed that effects so reg'ullr as t.lese, and ag'reinll so completely with those obtatiled ABSORPTION IN FIf1tlI,RN. 33AI wtitlt small quantities of other vapors, and even Awith small quantitics of the perlimanellt gases, can be due to the colnden. satiion of tihe vapor onl the interior surface. When, moreover, five inches of air were inr the tubc, less than one-sixth of t he vaptor necessary to saturate the space was presenlt. Th'lre dricst (lay would malke no approach to thiis dryness. That condensation, especially condensation which should destroy, )by its action upon the inner reflector, quantifies of heat so accurately pr(op)ortional to the quantities of matter present, shlould here occur, is scarcely to be thought of, (484) AMy desire, however, was to take this importanti question quite out of the domain of mere reasoning, however strong this might be. It, was, therefore, resolved to abandon, not only the plates of rock-salt., tbut also the experimental ttbe itself, tand to dlisplace one portioit of the free atmosph)ere by another. W+ith this view, the following arratlgement was made: c (ftig. 92), a cube of boiling wvater, is our source of Flo. O2............... l heat. Y is a hollow brass cylinder set upright, 3'5 inches wide, and 7t5 inches high'. P is fie tl e trmo-Clectt'ric pile, and o'a compn)satillng cube, between which and v is an adjusting screen, to regulate the amnouant of heat falling on the posterior 336 1IEUIAT AS A MOD)E, OF MOTION. surface of the pile. The whole arranglement was surrounded 3by a hoarding, the space with;il which w\as divided into comnl)arl'tmnts by sheets of tin, and these spaces were stuffed loosely \with paper or lhorse-hair. Tlhese precautiollns, whiclh required time to be learned, were lcccssary to le)rvent; the formation of local air-currents, and also to intercelpt the irregular act.ion of the external air. Tlhe effet to be measured there is very small, alnd hence the necessity of removin1g all causes of disturllance -which colld plossibly interfere with its clearness an:d plurity. (4t85) A rose-burnerl, ir, was placed at the bottom of the cylinlder Y, and from it a tube passedl to all India-rublber bag containing air. qThe cylinder Y as first filled witIt friagments of rock-crystal, moistened with distilled water. On subjecting tfhe India-rubber bag to 1)ressure, tihe air was gcFntly forced up amongr the friagments of q(uartlz, andl, having there charged itself with vapor, it was discharge( in the space between the cuble c andl the pile. Previous to this the needle stood at zerol; but, onl the emergence of tile saturated air from the cylinder, tthe nceedle moved and took lup a final deflection of 5 degrces. The direction of the deflection sho-wed theat thoe opacity of thle space, between the source c and tihe pile, was auingented by the saturated air. (486) 1lThe quartz fragments were now removed, and thell cylinder tilled ith fi'rCagments of fresh chloride of calcilum, through which the air was gently forced, exactly as in thle last experiment. Now, however, in l)assitffg through the chloride of calcilum, it was, in great part., rol)bed of its aqueous vapor; and the air, thus dried, displaced' the common air between the source and the pile. The needle imioved, (declaring a per'manent deflection of 10O; the direction of the deflection showed that tile transplarency of th;e space was augmented by the presence of the dry air. B1y properly timing the discharlges of tihe air, tle swing of the nceedle could be augmiented to 1.5~ or 200. Repetition showved no deviation from this result; tlhe saturated air always algmented tlhe opacity, thle dry air al TROPICAL RAINS. 33'/ ways augmented the transparenc~y, of the space between the source and the pile. Not only, therefore, lhave the plates of rock-salt been abandoned, but also thie experimental tlbe itself; and the resuilts are all perfiectly concurrent, as reg1ards the action of aqueous vapor ulOn rladliant heat. (486a) Many remarkable corroborations of tlhese views have, b)een published iby tlat excellent lncetorologist, Colonel Ricihatrd Striacheli, of tihe I.oyal ]:'nginceers. And his testimony is alil the more valtable, as it is b)ased on observations made long bofore the property of aqulleos vapor ihere developed twas known to have anl existence. From his implortant p)a)er, pll)lished in thle Plhilosophical Magazine for Jtily, 1866, I extract a single representative series of observations, made between thile 4th and the 25th of March, -1850; during }whXich l)eriod ~;lte sky remained remarkab)ly clear, while great variations in the quantity of vapor took place." The first: colunn of figures gives the tension of aqueous vapor, and the second the fall of t;ile thertmomleter from 6.AO Pt. r. to 5.40 A, n, T'lnsion of valpor. Fall of th\rmomtenoter. 0)88 inch... C0~ 081-9 ".....'1 0805".... 8.3 0'~49s...... 8'5 0"05 "...... 20 0135 "...... 15 1Th0-e general1 result is here unmistakable. In clear nights tlle fall of the thermometerl, whichl is aln expression of the enorgy of the radiation, is dettermined lby the amolunt of traanspnarent; aqueous vapor in the air.'The precssure of the vapor checks the loss by radliation, while its relmoval favors radiation and P)romlotes the nocturnal chill. (4t87) WVe shall subsequently add anoth.lcr potertful proof to those here given. Were the sulbject less important, I should not have dwelt up)O it so lonlg. It was thought right 15 338.111AT AS A ot00Dl, OF MOTION. to remove as fiar as possible every objection, so fliat meteorologists might ap1ply, without; the failntest misg'ivingl, the lesults of expe rilent.' Tli alipplications of these results to their scince ll lust bcl innumcrrable; and her I cannot but regret thlat the inaconmpleteness of mly kno'iolls... The sensibility of the instxt'umlnt is very striking, for the liquor in-: 342 tlIEAT AS A MODEt OF MOTION. cessantly falls and rises il the steml with every passing cloud.!Butt tle cause of its variations does not always app1ear so obvious. Under a fine blue sky thte t/hrioscope will sometimes indicate a cold of 50 mlillesimal degrees; rct, onl othle days, when, the air seems equally brightt, the effect is hardly 30o." This anomaly is simpl)l due to the diffternce inl the quantity of aqueous vapor present in the fatmlostphere.'Indicde Leslie himself colnnects thle efct witlh aqueous vaipor in these words: " The pressure of hy-grotetric moisture in tle air probablly affects tihe instrument.". It is not., however, the " pressure" * t[hat is effective; tihe presence of invisible vapor intereelpted the radiation from the wcthrioseopec, while its absence opened a door for the escape of this radiation into space. As regards experiments on terrestrial radiation, a new3 definition will have to be given for "at clear day;" it is tmatlifest, for example, tllhat in experiments with tlhe pyrhlelionleter,t two days of equal visual clearness may give totally dliflerent r'esult:s. Ware ase ao enabled to accont for thle fact that the radiation froin this instrulment is often intercepted,'when no cloud is seen. Could vwe, however, ltllake the constituents of thile atnlosphlcre, its vapor included, objects of vision, wve should see sufficient to accountl for this result. (4:95) Another interesting point, on whiclh this subjcct Itas a hbeariltf, is the theoo ry of serein, 1" Most authors," writes Me}lloni, "a attribute to the cold result,,ing froml the radiation of the:air, t1he excessively finle 1nain which solmetitmes falls in a clear sky, drtling tile fine setasoln, a felw lomenlts after sunset. 3ut," lie contlinues, "a s no fact is )yeti; k1nown which dircctly proves tlhe cmissive powCer of pure and transp)arcnt elastic tluids, it atIpetars to me )lore confor lable") eltc., etc. If the dtifficulty here urged against the theoLr of sere'in be its only one, the theory will stand, for t ransplarent elastic fluids - Possibly the word t prossure'" is a Iisprilt br " presence."'The illtrutnent is described ill OhnC)tCter XIII. Tlis.statentent indicates tthe statet of the science of thmeriotlcs in rtefer ctnc to the gaseots feorm of matter when these researches wre egun.lll, COLOR OF SKY. 343 are now lprovcd to possess tile hl)poer of radiation which tihe thelory assumes. It is not., however, to radiation from the tair that the chilling- can be, ascribed, but to radiation from thle body itself, whose condentsation p)rodtuces the screi?. (4:96) Let mte add the rcmark that,, as fr as < I canll at present judge, aqueous vapor and liquid water absorb tlle salime class of rays; this is another w\ay of stat ing that the color of!rarc water is shared by its vapor. In1 virtue of aqueolls vraporl the atmoslphere is therfore a b1luce edium. It ha ss been remarked that t he color of the firmamental blue, and of distant hills, eiepens Nwitll the anotuntl of aqueous vapor in the air; but tlhe substance which produces a variation of dep)t must b1e otfeetive as anl origin of color. Whether the tazure of the sfky-tlhe inost difficult question of meteorology. -is to bc thus accounted for, I will not at present stop to inquire.t*.Noer:.-.- -In a palcr recelntly published by him, ProfeSor Mtagrlus controverts the views maintained in the foregoing chlate1r regarding the action of aqueoulls vapor. I shall givet thte xperiments aid reasonings of mfly clminent friend (by which I am not convinced) due consilderation as soon as E canw command the time, NoT':, 1870......... The fifteentcc h ap)tr of this volume is devoted to tile color and polarization of thle light of the sky. * In connection with the investigation of tle radiationl andl absorption of leat by gases and vaplors, it gives mife pleasure to refer to the promlt and intelligent aid rendercd m:le by Mr. Beekcr, of the firml of Elliott's}, West St rand. Mtr. B3ecker is well acquainted with the applaratus lnecessary fbr tlhosei xpcrifile fear of beincg led too finr friom my snlbjeet catuses m t to ithhold all stceeulatioln as to th causoe of atmospherie polarization. I ay, howevert; remark, tiat thle lolar izationl of lcat was illustrated ly means of the mica-piles witih which Professor (nows Principal) J. 1)i. Forbes, irst succeeded in establishlil tle fact of polarization. f 4 4 IllEAT AS A MOD1E OF MOTION. APPE'. NIX)X t'O 01A [PTI'R XI,. VXA'TltATSC FEROM A D)ISCOUIRSE 0N RAD IATIO N THRiIIOUG(}! THII] ElART'II'S ATMOS'} [1* I.'11." " N on veY Over o1)ta;eind the idea of a line fooml lEuclid's definition of it —.'length without breadfth.' The idea is obtainedl fiom a real phylsical line, drawn by a penl or penceil, and therefore possessing widlthl; tle idea being afterward brought, 1)y a I'proess of abst ra'C tiol, m1olre 1nearly into accordance wit.h the conditions of the definitioln. So also, with regalrd to physical phlnomna; we must help ourshelves to a conception of tlhe ilvisible, by 1meanls of proper images derived filom the visible, afterward purifying our conceptions to tlhe needful extelnt. Definiteness of conception), cven though at some extpenso to delicacy, is of the glreatest utility in dealing w-\ith physical plhenomena. Indteed, it may leo questioned whethler a mind trained in physical researchl cal ant all eniljoy peace, without, haNving made cleatr to itself some possible way of conceiving those operations which lie beyond tlhe boundaries of Tsense, and ill wtich sen3iblC phienomellna originate. " W\ln itwe speak of radiation through the atmospl3here, we ought to be able to aflix definiteo physical ideas, bothl to the term atmtospliere and the tertn radiation. It; is well known that our atnmosphereo is mainly composed of the tNwo 0elements, oxygen and niitrogren. These elementary atoms may 1)0 figured as small lsp)heres, scattered thickly ill the space which imntcl diately sulrrotllunds tlhe earth. Th'lty constitute about.t 992 per cent. of tihe atmosphere. Alixed w it tlese atotms, we have others of a totally dififerent character; owe ha:ve t~he molecules, or atomic gitoul)s, of carbonic. acid, of ammtlonia, and of aqueous vapor. In tlhese substances diverse ato1ms hatye coalesccd, formling little systems of atolms. The im-olcecle of aqueous valpor, for example, consists of two atoms of hydroglen, united to o1ne of oxygeln; and they mInile, as littlo triads, among tho monads of' oxygfen anld hydrogen which constituteo the great mllass of the ntlmoslherle. D3ISCOURlSEf ON RADIATION. 346 "Theste atomX s andtl molecules Iare sel)arate, b)ut they are emlbraced by a common meditium. Within our altmosphere exists a seconld allnd a finer at m ttlosphere, ill whichl tihe atolms of oxygen and nitrogen haIng like suspended grains,''his fine' atmoslpher' unllites not nly atom with atomt, lut star witl star; and the lighlt of tall suns, and of all star;s, is in reality a kind of mullsic, propagtated through this interstellar air. This imago imust bo clearly seized, and then we have to advance a step. We must 1not only figulre our att0om8 uspended ill this mlldium, but vibratitng ilt it. In this 11motioll of the atoms colnsists lwhat we call thleir heat. i' What is elat in ts,' as Liock has pelfcectly expressed it,' is in tbe body h, ct It notlbinJg blt Ilotion.t Well; we m1ust figure thifs motion communicated to tile mttedilum in -vwhich thte atomls swing, and seClti i ripiplCes through it, with inconceivable velocity, to tle ounllds of space. Motion in this fil'or, unconnected with ordinary matter, but speeding through tlte interstellar i-ediun,- receives the niame of Radiant leant; andl, if completent to oxcito thle lnerves of vision, we call it Light.t "A1tiqueous vapor was defined to be an invisible gas. apolr was permitted to issue horizontally with considerable force) fiom a tube conected with a sit a ll boiler. The track of the cloud of condensed steam was vividly illuminated tby the electric light. WhaV\F}t was Seel, however\, was not rvapol, but vaplor condensed to watet. Icyolnd ttle visible end of tlhe jet, t1e cloud resolved itself into true vapor. A lamp was placcld unlder te jet, at varions u)soints; the cloud was tut shlarply off at tha}t point, and, when the fle tamle was p)laced ilnear thto cfiux oriic, the clounld entilrely disap)eared.' h ]cThe het of trhe al!mp) coaipletely prevented precil)itationl. 1The same vap)or'was ollndenlsed and congealed on tlhe surface of a Vessel contlaining a freezmlg mixture, from which it was scrapeld, in quall tities sufficient to form a small snowbll. The beam1 of thle electric lampn, more Cover, was, sent tlrough a large receiver placd on an air-pumpl). A siingle st:ioke of the )l)ump caused the precipitation of the aqueo.us vapolr withlin, whlich became bealutifuilly illumtinatedl by the b1eam; while, uponl a screeln behllind, a riclbly-colorcd 1halo, (dte to diffilactionl by the little cloud witin thle receiver, flasied ftrth. "The wavest of heat s!peed lfroi our eartlh through the at-mosplhcee toward space. These waves dash in their passage against tl, atolis of oxygen and nitrotgen), can(l against, thie nimolvei'les of faqlueolus vlalor. Thlinly scattered as these latter arve, we night natull y t lhink meanly of theml1, as barrietrs to tlhe waves of heat. WVe might iimagine tialt 34(36 IBAT A8 A MODE310 OiF' MOTION. the wide spaces betveen f te vapor-molecules would be all opeln door for tlhe passage of the n1undulations; anld thlat, if thlose waves Nwere aft all intercepted, it would lie by thte substances whilch form 099 - per centt of the whlole atmosphere. Tlree or four yeat's ago, htowevcr, it -vwas found by the speaker that t-his small modicum of aqueous vapor interceepted fifteen tilmes the quantity of lhcat stoppped by the whole of tlle air illn whichl it was diffused. It vwas atlerward found that the dry nai thent expriStnented with was not l1)erfctlyt 1)ure; and that, thle purer the air became, the mlore it approached the character of a vacuum, and the greater, by comparison, becamte the action of thle aqueous vapor. Tho vapor mwa s foundl to act with 30, 40, 50, 60, 70 timcs the energy of the air iun whlich it vwais difflused; Sand no doubt was entertained that the aqueous vapor of thle air which filled the Royal Institution Theatre, during the delivery of thell discourse, absorbed 90 or 100 times tlhe qua\tlity of radianf t Iet Vwhichl was ab)sorbed by thre m-lain body of the airt of the room. Looking at the single atoms, for every 200 of oxygen andl nitrogen there is about of aqueous vapor. This 1 is 80 tiimls more powe~rful thlban the 200; and hence, comparing a single atom of oxygen or iitrogen withl n single atom of aqueous vapor, \\we may infer that the action of t1he latter is 10,000 times tlhai of the former. "No doubt can exist of thle extlraordinary opacity of this sill)stance to the rays of obscuroe heat; pnarticularly such rays as are oemitted by the eartlh, after being wvarmled by thle sun. Aqueous va)por is a blanlket, more necessary to the vegetable life of E]ngland than clotlling is to man. Rlemove for a singlo summer-night t1he aqueous vapor firom the air which overspreads this country, and youl woutld asstredly destroy every plant cal)able of being destlroyed )y a freezing temperatulre. Thle warmth of our fields and gardens wtould potlur itself unrequited into space, and thle slun would rise ulol an island hlld fast in the iron grip of frost. Tlle aqueous vapor cotnstitutes a local dam, by which tlhe tempenrature at te earth's sutrface is dcepened: the dam, however, finally overflow\s, and we give to space all that, wte receive from the sun. "tTho suun raises thte vapors of the equatorial ocean; tlhey iise, but for a time a vapor-scr'cen spreads above land arolund theml. H3lt, the tigher they riso, the 1t101ro they come into the prese1nce of pI)reC space; and, when,,5by their levity, they have penetrated tlle valorscreell, which lies close to itle eartf's sturfacc, what Imust; occur'? "' It hais beent sail thitr, compared atom ifor attom, tlhe absorption D)ISCOURSt ON RADIATION. 347 of anll atomi of aqueous -vapor is 16,000 timhes that of air. Now, the power to absorb and tho power to radiato are perictly reciprocal and p'roportionlal. The atom of aqueous vapor will thete-lore radinto xwith,f(100 tinmies thlo entergy of an atom of air. tIlmine, then, this powerltl radiant in the presence of stace, adl with no screen aiovo it to check its radiation. Into space it pours its heat, chills itself, condenses, and the tropical torrents are thle clonsequence. The oxpansiont of the air, no doubt, also ecfiligceates it; lbut, in accoultilng for deluges, the chilling of the vapor by its own radiation must play a most impoirtant partt. p The rain quits the ocean as vapor; returns to it as wa ter. I ow are the vast stores of heatt, set free by theo chante from the vaporous to tlhe liquid condition, disposed of? Doubtless, i great plart, they are wastled by radiation into spaceo. Similar rtmarks apply to the cumuli of our latitudes. The warmced air, charged with vapor, rises inl columns so as to p)enetrate the xapor-scee-n w lwich hugs the earth; in the presence of s'pace, thle head of each pillar wastes its iheat by radiation, condenses to a tcunllls, \which constitutes thl visible ceaital of an1 ilvisible cohlumnw of saturated fail. Numberless other meteorological l1henoenll a receive their solution by reference to thte radiant altd absorbent propertics of aqueous vapor.1" The radiant lotwer of at vapor is l)roportional to its absorbent powver. ixpelrimeonts on the dynamiic radiation of dtried and u1n1dried air provo tlhe suporiority of the hatter as a radiator.'Thle followilng expelriment, perforuned by )Dr. F rankland in the theatre of the Rloyal JInstitution, showed the efect to a large audience: A chareoal-chauffor, 14 inches high and 6 inclhes in diameter, was pllaced in front of a thermo-eleetrie p)ile, and at a distance from it of two feet.. The radiation from the chaullbr itself was intercel)ted bly a metatllic screetn.'hll detlcctioln due to the radiation fi'om thte ascendingl co-tllumn of hot carbonic acid wavs then carefully neutralized by a constant solurco of lieat, radiating against tihe opl)ositeo face of tlhe pile. A current, of steam wtas then forced vertically througlt t:he chaulfer. The deflcetion of the galvanometer wavs prolmpt and powerfulf. When the current of stteam was intorrupted, the needle returned to zero. \tlWhen, instead of a current of steam, a current of air was forced throlughl the chauttllr, the slight etiect iproduced showdl thle pile to b1) chilled instead of warmi. ed. In. this experiment Dr. Frankland corlll)arel aqueous vapor, not w\ith air, but, with the more l owerl ti clar 348 lIEAT AS A MIODE OF1 MOTION, bonie atid, and demonstrated the superiority of tle vta)por as a radiator.* The following remarkablle passage from- looker's "lh-imalayan Journalds," 1st edit. vol. ii. 1). 407, also lbeals 11)ol tphe present slubject: "'From a multitude of desultory observations I conclude tlhat, at 7,400 feet:, 125'70~, or 67~ above thlt temperature of the air, is the acrage effcltet of the sun's rays on a black-bulb thermometer.... Thiese results, thouglh greatly above those obtained at Calcuttat, are not mueb)l if at all, above what mlay be observed on tle l)lainls of India. Th1e effect is muclt increased by eleNvation. At 10,000 feet, ill )December, at, 9 A.., I staw' tle 1 mercury miount to 1320, while tllh temperlature of shaded snow Ihard b)y was 22,. At 13,100 feet, in Janu1ary, at 9 sA. 3i, it has stood 983, with a dliTerencee of 68.2", and at 10,. r. w at 114~, with a ditllrence of 81.4~, while the radtiating thermometer on the snow hatt,tl4/en at sunEris to 0.N." Theseo enormous differences betwelen the shadfled and the unshlade(t air, andl between tie air and the snoxw, are, no doubt, duo to the comparative absence of aqueous vapor at theso elevations. T le air is incompetent to check either thbe solar or the terrestrial radiation, anld hence the maxinmum heat in the sun and the maximum 1cold in the shade must stand very wiide ap)art. T'l}o difflerence betweeln Calcutta and tthe laitls of India is accounted for in the same- way. 3Dr. Livingstone, i his "'1Travels int Soutlh Af'tica," as givel3n some striking mexamples of tile difference ill nocturnal ch:,illing whenl1 the air is dry and when laden with moisture. Thus lhe finds, in Southt Contral Aftric, during tlhe month of June, " the ftheemtomlter ear ly in tile mornings at from 42~0 to 520; at 0noon, 94 to 9 6, or a mean difflerenco of 48~ between sulnrise alnfd mid-day. The range would probably havo been found still greater lad not tlhe thertmometer bleen placed ill thle shadee of hlis tent, whicI was pitchlled 1underl tile thickest, tree 1he could find. Itff adlds, moreover,\l') T!the sensation of cold1 aIfter tle ]teat of thle (day was very keen. IThbe lBalonda at this seasoln never leave their fires till nine or ten in t he imorning. As the cold was so great Ilere, it was probably firosty at Lillyanti; I thereft'ore feared to expose ily young trees there.'" 1)r. Livingstono afterwardt crosses tl e continent and roeaies the river' Zamlbesi at tthe beginning of thel year. Itere the therlnonletrti rangeo is reduced fr'om 4108 to 120. Ite thllsl describes the change lie felt on entering the valley of t-le river: " We werle struck by tleo * P1111. Mag. vol. xxvii. p. 320. t fivingstonles Travels, P. 481. DISCOUUtSN ON RIAD)IATION. 3:409 fact tflat, as soon0 as Xwo eamno between the range of hills which flank tfle Zamlbesi, the rains felt warmll. At sunrlise tlhc thleClrmometeCr stool at foi'n 820 to 86~; at mid-day, in the coolest shade, name ly, in my little tent, under a shaldy tree, at 960 to 980; and at sunset at 86~. This is dilterenlt fronilom any thing we experieted i t i the literior." Proceedilg toward the mouth of the river, onl Jamtary 1G6th, t makes the follo"wing additional o)servation: "Th0e Zambesi is very b)road here (at Zum% bo), but contains many inhabited islands. VWe sldept opposite one on tile 16th, caltced Shibanllga. The nights ar'e warm, the temlperature never falling below 80; it was 910 even at sullset. On1e cannot cool thc water b)y a wet towel round the vessel...." t Inl Ontral Australia tihe daily range of the thermonmeter is still greater. The following extract is froml a 1 aper by Mr. A. S. Jevolns, "' OIn somie Da)nta concerning the Climate of Australia and New Zeta1land: " c,.. In the interior of the continent of Australia the tluctuations of templeraturl are himensely ilncreasc d., JThe }heat of the air, as described lb)y Caltain Stut, is fi rfi dliring summer; thus, in about lat. 30~ 50' S., and lon. 1410 18' lE., lie vwrites:' The thtrmonloter every lday rose to 11.20 or 1160 ill tite shade, while ill the direct rays df the sunt fron 15400 to 150, Again, At a quarter-past three r a.. oni January 2st (1815) th ther t ometer had risen to 1310 inl the shtade, and to:15t4~ inl teo direct rays of the suit.l'... It the winter the thermotneter was observed as low Ias 20,~, giving an Cxtremet ranlfgelt of 107~. Tite luetmnations of temper ature were often very great mltd stdden, andl were severely felt. On one occasiion (October 29Sth), the temperature rose to II~ t0lduring thle day, but, a squall comingl oin, it fell to 380 at the fotlowing sturise; it tlis varied 7 2~ in less thalt t:wient'y-ftlr n toll'rs..s. Mitchell, oit his last journey to tlte N. MV. interior, had very cold frosty nights. On NMay ay2.2d, t he tltermnometer stood at 1120 it the otpen aitt. Still, il the daytite, t}he air was tvwair-Im, and tlhe daily rtnge of temperaturl o was enormous.'Thus, onl Junte 2d, the thermometter rose ftiom 1.1~ at sunrise to O67 at four P,. m,.; or tlrough a ranget of 560. Oln June 12tih, the range was 530, and on manlly other da(lys nearly as great." Even at Sydney the averflage daily range of the therinomtieter is 21i, while at G(Treenwvicte the average daily 1ran>ge is Ionly 1t'. iIt thus atpeiars that evetn close to the ocean te mean1 daily range of tIme' tivitistonteos Travels, p). 57..t Ibid. p. Sf'It. 350 HlE"iPT:AS A MODE 0Q1 MOTION. Australian climate is very considerable. It is least inl thle autumn and greatest during tlhe cloudless days of spring." After giving a table of the seasonal varlation of the rainfill il Australisa, lr..Jevols remark' s tflat "it is plainly slown thaflt thCe mOSt railly seasonl of thie year onl thle east coast is t hleo autumn, that is, the tlhlee months, March, April, Mtay. Thte spring season 1appears the driest, sulmmer and wilter being intel'mediate." Withonut quitting lturop)e, w\re mixd places where, while the day temperature is very high, the hour before sultrise is iltensely cold. I hatve oftell experieneed thlis ill thie post-wag:ons of Germany; aandl I am informn-ed tihat theo I hngarian peasanit>s, if exposed at night:, take care, even in hot weaIther, to pr-otect thlCmstlves bgy heavy cloaks against thlo nlocturnal chill. The observations of tMM[. lr'avais and AMartins on thle Granld lateau of MAont lane have eenl already rcforred to. AT. Martihns has recently added to our knowledge lby making observations on theo heating of the soil at great clevations, and finlds on the sittlnit of the Pie du l idi the heat of the soil exposd to tilhe sunl, abov that of the ailr, to be twice" as great as ill ttle valley at the base of th]e mountain. "The immense heating of the soil," writes A. Mfartins, i" conpared with t.hat of the air on high mountains, is the mtnro remarkable, silce, during the nightlts, the cooling by radiatiot is thereo much gtreater than ill t:he plain." The obsotrations of to Mefssrs. SCellagtcntweit fi'urnish, if mistake not, flmany itllustratfions of tho action of aqlueous vapor; anld I do not doubt thlat, the mlore tliis quest>ion is tested, thle lnor clearly will it all)pear that the tadiant land absorbent po'wers of t:his substance enable it to l)lay a most important part in tho p)hcnolmina of meteorology. tAD)IATION TIItROUGOt LIQUIDS. 35 C(IIAPT1''It X1I. ABISORP'tON OP IEAT }1Y ~OLA'fIR. ItQUt:..S —At\SOBI$lOR1ON OP IREAT BY''I}t Y~APOt115 Or tHOSB lIMQUIDtS AT A COMMON IRS:.SURIt' —— AUSOX 1t:IONN OF JlgAT BY'TIM1 8SAME-1 VAtORf WVII:N'111) QUA''NTl:iES Or VAPOR. AlR)R PtROPOR'tlONAT TO'11ie QUANTI'lfIES OF JlIQUll). — COMPARAIV'TYi V'IEW\ 0' TIlE s ACT;ON O1F DRIQtUIDS O itAND T'11, VAt'tOR UPON RADIANT 1t1APT-..II.'ltYSI OAL AtTS Or OPACITY AND'S'RANSPAt-iRNO~Y-. INEIJUENCIg OF'XI:MP;tRAfUTPB ON'ltIP'IRIANSMISSION Or IAIASANT I,;PAT —CIIANGtENSF olrtOSlt1xN 5'JIROUtG! (CHOAN(1E3S or' I'M,1EKRA'"URE-B.....i- IAIO- lN i' O I: W'' RO LANMES.INIUXENOE O S O0SCILLAT.CAIN PIRPO1 ON'ilet TRANSMIXSIOXNOF RAIANTN OF CRTAIN BEStSUI S OF )fIMEIONI AND NOBTAUCHI (497) rlitlfi Iatural pllilosophy of the future will certainly.-. for the most part consist in the invcstigation of the relations subsistilig between the ordinary matter of tile untivers and t-he wonderfiul ether in w1lich this matter is immersed,.tRegardIing the motions of the cther itsclf, the optical investigations of te hast half-century leave nothing to be dosired; tbut regarding the atoms and molccules, whence issue the fun1dulkations of light and heat, and their relations to the medium in which they move, and b1y which they are set in motion, th-sco invcstigations tcach us little. TUJo come closer to the origin of the ethereal waves —- to obtain, if possibl)e, sonic expcrincutal hold of the oscillating atoms tlhemslelves-.-. has bcen the main object of those researches oin the radiation and absorption of heat by gases and vapors, which, in brief outline, lhave been skctchlcd efolre yotu. (498) These inquliries lhave made known the onormous differences existing bCetween different gaseous molecules, tas regards their power of enmitti. and absorbing radiantlt heat:. A'When a gas is condensed to a liquid, the molecules approachl 352M 1,iEAT AEc A 0tOl)Ii Of0' IMOTION. and grapple wvith eacth other, by forces N'whlich are insensible as long as the gaseous state is maintained. ]jBut tihought thus conldensed and entltral lled, the all-pervading ether still surrounds the, molecules. If, then, the power of radiat. ion and absorption dlepend upon them individually, we wmaya exp;ct thlat the deportment toward radiant heat; of the free molecule -will maintain itself a fteor thiat molecule hcas relinquished its fieedom and tformed part of a liquid. If, on thle otller hand, the sttate of aggrcgatio be of paramount impeortance, we mtay expect to find, on the part of liquids, a deportment altogether different from that of their vapoirs. N'hllich of these views corresponds with tihe truth of Nature, we have now to inquire. (499) AMelloni examined the diathermancy of various liquids, but hle employed for this purpose the tflame of an oiLlamnp, covered by ta glass chimney. Xis liquids, moreover, were con-tainled in glass cells; hencl, the radiation was profoundly modified before it entered the liquid at, all, glass being imper\vious to a considerable part of the emission. Mllonitl moreover did not occupy himself -with tfhe questions of molecular phyrsics, which to us care of paramount. interest. TI tihe examination of the question now before us, it was 8my wA5ish to interfere as little as possible with t}he primiitive emission, andti an ap)l)aratus was therefore devised in which a layer otf liquid, of any tbhickness, could be enclosed between two polished l)lates of rock-s alt. (500) The appllaratuls consists of the following' parts: I A. (fig. 94) is a plate of brass, 3'4 inches long, 2'1 inchess wide, and 0'3 of an inch thick. Ilnto it, at; its CorCIers, are rigidly fixed four upright pillars, firnished at the top) with screws, for thle recept ion of the niu4ts q v s t. l) i.5:v is a second plate of brass, of the same size as thle former, and )ierced with holes at its four corners, so as to enable it to slip over the four col11nn1s of the plate A C. IBoth these plates are perforated 3by cireltlar apertures,?m i alnd op, 1'35 inch. inl diaetenr. c;It is at third p1aite of glass, of tle same area as 1):l i t, and, like it;, havixing its centre anid its corncets l )perforated. The Iplate (G: Xi ROOt$-SALT 01E,LLS. 353 is intended to separate t.ho two plates of rock-salt which are to form tlhe walls of tle cell, and its thickness determines th;at of tho liquid layer. The se)artat:ing plate a:u x was ground Fro. 90. ri:'!'?.[i'_!.' i' i;'':' r';i'i~;i~: 4~I0i, |!|i:is'lt o,|li iCi!. _,|,,,j1:~ tS| ~*1 -0;i j with tite utlnlost accurtacy, tand the sutr'faes of thie plates of salt were )o lishcdt witlh ext r(mll care, witih a view3 to lrenderingl the contact betweenc the salt and the bra3ss watcr-tih'tt, inl practicc, however, it wtras found inecessary to introduce wtasihers of thin letter-paper lbctwccnl the plates of salt and the scparating plfate, (501) In arranging' thle cell for experiment, tie nluts q? 8 t are insrewed, and at wtasher of itndia-rubber is first place( on O. Oit thids \washer is placed one( of the jalates of. rock-salt. Ott the plate of roclk-salt is laid the washer of leter-lpaper, and on tli;ts again thl septarating plate o( it x. A sccottd washor of paper is placed oln this plate, t.hen comes the second plate 354 1IEAT AS A MODE OF MOTION. of salt, on which another india-rubber washer is lai(d. The plate I) -. ir is finally sli>pped over the columtns, and the wvhole trrangement is tight:ly scerewed together by the nuts q i' s t. (502) Thus, when tle plates of rock-salt are ill position, a circular splac, as wide as the plate o ji x is thick, is enclosed between thlem, and thlo space can be filled with any liquid through thle orifice k. The luse of the nldia-rubber washers is to relieve the crushing pressure which would be alpplied to the platets of salt, if tlhery were in actual contact with the brass; anltd the utse of thte )aplcl washers is, as already exl)laineld, to rendor the cell liquid-tight. After each experiment, tlle apparatus is unscrewcd, thC pl)ates of salt are removed and thorough'ly cleansed; the cell is then remountedc and in two or three minutes all is ready for a new experiment.l (503) My next necessity was a pefcctly steady source of heat., of sufficientl intensity to penetrate thle mllost absorbent of the liquids to be subjected to examination. This was found in a sl)i'al of platintllum wire, rendered incandescent by an electric current. Thei frequent use of this source led to the construction of the lamp shown in fig. 95. A is a globe of glass thrce inches in diameter, fixcd upon a stanld, which can be raised and lowered, At the top of ti1e globe is aln opCllning, into whicll a. cork is fitted, and through tlhe cork pass two wires, the ends of which are united by the platinuml spiral s. The wires are carricd down to the binding-scrcws a b, -which are fixedl in the foot of the stand, so that, whenl the instrument is attached to the'battery, no strain is cvcr exerted oni the wires which ctarry the spiral. The ends of the thick wire to which the spiral is attached are also of stout i platinum, for when it was attalched to coppert wires unstcadinless was introduced throughl oxidation. The heat issues from the incandescent, spiral by the, opening da, which is an inch and a half in diameter. B3ehind the spiral, finlly, is a metallic reflectorl -', which augments the flux ofT heat; withoult sensibly llfanging its quality. In tihe openf air the redt-ot spiral is a capricious APPARATUS. 356 source of heat;, but surrounded by its glass globe its steadiness is admirable.* (504) The whole experimental arrangement will be inlmeFla. 95. diately understood fromn the sketchl given In fig. 96. A is the platinum lamlp just described heated by a current from a Grove's battery of five cells. Means were devised to render this lamp perfectly constant throughout the dtay. In front of the spiral, and wvith an interior reflecting surface, is the tube ii, through fwhich the heat passcs to the rock(-salt cell c. This cell is placed on a little stage, soldered to the back of the pCrforated screen s s', so that the heatr, after having crosscd the cell, passes through t;he hole in the scren, and afterward imI hlave had also la mnps constructed inl whichl the spiral Nwas placd in vaeuo, itRs rays passing to sxternal spaco through a plate of rock salt. Their steadinces is porfect, A..~. at ) Cn~ S i' f-w:::: -\i _ _: 2: -, V~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ii V C'~~~~ *~ ~~~ r.- nox or HElAxT BY LIqU1IDs. SoURCE Or HEAT1': PLATINl'UM S'IR'AL, lRAISED) TO BllR(XIlT' REDI-;NESS BY' A'VOLTAI GlUI;W'NT. Thldckess oi3f liquid i pt'rts of fan inch. Liquid.......... 0.02 0- 01 0 001' 01 0 27 Bisulplhide of carlon........... i S 126 1''2 183 Chlorofotrn.................... 160. 26'0 23'0 400 4458 Io(li1de of mIettlyl.. 36'1 416' 653'2 65' 68-G Iodide of ctlv................ 38S2 60 690 690 1' B enz ol 43.4 5"'1 0 2''I' lv38' Anylee..............,,......8'3 65a'/3' J7 82'3 Stllphuliie eite............... 63'3' t1' 8' 85'2 Acetic othei................... /,4 0'80 82'0 8601 F'ormtic the................... 652' 3 f0 8;40 8'.0 Alcohol....................... 63 0 t18 6 83G 86 3 801 Wa t e r.t.............. 80 86'1 88'8 910 91'0 (507):ere, for a thickness of 0G02 of an inch, we find the absorption varying from a minillmum of 5'5')er cent. ilt the case of bisulphide of carbon, to a maxilmum of 8017 peri cent. in the case of water. The bisullIphide, therefore, transmits 94'5 per cent., while the water —a liquid equally transl)arelnt to light-.transmits only 19'3 per cent, of the cniire radiatioln. At all thicknesses, water, it will be observed, asserts its predominance.' Next to it, as an absorbent11 stands alcohol; a b)ody which also rcsembles it chemically. (508) As liquids, thcn, those bodies are shown to possess very different capacities of intetrcepting thie heat emitted bty our radiating source; and we lhave next to inquire viwheteflr these differences contitlue, after the molecules have been rcleascd fronl the bond of colesioln and reduced to the state of vapot). Awe lmust;, of courise, test the vaplors by wtaves of the same period as those applied to the liquids, and this ourl mode of cx)perienll renders easy of accomj)lishment,. The heat generated t a wire byl' a current of,a given ~ st rcength being intvariable it was o0nly nlecessalry, 3y means of a. tangent comi)ass and theocord, to keep the current constaunt from (lay to ABSORPTIfON OF LIQUID)S AND VAPORS. 359 day, in' order to obtain, both as reards qutaltity land quality, an ivariall' e l source of hleat. (509) The liquids fi'om which the vapors \were deri\ved were 1)laced in small long flasks, a separate flask b)eiing devoted to each. The.l air above the liquid, and within it;, beilng first carefully removed by an air-pump, the flask was attached to the experimental tube, in which the vapors were to be examined. This tube was of brass, 49'6 inches long, and( 2'4 inches ill dizameter, its two ends being stopped by plates of rock-salt. Its interior surface was plolishcd. With the single xccption thflat the source of hcat was a red-hot platinuml sailal, instead of a cube of hlot wrater, the arrangement was that figured in Plate 1. At the commencement of etach experiment, the bIass tube being thoroughly exhausted, and the radiatlion from the spiral being neutralized by that from the ompenl)nsating cube, the needle stood at zero. The cock of the flask( containing' the volatile liqluid was then carefilly turned on01 and the vapor allowed slowly to enter the, expeorimental tube. When a )ressure of 0O' of an incht was obtained, tl}he vat)or was cut off atind the plermallelnt deflection of the needle notcd. Knowing the total heat, the ablsor)ption il 100tths of the entire radiation could be at once deduced from the deflectionl, The following table contains the restlts: IRADIATION OF 01}:EAT Tt11OU(Ii VAPons. OURCt-: gn:ttlO'r 1 LfA'IATINUMt StI,,Ar. ]}'I\Ssuln1u, 0'5 OF AN IXNC. Absorption per ccnt. ]3isulttphide of carblon... 47 Chloroform.....6 Iodide of methyl.. 9'6 Iodide of ethyl.....1 3Benzol,..... 20' Ay lto e......2'*t' A1n1ntienl..,,., 28. 1 Alcohol.C... 8, 31 Formnic ether..... 314 Sulphuric ether'.... 31'9 Acetic ether. 3 ~6 Total lheat.... 1o0t) 360 lJIAXT AS A [MODE OF MOTION. (510) e are now ill a condition to compare tlhe action of a series of volatile liquids with that of the vapors of those liquids, upon radiant lheat. Commencing with tile substanllce of t1he lowest absorptive energy, and proccc(ling to the highest, we have the following orders of absorptionl JAqulds. Vapors. B1isullhiite of carwlon. Bisultl)pid of carbon. Chloofolfoti. Chlolroform. Iodide of methyl, Iodide of methyl, Iodide, of ethyl. Iodide of ethyl. Beinzol. Benzol Amylete., Amylone. Stlpluric ether. Alcohol. Acetic ctlcer. Formic other. Formic 0ether. Sulplhrio ether. Alcohol, Acetic ether. Wrater. (511) HIere, as fair as amylone, -the order of absorption is the same for both liquids and vapo0sr. iut; from amylene downward, though strong liquid albsorption is, ilt a general way, paralleled by strong vapor ab)sorpt;ion, the ordcr of both is not the same. There is not tlhe slghtest doubt tihat, next to water, alcohol is the most powerfutl absorber in the list of liquids; but there is just as little doubt that the position which it occupies inl tile list of vapors is the correct one. This has been established by reiterated explrimonts, Acetic thler, oni tlle otffer hand, tholugh cCrtailly thle most enCergetic absortber in tle state of vaplor, falls behind both formic ether and alcohol in tthe liquid state. Still, onl flte whole, it is perfcctly impossible to contemplate these rcsdlts, without arriving at the conclusion that t1he act of labsorption is, in the mail, molecultt, anl( that thle molecules maintain their power as absorblers and radiators when they clhange their sttate of aggregation. Should any doubt;, however, linger as to tile correctness of this conclusioln, it will speedily discappea'r. ORDI) OF1 ABSORPTION. 361 (512) A moment's refilction will.show that the comparison hrc institulted is not a strict onel. We havF taken the liquids at a common thickness, andl thc vapors at a common volume and )pressure. But), if the layers of liquid emplloyed were turned, bo dily, into vapor, the volumes obtained.would -iot be the same. lencc3, the cquantities of matter traversed by the radiant heat are not p)roportional to each other in tlhe two cases, and, to renlder the comparison strict, they ought to he proportiounal. It is casy, of course, to make them so; for, tlhe liquids being' examined at a constant volume, their specific grlavities give us the relative quantities of matter travwersed by the radiant heat,, and, fr'om theic and the vapor-densities, we can immediately deduce the corresponding volumes of the vaNpor. I)ividing, in fact, the specific gravities of our liquids by the densities of their vapors, we obtain the following series of vapor volumes, whlose weights are lproportional to the masses of liqulids emlloyed: TAilfkl 0' OF ROPORlTIONAL VOL YOUtiqS. Bisullpide of carbon. 0,48 Chloroform... 36 Iodide of methyl... 0'46 lodide of ethyl.... 0'36 Beinzol.., 0'32 Amlylne.. 0'26 Alcohol-. * O60'5 Sllplin.ic ether0...0'28 Formic ether-.-... 0-36 Acetic ether.29.. 0'2 Water- 1-.'00 (513) Introducing the vapors, in the volumes here indlicated, into the experimental tubl, tlhe following results vere ol)taied: 1O 362 ltSEAT AS A 3t01)E OF MOTION. RADIATION Or 1]:AT'ItROU;Ilt VIAPORS. QUtANi-fTY OF APOR'ROfOR1TIONA'T TO TH1AT OF LIQUID, ireSSm'o l in parts Ab)0orption, Nane of YVapor. of an inch. per c(lit. Bisulphide of carbon...,, S 43 (hloroform...... 036 6 Iodide of methyl..... 0406 102 Iodtide of ethyl... 0'36 1654 Beuzol.,.. 0382 168t Amylene.... 0'26 19'0 Sullthnrie ether...... 02' 291lt Acetic ether,. 0, 29 22.2 Formic ether 0.....,.'5 Alcohol 0. 50 22`1 (51::) Arrangi ng b)ot;ll iquids and val)ors in the order of thelir absorpition, we now obtain the followinlg result LiquiLds, apors. Bisu1lphide. of carboll. MBisulphide of carbon. Cthloroformu. Chloroform. Iodide of met1hyl. Iodide of methyl. Iodide of ethyl. Iodide of ethyl. =%nzol. Benzol. Amylene. Amylelne. Sulphuric ether. Sull)Iliic. ether. Aectic other. Acetic ether, Formic ether. Formic ether. Alcohol. Alcohol. Water. (515) Hcre tfhe discrepancies revealel by our formner series of experiments entirely disaplpeaft, and it is provedf thlat, for heat of the same quality, tihe order of absorltion for liquids and their vapors is the same. We samay, theriorc, safely infer that the position of a. vtapor, as an absorber ori a radiator, is determined by that of the liquid firom which it is dcrivcd., Grcanlting the validity of this inference, the position of zwater fixes thlat of aqueous vapo. But we hare found tthat;, for all * Aqueous va)or, unimixed witNh air, condenses so roadily that it cannot bo directly examined in our experimental tube. VIBR1ATING X PEiRIODS. 363 }hi:kncesses, water ecceeds the other liquids in the energy of its absorption. I:cencc, if 1no single cxp)erimllclt oin tihe vapor of water existcd, we should )be compelled to conclude, from tle deltportllt nt; of its liquid, that., vweight for wcightl, aqueous vapor transcends all others ill absorpt,ive lpower'. Add to this the dircct and multiplied expelriments, by vwhich thle action of this substance onl radiant theat ]has 1)oon established, and \we have before us a body of evidence sufficicnt, t trust, to set this question forever at rest., and to induce thle mcteorologtist to appl)ly tite result., wit{hout; misgiving, to tlec l)lIenoimlcell of his science. (5t16) AVc must lnow )r'l)parc the way for the considerattion of an important question. A pendulumt swings at a certain definite lrate, which depends Utl)Ot tile lengthl of thle pcndulum.l A spring will oscillate at a rate irwhich dpelnds uponl the wrcight and elastic force of the spring. If we coil a wire into a long sliral, and attach a bullet to thle cnd, the bullet may be caulsed to oscillate p) and down, at a rate which depends upon its wceighlt, and upon the elasticity of the s)irtal. A musical string, in like manner, lhas its determinate, rate of vil)ration, which deplends upon its lcn gth, weiglht, and tcnsionl. A beaml whticel bridges a goirge has also its own rate of oscillation and we can oftenl, by tiling our movements on such a tecal, so accumulate tlhe im)ulses as to cndantger its safety. Soldiers, inl crlossilng ponltoon bridges, tlread irregularly, lest the motion implarted to the p)ontoons should accumulate to a daingerous extelnt. The step of a person carryintg \ater onlt is ]head in nll ope)n pail sometimes coincides withl the o.:cillatioin of ttie wvater from side to side of the vessel, luntil, imll)ulsC being added to impulse, tlhe liquid finally splashes over thle rim. The watct arrtaicer instilnctively alters his stcp, and thus reduces thile liquid to comparative tranquillity. You have heard a particutla' 1)ale of glass respond to a particular note of anl orgaln; if you open a piano, anl sing into it, some one string will also respond. Now, in the ecase of the orgaln, tlhe pane re. spends, because its p)Ciiod of vibration ha)ppens to coincide 364 H]EAT AS A MODE, OFl0 MOTION. with thle period of the sonorous waves tat imnpinge upon it.; andtl in the case of the piano, that string respotnds whose 1)criod of vibration coincides with tihe period of the vocal chords of the singer. lin each case, there is an accumllllation of the effoet, similar to tllat observcdl when you stand upon a plankbridge, and time your itll)ulses to its rate of vibration. in the case of the singingp' flatamce, alreadtlcy rcfel rrcdt to, you had the infiluence of period exemplified ill a very striking mannert. lt responded to tlhe voice, only when the pitch of thle \voice corresl)ponded to its own. A higher and a lower note wcere cually ineffective to put lte flame ill motion., (517) These ordinary mcchlanical facts vill hclp) us to an insight of tlhe more subtle p)leolntcha of lighlt and ra(lianlt heat i havre shown you the transparency of lamp)-black, aild, thle far more wonderful transparency of iodine, to the purely thcermal rays; and we have now to inquirle why iodille stops light and allows heat to pa5ss. The sole dilffrence bet ccln light and ra(lianlt hesat is one of periodt. The waves of the one are short and of ral)id recurrence, while those of thle othecr are lotng, and of slow recurrnce,. The former are interceplted by ithe iodinel, and the latter arc allowed to pass. Whlty? Thlere can, I think, be only one answer to this question' that. tle interceptcd waves are those whose pchriods coincide Nwith the periods of oscillation possibllc to the atoms of the dissolvcdl iodine. The wavcs. transfer their motion to t the atoms lwhich synchbronizc with them. Sulpposingl waves of any period to impinge upon a an assemblage of molecules of any other period, it is, I thlink, physically ccrtain that a tr'temoi' of greater or less intensiity will be set lup among' the moleculcs; but, for the motion to (ccmtmula-te, so as to produilce stnsible absorpt.tion, coincidence of period is necessary. Briefty defined, tlerefore, traxasllP)ar'1ncy is synony1mous with discoWrd, while opacity is synonymous withl accor'd, betwee n tChe le)riods of thl waves. of cther and those of the molecules of the body oin which they impinge. Thcl opacitry, thenl, of our solution of iodilte to light, shows thlat its atoms are completent to vibrate in all periods M:tOIEfJULAlt ACCOUD AND DISCOR]) 365 which lie withill the limits of thle vlsible spectlrum; wh]il its transpl)arenlcy to the extrsa-red undiulations demlolnstrates the ilncoml)etoncy of its atoms to vibrate inl u1nison wit.h tthe longcer watves. (51$8) Thec terml "quatlity," as applied to radiant hleat, htas bceen already defined; thle ordinary test of qualit-y being the power of radiant heat to pass tllrough l iathlc, ice bodies.'If the heat of two beams be transmitted by t he slf-same substance in diftleret proportions, thl two beams are said to be of differcnt qualitics. Strictly speaking, thlis question of qultality is one of period; and, if t he lheat of olne source be more or less copiously transmitted than thle }heat of another soturce, it is because the wavcs of ether xcited by the onle liar differentl in lcngth and period from thosc excited by the other.'Wlhenl we raise tile templ)ratur of our pl)atimlnl spiral, we alter tihe quality of its healt As the templerature is raisedl, shorter and cver shlorter waves minle in the radiation. 1),'. Draper, in a very beautifill investigatiion, has shown that, when platinum filrst aplpears luminolus it emlits only red rays; but, as its temperature augments oaruge, yellow, and greccen, arte ssuccessively added to the r adiation; and, -when thel, platinuml is so intenisely heated as to (e)mit white light, the decom)positio of that light gives all the colors of the solar spectrtu lm. (519) Almost all the vapors which we haxve hitherto cxaminelc are transparenit to lightl, while all of them arc, iln some dlgree, opaqul to obscure rays. Thlis p1iroes thc incompctcnce of the molecules of these vapors to vibrate ill visual pleriods, and their competence, to vibrate in tile slower pcriods of tle wtaves which fall beyond the red of the spectrum.i Conceivce, then, our l)latinlm sl)iral to be gradually raised from a state of obscure to 1a state of hluminotus heat; thle chantige would manifestly tend to p)roduce discord bletween the radilting platiumtt i and the molecules of our vapors. lAnd tlhe Iighlerl we raise the temperatlure of our platinuml, the more decidc( will bh the discord. Onl a priiotri grotlds, then, we should infer that the raising of the tcmperaturtl of the platlinum 36a6 HEIAT AS A MODE ] OF MOTION. spiral oughtll to al uginent the power of its rays to pass through our list; of vapors. This conclusion is entirely verified by t1he experiments recorded in thle follotwing tables' ADIJATION THRiOUO VAPOR. SOURION OF ]EAT: PLATINU.M rPIRtA, BARELY vYIStILE IN TI: 1D),ARK, Namo of Vap'or. Absorption per cent. Bisulphide of carbon.l.. 615 Chlorotform 91l Iodide of methyl.... 12' lodide of thyl..... 21'0 1lenzol l.... 25-4 Amylo ne.....368 Sultphi\uic ether... 43t Formic ether.. 45'2 Acctic cthcr... 49'6 (520) With the samie llatinunl s)iral raised to a wNhite heat,) the followingl results wcere obtained RADIATION tIROUGH VAPosR., Souett oF Ift: EA WHITE-ioT PATINUM Nalno of Vapor. Absorption per cent.. 1Bisulphide of carbon.. 2'9 Chloroform..,. 60o Iodide of methyl.'8 Iodide of ethyl... 12'8 J-Benzol.,... 16o Aylenc..... 226 Formic ether.... 251 Sulphuric ether... 2659 Acltic ethe..... 2/'2 (521) Withl the samle spiral, brought still nearer to its point of fusion, the following restlts wcere obtained with four of the vaplors: RA)DIATION TiHRlOUO1i VAPOlS. SOURCE: PLATINUM SPIRAL AT ANN IN ENtS' W'IIT: IH EAT. N'amto of Vapor. Absorption. Blisulphide of carbon... 2,5 Chloroform... 3'9 Formic ether... 21'3 Sulplhuric ether... 2328' IN1FLURJNCh OF',O TE1MPERATUlN'. 36'/ (522) Placinlg tile rcsults obtained -with the respective sourlces side by side, thec influence of the vibrati' period on the transmtissionll comes outl in a very decided mannetlr: AlfSOr0l'IO N OF ]H[AT I )Y V,uPORS. Namo of Na por. 8oitrec: llitinum Spinal. ]artk' visiblo. JBright rct. \tWhite hot. Ne;r fusion, Ilisulplhidc of carblon G6' 5 4i7 2'9 2 - Chlo'roform.. 91 6'3 5'6 3'9 Iodide of methyl. 12'5 906 1'8 Ioditde of ethyl. 21'3 1'71 12'8 ]nezol,,, 26'4t 20'6 1065 Amylent, c. S3'8 2'1- 22'1 Sulphuric etllher 43',t 31-4 2569 23'1 Formtic ether,. 452 31'9 25'1 21'3 Acetic etther.,'196 34 27'2 (523)'.lhc gradual auglenltaltion of l)ecletfrative p)ow(er, as the tenmperaturle is aurtlgcnt is hre vcery manifest. 3y raising the sl)iral from a. barel)y visible to an1 inltense white icat, we reduce the I)rolooltionate absorl)tion, in the case of bisullp)hide of carbon andl chloroform, to less than one-half. At l)ar'ly visible redness, moreover, 56'6 and 54-'8 per cnt:. palS tlhrough sulphlric and formic etlher rcspcc tivcly; vwhile, of thle intelscely wlhite-hot spiral, 763 andl 78 7 pcr cent. pass th1ough thie same van)pors." T.l'us, by augmenting the temperature of the solid tlatinum, we introduce into the radiation waves of shorter )erioCd, whlich, being in discord withl the p)eriods of the vapors, pass more casily through them, (524) Itunning the eye alongt the niumlbers \which express tile absorl)tionls of sulp)huric and formic ether il the last tablet, we find that, for theo lowest lheat, the absorptioll of the latterl exceds th'at of the former; for a briht-rci l heat they are nearly equ(Ial, but the formic still retains a si t lit predlolinanle at a white heat, however, tIhle sulplhuric slips in tadvance, and at the heat near fulsion its predomialnclee is decided. i: hiave testedt this result in var'ious ways, a ld by nmultiiplied cxleri-' TXhe ransmi,?don is t'ound b1y subtracting thlle absorption fr'om 100. 368 IlUAT AS A MODEW OF nMOTION. melits, and placed it beyond doubt. Wle may tat once infer from it thatt the capacity of the molecule of formic ether to enter into rapid vibration is less than thlat; of sulphuric, anld thuls we obtain at glimpse of the inner character of these bodies. By laugmenting the temperature of the spliral, wve produce vibra-tions of quicker 1periods, and, the more of these tlhatt arc introduced, the more opaque, inl comparison witl formic ether, does sulphuric ether become. The atom of osygen which formic ether possesses, il excess of sulphuric, rlndes it mlore sluggish as a vribrator. Exl)eriments made witht a source of 1000 C. cstablish more decidedly the preponderance of the formic ether for vibrations of slow period. iAUnDIATION THROUcu VAPORS. SOURCe-: LESI-E'S CUBw, COATED WITti LA Mr-BJ, xt,1 TPE ls\, A212 A ll, Same of Vapor. Absorptiol pier cent. Bistllphide of carbon... 6'6 Iodide of methyl... 188 Chloreofo rm.. 21.6 Jodide of thyl.... 29'0 Benzol.... 3' Amtylne....... 4't1 tulphuric ether..... l:t'1:ormlic ther.... 00 It Acetic ether 9..,.. 699 For heat issuing friom this source, thle absorption by formic other is 6'8 per cent. ii excess of that by sulphuric. (525) ]But in this table we notice another case of reversal. In all the eXl)crimilnts with t.he platinum spiral thus fatr recorded, clltoroform showed itself less energft tic as an absohber than iodide of mcthyly; )but here chloroform shows itself to be decidedly tle more powerful of the two. This result has been placed beyond doubt by rep)eated exp)eriments. To the radiation emitted by lamp-blackl, hehis composite souree amounteld to $8' l)er cent. 37 2 1,t! i'A] MASi A 3 MODIE 0OF IMOTION. On humid days, the absorption of the heat emitted by tt hydrogen flame exceeds even t.he above larte ftigure. Etnployinwg the same experimental tube and a new burner, the explerilents were repeated sonme days subsequently, with the following result: RADIATION TIttIOUOtu Alti. SOUROiE: ]'DIRIOGEN' FLANW. Absorption. Dr1y aic... 0 ndrid air.... 20'3 (532) Tihe physical causes of transparency and opacity have been already pointed out,; and we may infer from tlhe foregoing powerfull action of'atmospheric vatpor on the raldiation from the hydrogen flamie, that accord rcigns between thle oscillatinfgi molcecules of th;]e flame at a temperatlurl of 5898S ]iahr. and the molecules of aqueous vapor at a telnl)eratur. of 60~ ]lahr.''l i enormous tcmpcrattur of the hydrogen flame increcases the amplitude, but docs not change time rate of oscillation. (533) reW\ must devote a moment's attention, in p"assinlg, to the word " ampltitude " here employed. Thl pitch of a note depcends solely on thC number of ali'ial \\aves whichi strike tlhe,4r in a scecond. The loudncss, or intensity of a note delpends upon the distance within which thle sclarate atoms of air brate. T'h]tis dcisttanec is called thle (annitpbte, of the vib)ra-, tion. Whcn we pull a, htarp-string very gently aside, and let; it go, it disturbs the air but little C; the amplitude of the vib)rating; air-atoms is slmall, and the intcnsity of the soiund feeble. But; if we )ull the string vigorously, aside, on lctting it go, we have a note of the same pitch as before, bult, as the anltmplitutde of vibration is greater, t]he solund is more intense.,hile thelln, the wave-lcngth, or picliod of recurrnce,, is independent of the amrplitude, it is tis latter which lctermlines thi loudness of thc sound. (534) Thle same holds good for light and radiant heat. [Here the individual therl-partieless vibraite to and fro across IARBONIO ACID THIROUGHl CfARBONIC ACID. 3'J3 thle lilne of propagation; and the extent of their excursion is called the ampllitude of tle vilbrationll, e mat as inl tile case of sound, lhave the same wave-length wiith very dillerent amp)litudes, or, as in thie'nse of wvater, w\e may have higlh waves anid low waves, with the same distance be)tween crcst and crest;. Now, while the color of lig-ht, and fthe quality of radiant heat, depend entirely upon the length of the cthereal waves, the intensity of the light and heat is determined by the amplitude. And, inasmluchT as it has beenl shown that the periohds of vibration of a hydrogen-flame coincide. with those of cool aqueous vapor, we are compllced to conclude that tthe enormous temperature, of the flame is not dlue to the raplidity, but to the extraordinary amplitude of its molecular vibration. (535) The other component of the flame of Bunsen's burtner is carbonic acid, and tic radiation of this substance is immediatcly obtained from a, flaime of carbonic oxide. Of the radiation from this source, the small amount of cattrbonic acid difflused in the tair of our laboratory absorbed:13'8 perl cent. This high absorption iproves that the vibrations of the molectles of carbonic acid, within the flame, are synchronous with the vibrations of those of the carbonic (acid of t he at;mosphere. The temperature of the lame, however, is 5508 F11ahr., while that of the atmoslphere is only 60. But if the bchigh temperature is incoml)eCtCnt to change the ratc of oscillhtioit, we Ilmay oxpect; cold carbonic acid, w\\len used in large quantities, to be highly opaque to the radiatlion from the earbonic-oxide Ilamltc. [cre follow the rcsilts of experiments cxecuted to test this co0nclulsion' RAnIATroN'tHROUOt[ D}l~Y CAhatos"1;0 AACID. SOUlaC:A:,hRNtlOSCOxIt: I) f,A Lr l'ressure in inches. Absorption. 2'0 55'5 3'0 6003 4t0o 65,.1 510 0686 10.o 14'3 374 1HliAT AS A MODE 01F MOTION.'For the rays emanating from th.e hcated soids employed int our former rescearches, carbonic acild p)rovd to b)e one of the most feeble absorl)brs; but hcre, when the waves scnlt into it clmanat from molecules of its own substance, its absorbent eneCrgy is enormous. The thlirtilth of an atmosphere of thie gas cut;s off half tile entire radiation; while, at a )iessure of 4 inches, 65 pelc cent. of the lcradiation is intrcceptced. (536) Ihc elncrgy of oletfi t gas, both as an ab)sorbcnt. and a radiant, is now well 1t11known. ]or the solid sources of ]leat just reflrred to, its powcr is incomltarably greater thall that of the carbonic acid'; but, for the radiation from the carbonicoxide flame, tile powcr of oleflant gas is feeble, when compardln with tlhat of carbonic acid,'This is proverd by the experiments recordcd inl the following table: A)ADIATION T1iROU(tII DRY OLJFIANT GAS AND mlY niAtlloNif AcID. o80UseC: (AnnoRI;Oc-OxtnD FtxAMEw. Olefianlt-gas C'trbonlllie-acid P'ressuro in inches. absorption. albsorption. 1'0 23"2 480 20. 34,1 55.5 3'0,440 60c3 4o0 5016 6541 5'0 5-1 08 10'0 06B5 q43 (537) B3eside the absorption by olefiant; gas, I have placel that by carbonic acidt derived from the last table. The superior power of thle acid is very decided, and most; so ill tbhe Smaller 3pressures; at, a 1ressure of all inch it is twice that of the olefianlt ges. The substances approach each} ot.hcr 1 more closely, as thle quantity of gas augments. H1cre, in fact, both1 of thncll appr)l'oacl perfect opacit)y anl(d, as they draw near to this comm101on limit, their absorptions, as at matteri of course', approximate. (538)''These experiments prove that tthe prel)senlc e of all ilifinitesimal qu(anttity of carl)bonic-acid gas mnig'ht lbe detected, eby its action oni the r1ays emitted by a carlbo)lic-oxide flame. The action, for example, of the ctarbonlic acid expired )y flle PIYSICALI ANALYSIS OF IiUMAN BIRE1ATI a. 375 lungs is veryAdecided. An India-rubber bag Nwas filled friom the lungs; it contained, therefore, bo)th the aqueous vapor and the carlbonic acid of the brlcath. The ailr from the bag' was thenl conducted through a drying apparatus, the moisture beinlg tlhus removed, anld the neutral air and active carbonic acid pcrmitted to enter thle experimental tube. the following results were obtilned: Amlt rax fi (.s COrAS s( LUNS CONA O. 8outcG:: 0ARsoNte.OxxIou Prestutro In ilnches. Absorption. X 12'0 3 26'0 5 333 30 60'0 (539) Thus, t;he tullc filled wlith tihe dry exhalation fi'om the lungs interceplted 50 per cent. of the entire radiation from at carbonic-oxide flame. It is quite manifest thatl we lave here a means of testing, \w\itl surpassingl delicacy, the amnountt of carb)onic acid emitted under various circumstances fi'om thle lungs. (540) The atl)lication of radiant hecat to the determination of the carbonic acid of the l)reath has been illustrated by my late assistant, Mr. Blarrett. Th'le deflection p)roduced by the breath, freed from its moisture,but, retatining its carbonic acidl, Nwas first determined. Carbonic acid, artificially preplared, was then mixed with perfectly dry air, in such l)roportiolns that its action upon the radiant lheat was t;he same as that of the carbonic acid of tlhe breath. The percentage of the former being klnown, immediately gives that of thte latter. TI here give the results of three chemical analyses, determined by 1)~r. Frankland, as compared with three physieal analyses performed by my late assistanit: PmRCfEsrxCA: or,uARtoNf', Aom iN HIUMASN f AT'rLf, By ch:etmical nslysis. By ihysial p analysis. 4'311.1 00 4'0O 4'50 5'33 f",22 R3116 1f1,]HUt1EAT AS A MODE OF MOTIO(N. (,541)'lThe agreement, betweent thle results is very' faith. )oubtless, with greater p)racti;e a close'r a-gr'ecmentl could be attained. rec thus findl in the quantity of ethereal motioll which it is competent to interccptl. an accurate and practical meatsure of thle anbmount of carbonic acid expired from the hiuman lungs. (542) XS\ater at moderate thiIkness is a very transparent substance; that is to say, the perio(ds of its 1moleculesJ i're iln discord with lthose of the visible s.pectrum.'It is also highly transxparcnt to the cxt'tr-violetf rays; so that we may safedly inferC from t hel deportment of this substance, its incompletnce to enter into rapidt molecular vibration. WhVlen, however, we once qcuit the visible spect truml for the rays beyond thle red, the opacity of the substance begins to show itself; for such rays, itndccd its absorbent power is unequalled. The synchlronism of thle periods of the water-molecules with thoset of th e extrat-red waves is thus dlemonstratecd. We have already seen thlat, undried atmospheric air manifests an ext-raordinai y o)ancity to tlhc radiation from a hydrogen-flame, and from thlis dteportment we inferred the synchronism of t;he cold vapor of t]he lir, and, the hot vapor of the fulamc. B1ut if the pecriods of a vapor be the same as those of its liquid, we ought to lin(d water highly opaque to tile radiation from a hydrogen-flame. IHere are the c rscsults o btaincd with flve different thickncsscs of thle liquid RADtATION THIRllOt rol VrATERI. SOtuCE: Y!oGEts-iEN'-FLANtME. tlhicFkn.esis of liquid.,,?4 _................,...... _e.... #.......,.;,.....~-~, _~~,:~~~~,~, _~,....... 002 inchl. O001 inch. 0'0? iltch. 014 inch., 0,2T inch. Transmimission pet cent. 8 2'8 1'1 0.. 0.0 (543) Thirough a layer of water 0'36 of an inch thick, AMelloni ifound a trranstlission of II per centl. for the heat of an Al and lamp. [cr N we e mploy a source of higher temperature, and a layer of water only 027 of an inleh, and find the whole of thle lceat intercepted. A lacyer of water 0'27 of fan inch in thickness is pcrfctly opaque to the radliationi from a, VIBRATING PERIODS. 37 hydlrognl flame, while at layer about one-tent}h of the thickness emplloyed by Melloni cuts off more thlan 97 per ccnt. of the entire radiation. Hence wve may ilfCr tei coincidence in vibrating period between cold Nwater and (faqueous vapor heated to a tempelrature of 5898~ ]Far.. (3259~ C.). (544) F1rom thle opacity of water to the radiation frol aqueous vlapor, we may infer the olpacity of aqueous vapor to the radiation from water, and hence conclude that the very act of noctfurnal rcifigcrationl which causes the conldensation of wrater on thle cart.h's surtface, gives to terrestrial radiation thlat p)articular ctharacter which rlnders it most ltiable t6 be intercepted by our atll mosphere, and thus prCvcnted from wastilng itself ill spacc. (545) This is a point which deserves a mIomcnt's furthcr consideration. I1 find that olefiantl gas, conttained in a polished tube4 4 feet long, absorbs cabout 80 per cent. of the radiationl from all obscure source. A layer of the same gas 2 inches thlickl absorbs 33 per cent., a layer I inch thick absorbs 26 per cent., while a layer -1t.-th of an inlch ill thickness ablsorbs 2 per cent. of lte radiation. Thus the absorptioni increases, and the quantity) tranismittcd diminishes, as thte thickness of the gascous layer is augmented. Iet, us,now consider or ort lmomenllt the effect iipont thlle earthb's teml)erature of a shell of olefitant, gas, surroundling our planet at a little (istance albove its surface. JIThe gas would b)e t;ransparentl to the solar rays, allowin tlel, without sensib }lie hinderance, to reach the earth. I[ crc, however, the luminous leat of the slunl wotuld be converted into non-luminous terrcstrial heatt; at least 26 per cent:. of this heat would be intertel)ted by a. layer of gas one inch thicl<, and iln great part returncd to the e-arthl-. Under such a canlopy, trifling las it may alppear, anl t )erfectly tranas)paremnt to the eye, the earth's surface would be maintained at a stilling temperature. (546) A fewtt cars tago, a work, possessingt great chtartms of style and ingenuitty of reasoning, was written to prove that the moro distant planets of our system are uninhabitable. 3'78 1:XEAT AS A MODE OF MOTION. Ap)pl)yig the law of inverse squares to their distances fi'rom thile sun, tte dliinut tion of temperatrure was found to be so greftt as to preclude the possilility of human life in thCe more remote members of tile solar system. Biut in those calculations tlhe influencce of an atmospheric envelop was overlooked, tan1d this omission vitiated the entire argumentl. It is perfectly possible to fil d anl atmosphere which Nwould act the part: of a bat'b to the solar rays, permitting their cntrancc tow\ard the p)lanct;, but prevent;ing their withdra wal. Jf'or cxampll, a layer of air two incces inl thickness, and saturated wfith the vapor of sulpllhuric ct her, would offer very little resistance to the l)assage of the solar rays, but ] find that it wolmld cut of' fully 35 per cent. of the phlhanettary radiation. It would requni e no inordinate thickcnling of t he layer of vapor to double this absorption; and it is perfectly evidentt that with a p)roteCtilng envelop of this kindl, permittintg tihe hleat to enter, butf lprcvret.inug its escape, a comfort able temperahture mightl be obtained on t.he surface of tlhe most distant plaInet. (547) D)1. Ailler was the first; to infer, from the inability of the rays of bu)lning hydrogen'to pass through glass scrceens, that, the vibrating )eriodls of the flame must be extra-red; and that, conscquently, tLhe oscillating periods of the lime-light nust be more rapid than thl;so of the oxyhydrogen-flameC to fwhich it oves its incandcscelnce.* As pointed out by D)r. Miller, the lime-light; fiurnishes a case of cexalted refirangibility. The stame remark applics to a platinum wirel plunged into ca hytdroge'n flame. We have, in this case also, a convl rsion'* After retirring to the researchles of Professor Stokes on "degraded" refilangibility, D)i. Miller saPys: "i Heat of low refanlgibility mlay, however, bo converted into that of hiefher refrangibility: for examuple, ajet of mixed oxygen and hydrootgeln gases fhtlrishes a hleat nearly as intense as any which art can co0nnllaud, yet it ( does not emlit rays which have the power of traversing glass in anlly colsideirable (futntity even though a leis hbe emtployed for their concentration. Upon nintrodulcit a cylitnder of limo into time jet of blrlilg gaset, though the amount of lieat is not thus increased, the ligh1t becometc toobrllght for the unprotected( eye to enldure, and the thermie ra ys ae(tuire tile property of traversing glass, as is s;howtn )by their action upon a thermuometer thel bulb of which is placcd iml tihe focus of the le1ns." -. —.lChemical 1h1,ysics, 1855, P. 210. RATISING OF TIPlt RIATE, OF VIBRATION. 3/79 of unvisual p1eriods into visual ones. This shortening of periods must augment the discord betwceen the radiating source anld our series of liquids (~ 506), whlose periods are slow, anld ihence augment their transpiarency to the radiation.'Thc conclulsion w as tested and verifited by experiments oin layers of the liquids of t\wo diffcrent thlicknesses. RADIArTION TrlIROUGOH lIquIoS. S OURvE s: 1. [IYDRoXN —F',,\AS; 2, HiYtmoGYNsx-F, AUK AND) IATI,, L'VSI-~1iIAL...'an.sm.l....on..-......................................................... NIao of i, -T l Thilcknless of llqutd 001 inch: Thtlcknes, of liquid 0'01 Inch: Flo i, only.'Flame and slital. Faie olty. Flame and spiral. Bisulpihidf1 of carbon ii'??7 8s'I2 Q. 04 86'0 Cthlorofo1rm.. 54'0 12'8 607 609'0 Iodide of mncthyl. 31'6 42'4 26'2 36'2 Xodide of ethyl 303 36'8 21t"2 32'(S lBenzol.., 24t1 320G 11'9 28'8 Amylcne.. 1,'9 25b8 12'4 2.4'3 Stull)hutio cthcr. 13'1 22'6 81 22'0 Acetic cther., 1O'1 18'3 6' 6 18S6 Alcohaol.. 94 1,1' 6'S8 12'3 W\ater.. 3'2'1 2'0 604 The transmission iln each case is shown to b)e considerabtly augml ented by the introduction of the platinum wire, (548) Direct experiiments on the radiation friom a hydrogecr-flame completely verify the inference of Dr)1. AMiller. I had constructed for mne a colmpllete rock-salt train, capable of being sub)siituted for tile ordinary glasss train of the electric laln). A louble rock-salt lens placed in tile camera, rendered tlcle r(ays parallel: they then tpassed tllrough at slit, and a. second roek-salt lens placed withoutl the camerl a l)rodluced, at an apl)r'oriate distance, an imalge of the slit. 1ehblind this lens was placted a. roek-salt prisml, while laterally stood tihe linear thermo-elcetric pile already described (~ 309). WVithin the camcra of the electric lamp was placed a blurner with a single aperture, thle flame issuing' firom it ocel)yinyg tlle position usu ally taken up by the coal-points. lThis l)l ntCr w\as conncoted with a Tf-i ce, fl'om which two pieces of ndia-rubhber 380 EtIIAT AS A IMODE) O01' MOTION. tutbiig were carried, the one to a large hbydrogen-holder, the other to the gas-pipc of the laboratory. It was thus in mly power to have, at will, either tlhe gas-flame or tle hydrogclnflaml. W\\hen the former 1wats cmployed, a visible spectrum wus produced, which cnabled mc to lix the thermlo-elcetric pile in its proper position, To ob)tain tthe latter, it was only necessary to turn on th te hydrogen uilil it reached the gas-flamet anld was ignited; thecn to turn off the gas tand leave the ]hydrogen-flamt belind. Iri thls way, rindeed, tlhe one flame could be substituted for thle otrher withlout. opening flic door of tile camera, or Xproducinlg any dagll ill t}he pmositions of the so1urce, the lenses, the prisllm, and tile p1ile. (5:49) The splectrum of t}h Iluminous gas-flamc b)eilng cast upon thie lrass scrccn (whicl, to render thle colors morc visible, was covered with tin-foil), the lpile was gradually moved until thle deflection of the galvarlomleter became a mlaximlunl. To reach tthis it wass necssairy to pa1ss to solme distance beyolnd thte red of thle spectrum;tl the deflection then observed Was 30O. WVhen tte pile was moved in eitier direction from thlis positioln, tlle deflection was dintinishecd. (550) T'lie hydrogen-flame was now\ substituted for the gas-flamne; the visible spectrum disappe.arced, and t he deflcction fell to![clnce, as regards ray3s of this alrticulear refrangibility, the enlission fri')om the lumnltlltous gas-flamle was t;o and a half tinies tl}at froml the hydrogen-fltlle. (551t)'hie pile was agcain moved to andtl fro, thle lovement in bloth directions being accoml)anied by;a (dliinisised dct lection. \welve degrees, thllerefore, was tle maximtum deflcction for the by drogcn-flamt l; and tle posiltion of the pile, (tctermined previously by lfyea1s of t-le lutninous flame, proves thlat ftils deflection was producetd by extra-red undulations. I SPfCTRltMT OF I[YJ)ROGINd-', AiiXt:. 381 moved( tie pile a little forward, so as to reduce the defleetion firom 12~ to 4~ and thln, in order to ascertain the rcfit'langibility of tlhe rays which produced this small reflection, relighlted thle g'as. The face of tlhe pile vwas fotund invading' the red. When thel pile was caused to pass successively tlhroughl positions corresponding to tlle various colors of the spectruml and, to its extra-violet ray;s, nlo mIneasurablo deflection was produced by teC hydrog;eln-flame. (252) It; is thus conclusively proved that the'radiation from a hydroecn-fiiame, as far as it is caplable of ilmasuremll nt by oure, delilcate arrang'ement, is extra lred. The othe;r constitnents of tie radiation are so feeble as to be thermally insensible. (553) And here we findl ourselves in a, position to offer solutions of various facts, which have hitherto stood out was enigmtnas i researches upon radiant heat. It was, for a' tinme, generally supposed tlhat the power of ]lat to penetlrate diathlermic substances augmented as t he ticmperaturl of tfie source became more elcvated. Jlnoblauch contende:d lagailst this 1lotion, showing thait tilq healat emitted, by a platinumr \ire, plunged into tan alcohol-flame, wtas less ab)sorbcd, by certtin diatheimic substainces, t}han tile heat of thle flame itself, and justly -arguing th;at the teml)ratttiur of tlhe spiral could not be lhigher tlhan thlAt, of the body from vwhich it derived its heat. A plhate of ttransparcnt glass being' introduced bctlwcn his itncanttdescent platinum spiral and his tlhermo-lcctric pile, the deflcotion of his nccdle fell from 35~ to 19~; Nwhile, whlen the source was t1he flame of alcohol, withoutt tle spirali, thle deflection fell from 35~ to 1.60. This l)rovcfe tfhe radiation from the flame to be interceptel more powei'filly than that from thle spilral; or, in other words, that thle heat cmanating fiom the body of highesl t templerature ptossssed the least plenctl'ative power. AMelloni afterwalrd corroborated this exprilment;. (554) Transparentl glass allows the rays of tile visible spectrumt to pass freely t hrough it; but it is well knlown to be highply oplaque to thle radiation firom obscuref sources; or to waves of 382 lIALT AS A MOD100:, OF MOTION. long perio(d. lA plate 0'1 of an inch thick intercepts all the rtays fi1rom a source of. 100t (o, and transmilits only 6 )per cenlt. of thle heat emitted tby cotper r aised to 41:000 0. Now, the products of an alcohol-flame are aqucotus vapor and car'l)Onlic acid, whose waves have been piroved to be of slow pelriod; of the particular character, consequcntly, most l)owerfutll inteicepted by glass. ]ut,I by plunlging; platinullm ire into suchll a flamett, virtually colnvert its heat into heat; of higher rcfirngibilit;y; e changel the longl periods into sorter ollnes, and thus establish the discord between the perio(ds of tthe soulrce and the periods of thle diathetrmic glass, wllicl, as be. fore delfied, is tlhe physical cause of transparency. On purely a priori grounds, therefore, we might infer that the inteodlction of the p)latitulI spiral woultd augment the lpenctrativ powcer of thle heat. W\ith a plate of glass, Mctlloni, in fact, found the following trlansmlissions for t.he flame and the spiral: For the flamo, For the pihatlimm, 41'2. 52'8. The same remadl S apply to the tfransparent selenitc examilc d by M11lloni0,'lhis substance is highlly opaque to the extra-red tudulations; but; t!he raldiation from an alcohol-tflame is mainly extia-red, and hence tihe opacity of tile selenite to t.his radiatIion. The introduction of the plat.inlll spiral shortelns the p)eriods and aulgments tho t'allsmissiot. Thus, with a speci1men1 of seolteite, Melloni found the trallsmissiolns to be as follows: F~fmofu, Platinumi, 4a4. 19. (555) So far, the results of Melloni coincide with tflose of Knoblauch; but the Ittalian philosophlter pulllsues the matter fulrther, and shows that Knollauch's results, thloughl true for the l)artieultar substances examined1 by him, are not true of diatheirmic media generally. AMelloni shows that, inl the case of bltck glass and black- mica, a strikingl inversion of tie effect, is observced: trough these substances the radiation from tlhe ])IFF;WIUJLTI.S F"XPIA1NE)D. 383 flame is more copiously translnitted lthan that from tihe platinum. For black glass he found the following transmissions From tho flame, From the platillmtmm, 52'6.,12'8. And for tmce platt of l)lack mica the following transmissions: fl'rorml tle flame, - From the platisImtll, 02'8. b2'. (556) These results werc left uncexplained by celloni, but the solution is now easy. The black glass an(l the black mica owe their blackness to tihe carbon incorporated in them, and the opacity of tlis substance to light., as already rmcnarlked, )'rovcs the accord of its vibrating periods with those of the visible spcctrum. ]But it has bencc shown that carbon is, in al consideralde degree, pervious to the l waves of long period; thlat is to say, to such waves as are emitted by a flame of alcolhol, The case of the carbon is, therefore, precisely antithetical to that of thle transparent glass, the former transmitting thc helat of long period, and tllhe latter that of short period most freely. Heince it follows tiat tlhe intlroduction of the platinulm wire, by convcrting the long periods of tlhe flame into shlort o1nes, augments tfie transmission throutgh the tralsparent glass and sclenitc, and ditinislhes it tlhrough thle opaque glass and mica., 384 JlItA AS A 510D1)1E OF MOTION. CItfAPTEl R XIII, )IRCgOOYRY OP D)ARK SOAlA. RA...:.tI-; gg:lPlEJ, AND tUllR,}tI'S }EXP':PRIMTPSNI- 15-} —: OP-r INr'PSSlYl \V\''1Xlt{ ItAf.llUR}-:-..IAT O-:-OI 8P::OU.......lY-}'rII;S 81 tI[O TIE' }:1EOYP' RIO {ItF111'-. -R.iSANSMUTAtON OF' lAYS — R1..tMtAf, IMXA(I,: PRENEI)EtEX. IUMINOU.S -.COMBITSI ON AN1D1 iN.,Nt)ESCEN': i'v DAIK ANAYS -.....LO tE:SCOENCI A WN) -AI.ORfS,0OEES. —N- -D)ARPK SO, LAR R}AYS —I),ARK Il-l)-TAMJIET RAYS —.-'RANKi N1S }'XPRESMImtE:NT ON COLORS... -I S ANA.LYSS ANE) }I'PLANAI'ON,. (557) )N a folrmer occasionl 1 )rOlised to akeI known\ to you the progress of recent inquily as recgards the subject of invisible radiattion. A hope wats expressed thlat I shouldl be able to sift in your presnce tlhe composite emissiol of the electric lamp; to (letach its rays of darknless fiom its rays of light; andl to show you t.he powcr of those dlark rays when they are properly intensified and concentrated. (558) The hour now before us shall be dcevoted to an attetmt) to redeem thlis promise anlld realize this hope. And, in the first placc, it is ncccssary t}at -we should havle distinct notions regarding these ar]k ratys, or obscure rays, or invisible rays- -- all ithese adjectives have been applied to tthem. Ve lhave defincd liohit as wavc-ymotion; we have learned that thediffcrent colors of light are d(uc to waves of different lenglths; and wvc Nave also learned thati, side by side wiith the visible rays cmitted by luminous sources, we have an outflow of invisible rays. This, accuratly expressed, m-eans that, fogcthcr with t.hose waves which cross the humors of the eye, ilmptinge ut1)On the retina, anld excite the sense of vision, tlhere are others which either do not reach the retina at all, or which, if they (1o, arc not gifted with the powter of producing that spc 8IR1 W I. l11ETRSOfEII$S EXPE]JIllMENTS, 385 cifte motionl ill tile optic nerve which results ill vision.'tWhcthtr, and in what degree, the dark.rays of the elcctric light reach the retina, slhall be decided sulbsequentlyy; but, no matter what may be th]e cause of their intlfficacy, whether it be, du(t to their being qulenclled inl the hulnors of the eye, or to a specific incompctence on their part to arouse the retina, all rays whicll fail to cxcite vision are called dark, obscure, or invisible rays; while all rays that can excite vision are called visible, or luminouts rays. (559): It musst be confessed that there is a. defct in the terms cmploycd; for we cannot seeG light. ]in interstellar sp)ace wc should be plunged in darkness, tloilgh the wavcs from all suns and all stars might be speeCding through it;. \\VT slshould see the stuns and we should see tlhe stars themselves, but the moment we ceased to face a star, the moment we turned our backs upon it, its light would become darkness, thtough thle ether all around us might b)c agitated )by its waves. re cannot see the ether or its motlions, anlld }hence, strictly spelaking, it is a mllisuse of languatge to speak of its waves or r'lys being visible or invisible. JThis form of expression, howcver, has taken root; its convenlliellce has brought it into gcncral use a, and, understanding, by the terms visible and invisible rays, wave-motions which are respectively omlpetent and illncompetent to excite the opLtic nerve, no harm can result from the employment of the termns. (560) To the detection of thlose dark rays in the emission of the sun reference has been already made, and their existoece inl the emission of that source which comes next to the sunil ili power.-tlhe electric light. —.has also been c(lemonstrated. VThe discoverer of the dark rays of the sln was, as you have, been already ilformed, Sir W~illiam Hlerschel. Hiis incalns of obsorvation were far less perfect than those nlow at 6r comnln Id; but, like Newton, lie could Cxt'ract from Nature great rcslts Nwith very poor appliances. 1ie caused thernmometersl to pass tlrough tile various colors of the solar spcct'umn) and noted the temperaturl corresponding to each color..He 17 386 RlIEAT A A MODE40t1 OF MOTION. pushed his thermometers beyond tlhe extreme red of the spectrum, and found that tho radiation, so far flromn terminating with the visible spectluim, rose to its tmaximum cnergy beyolnd the red. The experiment. proved that., side by side -with its luminous rays, the sun emitlted others of lower refrangibilitfy, whlichl although thelr posssscesse high calorific plowvcI, were incompetent to excite tJhe sen-se of vision. (561) Now, the rise of thle thermometric colum, when the instrument is placed in any color of the spelctrul, may be represcuted by a sttaight i inle. For example, if a line of a. certail lengthl be tah1y, or, in othler words, it generates udulflations which are too long, and of too slow recurrence, to excite vision. Conceive its tempclrature gradually:augmented. With thle increased teml)erature Fl. 98. more rapid vibrlations are introduced'among tlhe molccules of thie body; and, at a, certain tempcrlturce, the vibrations tre suflliciently rapid to affect thle eye light.'Th'e body glowvs, land, first of all, as proved by 1)Dr l)raper, the light is a pure 388 lIEAT AS A A'MODE OF IMOTION. red. As the temperature heightens, orange, yellow, green, and tbl)ue, fare introduced in succession. (564) Thle vibrations correspondiing to these successive colors are essentially new vibrations. lhit,, simultalleously with the introduction of each new alld more rapid vib)ration, wve have an initensification of all those vilbrattonzs whlich preceded it.' ihe vilbration executed when our ball was at the temperature of the air, continues to be executed vwhen tle hball is whlite hot, ]u,ttt, vlile tle period remains thus constant, tlle amplitude, on whichl tilhe intensity of the radiation depenllds, is enormously increased. F1or this reason, the rays emitted by an obscure body can never a)pproach the intensity of th;e obscure rays of thlc same refrangibility emitted 1), a highltyluminouts o01e0 (565) LTet me rivet this subijet upon your attentlion ly a numerical example of the risc in the intensity of a. special vibraltioll, while more rapid ones are being introduced, A sp)iral of platiinlun wire was placed inl t his calera, and in frontl of tihe camera. a slit.. A voltaic current was senlt tlrough the slilral, but not in sulfficicnt strength to make it glow. ]By means of lenses and prislms of pure rockt-salt;, anld b3y other suitable d'evices, an invisible spectrium of the rays emitted by the )latinlml wire was obtained. A thin slice of tllis specrtrumn was permitted to fall upon the face of the linear thermoelectric pile already described. The'lh, band of the spel)ctrum was.so narrow and thle radiation so w\eaik, thait the deflection of the galvanometer was, in the first instance, only one degree. Without altering tlhe position of any portion of tlle apparatlts, the current was gradually strengtlened; raising tlhe teomperature of the wire, causing it to glow, andl finally raising it to an intense white lnat. Whten this occurred, a brilliantl light-s)pectrum was projctcd on thle screen to which tlhe pile was afttached, but tile pile itself wvas outside of thCe spectrum. It received invisib)le rays alone, and, throughioutl tile xperimentl, it contlinued to receive tlose )articulair vibrations whichl first affected it.'The rate of vibration being determinled PER:SISTE8NCOES OF RAYS. 389 by the position of'the pile, as thlis l)osition remaill ed t.hrougllhout unchanged, tlle vibratiol wats unchanged also. (566) Thle following colunt of numbers shows the rise of intensity of tt t)artiicutlar ol)scure rays falling' on the pile, as the platilnumi spitral )atssed through its various degrees of incandescence ull to white lheat. Alppearance l~adiatio of )aint r d.... t0 ul red...... 13 XFull red.... 2 Orange....... 60 Yellow.... 93 Full white.. 1,22 Thiltus we )provet that., as tlle new and more, rapid vibrations are introduced, the old ones becolme more intensc, until at a whltite heat the obtscure rays of a special refrtangibility reacht atn intenlcsit~y 122 times that possessed by themll at the comleccmentL This abiding' and atiugmcentation of the dlark rtays whenlO the bright ones are introduced may b1e Cxpressed biy the p'aseC persiste ce oQf ra1yS. (567) thait has been herc demonstr'ated rcglarlding an illcandlcsc:r nt platinum spir-l is also true of the electric light. Si(de by side writ, h this outftow of intensely Iltuinlous rays, we thave a corlesponding outtflow of obscure ones, The carbonp)oints, like the 1)latiumtl sptiral, may be ralscd from a state of ol)seurc warmth to br illintcey almost cqual to that, of thte sun, and, as this occurs, the obscur'radiation also risCs enormlouslAly in intensity. T lie investigation of the distribution of heat in the spectrium of the clectric light will yield us implortant rcstults, and will fitly prep)arie the wi for tihose experimnts onl invisible rays to wthicah I shallt siil)scquelntly direct yourl attelntioll, (568) Tlhe thero-clectric pile employced is tlhis beautifiul 390 IhEAT AS A 3MODE, OF MOTION. ins'trument already referred to as constructedl bIy ltuhlnolr-ff It consists, as you know, of a single row of elements properly mounted and attachedt to a double brass screen, It has in fiont two silvered edgecs, whic!h, by 11lmeans of a screw, caIIn be caused to close upon the p)ile, so as to render its face as nllalrow as desirable, reducing it to thle vwidth of t1he finest hlailr, or, indeed, shutting it off taltogether. 11y3 mlleans of a small handle and long screw, the plate of brass and the pile attached to it can be moved gently to and fro, andl thus the vertical slit of the pile can be caused to traverse the entire spectrum, or to pass beyond it in both directions. Th'e width of the speet rumn was in eaclh case equal to thile length of thle face of the pile. (569) To produce a steady spectrumn of the elcotric li}ght, I employed a regulator devised by AMT. Foucault and conlstructed by Duboscq, t1he constancy of whicth is admirlable. A coml)lete rock-salt train was constructed, tlei arrtangilent of which has been already iltdicate. In the fi'ont orifice of the camltl era which sulrrotmds the electric lamp was p)laced a lens of translparent rock-salt, intended to reduce to parallelism the divergenlt rays proccedilng from tlhe carbon-points. Thle })arallel b)eamn was!)ermitted to pass tlhrough a narrow slit, in front of which was placed anothler rock-salt lens, thle position of thlis lens, being so arralngred that a slharly-deilned ilmalge of tihe slit was oltainined at a distance beyond it cqual to that at whlich the spectrum was to be formed. Immlediately behind t1lis lelns was )laced a pure rock-salt prism (sometimes two of tlhem)..lThe bet1am was thus decomposed, a brilliant horizontal spectrl1m being east lon tim sicree n whicuh sore t h e thl 1the1oelectric pile. ]By turning the handle already referred to, tthe face of the pile could be caused to trtaverse the spectrum, an extremely narrow band of light or radiant heat falling upon it at each point of its march.* A seonsitivre galvtanollmet'er wzas connected with the pile, and fi'omn its deflection the heatint, — power of every part of the s)ectI-rm, visible and invisible,% was inferred. *'The width of the linear pile was 0'03 of an inch, IltI'T Ol EIECP tR1IO S)ECTRT1UM. 391 i(570) Two modes of moving the instrumenlt were priaCtised, t{he ctescription of one of whicht will be sufficient here. The face of the pile was brought to the violet end of the spee. tltrum, where the heat is insenlsiblc, and then moved, as I nlow move it, throulgh all thle colors to the red; tlen past the rce ut1) to the position of maximum I heat at ar atfterward beyond this position until the hcat of the invisible spectrum graIdually faded away.'1'Rhe following table contains a series of imeasuremt ntes executed in this manner. The motion of the pile is measured by turns of its handle, every turn corresponding to the s-hiftilng of the face of thle instlrument; through a space of one millimetre, or l 6th of an inch. At the beginning, where thle increment of heat vwas slow and gradual, tle readings were taken at every two turns of the handle; on quittting thte red, where the heat; suddenly increases, the intervals cere only half a turn, while near the maximum, \'where the changes wcre most suddeil, the intervals were reduced to a quarter of t turn, which corresponded to a translattion of the l)ile throughll, -1 1l of an inch. Tnte-rvals of one and of two turns were nafterwa rd restned, unt;il the heating-power ceased to be distinct. At everly halting-place the deflcction of the needle was notcd. Calling the maxinmllunmti effiect inl each series of experhimen}tits 100, the c-olumn of figure's in the following table expresses tlte Iheat of all tle ottther parts of tlte spect.truml )IsrTnTot;oN Ors H[EAT IN Sp'EUFrtU. o,' l,,ECTRIC,iOlIGHT, MC.1orlfle. intiellsity Iovemletnlt; ofptle. i ll 00t1 of the 11iaxitlnuiI. llefore starting (pile in the hiuc)... 0 Two ttu'ns forward (grccn entered) 2 "c (red entcrced)... 21 "c (M(extrcme red)..... 4 I[alf turn forward..... 0 "'. s..~... 85 392 II{'AT AS A MODE OF MOTION. Calorifie hltetllstvy Movlement ofrpile. It tOtl of thl 1tlX}~lt]tUlf, f{alf turn llrward....,,.. 96 Quarter turn forward) maximum. 100 "...t.. 97 f........ t62 ~,' *..... * * 36 Two turns forward.. 14'4 r. 9.. * O ( -.....,, J~ 5 i oards ~wbgn h i,)2 (571) itere, as already stttted, we b)egin in tnhe blue, and pass flst throughlt the visible spctrum. {Quittilg this at thet p)lace mnarked (" extreme rcd'"), we enter the invirsible calorific spectrulmt ancld reach the position of maximuln }eat., from which, olnwvard tlhe thertmal power falls till it Ipractically dis. ap)pearS. (572) More than a dozen series of suchl measurements vere executed, eacll series giving its ownt curve. On su)erposing the diflerent curves, a very close agreement was found to exist between i them. The annexed figure (fig'. 99), which is the meantil of several, expresses,'ith a close aplroxilmation1 tot accuracy, this distribution of heat in tile spectlrumll of the electric light from fifty cells of Grove. The space A BO c 1) represents the invisible, while C 0) ]: represents the visible radiattionl. W'Ve lhere see the gradual augmentation of tfhermal )ower', firom thle blue end of tile spectrum to the red. But in the region of cdatrk rays beyond the red the curve shoots suddenly ulpward in a steel) and massive peak......... a kind of Maftterhorn of lIeat; -...wliich quite dwarfs by its magnitude the po1rtion of the diagram rc)rcselnting tthe visible radiatio)n. (573) T'lhe sun's rays before reaching the earth have to pass ])IAGR1A-M OF JI[AT SL'PECTRUM. 39 l 4 ~b 394 HIEMAt AS OA AO1) OF MOTttOI. tlhro-ugh our at,1mosphere, tlhe aqut ous vapor of whichl exorciges, a pow'rful absorl)tion on the invisible calorific rays. Fromn this, apart from other conitsiderations, it'would follow tlat the ratio of the inuvisibtle to the visible radiation, inl the case of the sun, must; be less than in the case of the electric light., sxlerimenlf, we see, justifies this conclusionl; for, whereas fig. 98 show's the inlvisible raliation of the sun to be about; twicc the visible, fig. 99 shows the invisible radiation of the electric light to be nearly cight, timcs the visible. If we cause the belam from the clcctric lampl to pass through a layer of Qwater of suitable thickness, wve place its radiation in appl)roximatly the same condition as that of the sun; and oni decomposing the beam, after it has been thus sifted, Awe obtain a distribution of heat closely resembling that observced in the solar Sl)c-:l'ltlll, (574) The curve represcntting the distribution of heat in tlhe electric spectctium falls most steeply onl that side of flte maximum which is most distant from the red. On bothl sides) however, we havc a continuotts falling off. I have made numerous experimnents to ascertain whether there is any interrupt,ioni of continuity in t he calorific spectrum; but atll the mefasurements hitherto executed with art ificial sources reveal a gradual and continluous augmentation of heat firom the point where it first become.s sensible up to the mnaxinnmlium. (575) Sir Jolll I:[erschel has shown that this is not. thIe case with the radiation from the suln when analyzed by a flint-glass p)rism. Permitting tlte solar spectrum to fall upoll a shecct of blackened paper, over which had been spread a mwash of alcohol, this eminent philosopher determined by its drying-power the heating-power.of tlte spectrumt. jIf found that time wet surface dried in a series of spots re)presentilg thermal maxima separated from each other b)y spaces of commaativcely feeble calorific intensit Sy. No such mlaxima and minimi a wvere observed in the spectrum of thle electric light,, nor in t he spectlrum of a platinum wire raised to a Awhlite healt by -a voltaic c, urrent, risms 1and lncses of rock-salt:, of crow mt 8lIFTING OIF TI;1 JXT1'RIC IGIIT, 395 glass, and of flint glass, Awere employed inl these cases. inl other expleriments tihe beam intended for analysis wvas caused to pass through layers of water and other liquids of various thicklnesses. ases and vapors of various kinds were also in. troducced into the path of the beam. In all cases there was a general lowering of the calorific power, but the descnt of the curve, on both sides of the maximtuml wavs unbroken.* (576) The rays from anl obscure source canlnot, as already remarlkc d comlpete in point of intensity Awith the obscure rays of ta luminous so1urce..No body heated under incandescence could emit rays of an intensity comparable to those of the maxilnmul region of tile electrio spectrulm. If, therefore, we wish to produce intense calorific effects by invlisible rays, vweo must choose those emitted;>by an intensely luminous source. The' question then arises, I ow. are the invisible calorific rays to be isolated from the visible ones? (577)'l'he interposition of an opaque screen suffices to cut off the visible spectrum of thle electric lighllt, and lceaves us the invisible calorific rays to operate upon at our pllasurlle. Sitr'W.illiam I crshchl experimented thus whrlen he soulght, by concentrating them, to render the invisible rays of tihe sun visible. But to for1ll a spcotrulml il which the invisible rvays shall be completely separatedl from the visible ones, a narrow slit or t small apelrture is ncess-ary; and t.his circumstance rcnd6rs tlhe 1amololtlt of heat separable by prismatlic analysis very limited. If we wis-h to ascertain what the intensely concentratced invisible rays can accompllish, we mustlt devise some other mode of detaching fltem from their visible companions. W~e must, inl fact, discover a substance which shall filter the composite radiation of at luminous source by stopping tile visible rays and allowing the invisible ones free trantsmission. (1578) Thie main object of these researches was, as already intimated, to make radiant heat anl explorer of molecular conldition, and thle marked difference betwveen elementary and At, a ftltreo day I hop to sutbject thlis question to a tmorl severe e8( xamina tion. 396 Dg EAT AS A MODE OF MOTION. coipound bodies which the experiments reveal is, ill my estimation, a point destined to be firuitfill itl importanlt consequencets. As soon as t.his dlifikrence came clearly out in the case of gases, liquids were looked to, and the action of sutch as I wtas table to examine fell in surprisingly with tle previously observed deportment of gaseous lbodies. Could we thenl obtain a black elementary body thoroughly lhomogeneious-, and with all its parts iln plrfcct optical contalct, we should probtably findt it all effectual filter for the radiation of the stiun or of the electric light.. While Cuttilng Off tile visible radiation, tlhe blackl clement would, probh(ably, allow the invisible to pass. (579) Carbon in the state of soot is black, lbut its parts are not opticatlly continuous. In black glass t}le continuity is far more )cerfect,; and hence the result cstablished by MAelloni, that black glass possesses a consilertable powecr of translissio0n. Gold in ruby glass, or in a state of jelly )rle)arcd bry 1Mr. ftaradlay, is exceedingly transyiparent to the invisible calorific ray-s, but it is not black enough to quench entirely the visible ones. Tie dlc!sey-brown liquid brolline is better suited to our purpose; for, ill thicknesses sufticiertt to quench tile light of ourl brighlitest flames, this element displays extraordinary diatlhernlalle)'. Iodine cannot be applied in the srolid condition, }bult it dissolves freely inl various liquids, the solution in somte cases being intensely da1rk. I ere, lhowevcr, the action of the elcmeait may be masked b)y lthat of its solvent.l Iodine, for examplle, dissolves freely illn alcohol; but alcohol is so destructive of the extra-red rays, that it w0uld ibe enti rely utifit for experiments the object of which is to retain these rays, while quench-ll ing the visible Ones. The same remarl alpplies, in a greater or less dlegree to mallny other solvents of iodinle. (580) The deporttmentt of bisulphide of earbon, bothl as a vapor and a liquid, sug'gests t1he thought that it w ould fotrm'a most suitable solvent. It is extremely diathermic, and there is lhalrdly another substance ablle to hold so ]tlarge a. quantity of iodine iln solution. ICxperimclts al ready recorded (~ 50(6) prove: thlat, of thte rays clmitted:y a red-hol t )latiltiutn spliral, B}ISUILPIXDE OF CARBON. 397 941:'5 per cent,. is tralsmitted by a layer of the liquid 0'0k of ant inch ill thiclness, thile'tanllmissionll through layers 0'07 andl 0O27 of tan inchl thiclc b 8eing 75 anl 82'S respectively. Another experiment with a layer of greater tliccknss wvill exhibit the deportment of thle transparent bisulplhide toward the far more intense radiation of the electric light. (581) Thlis cylindrical cell, 2 inches in length and 2'8 inches in diamleterl with its ctlds stopped by pla-tes of 1)crfectly transp)arent rock-salt, was p)laced empl)ty in frolnt of an electric lalmp,; tfle adiation from tlme lampl, after having crossed the cell, fell upon a thellc o-clectric pile, an1d prodluced a deflect,ion of 73. Leaving the cell unldisturbed, the traul)arlcnit bisu:lliCide of carlbon was lpourtc d into it: tlhe deflection fell to 72~. A rleetition of tihe cxperimcntt gave the following results: ofilectson.'Through ept-l)ty cel.....'t Throughl bisulphidc. e. *. Taking' the values of thecse deflections fr'om a table, of calibration iand calculating tlhe trlansmission, that thlrolugh tile eml)pty cell being 1,00, we o)tain tlhe following rcesults: Tra)smission. From thl firtSt experiment. 9'9 per Cccnt. From. tile second cxperimtl t... 9lt'6 " Mean...9, t 8 tiellce the introduction of the lisulpthi'de lowers the thranlsmission only from 100 to 94'8.* (582) A pea:fet solvent of thle iodine would be entirely neutral to t;he total radiation; and tle hbisullphlide of carb-on is *x Thet diminution of the. refliction from thle sides of the cell by tho int, roduction of the bisulphide is not; here taken into account. 898 IElJAT AS A M0OD1 OF MOTION. shown by t the foregoing experiment to approach veriy near l)ctlfection. AVC have ill it a body Capaltt)le of transmlitting with little loss the total radiatioll of thec clectric light. Our object is now to filter this total,by tlhe introduction into the bisullphide of 1a substance competent to quenchll the visible and transmit the invisile rayst, L'at iodine does thllis with marvellous snarp'ness it is now mly bulsilness to prove. (583) A rock-salt cell, filled witl thee rtansparclent bisulphiidc of carbon, was pllaced iin front of the camera which containetd the whitc-hot platinum spiral. The transplarent liquid w\as then dralwn off, and. its place sulpplied by the solution of iodine. The deflctions observed in the rcsp)cctive cases ar as follows: ltRAIATIONs ROM V \F V I}[TEO-HlT PLATINUM. Through htanlslumant liquidt,. Through opaque liqutid.j3.9o 1388~ 1390 2' (5841-) All the luminous rays passcdi tllrough te tr lans)arent; bisulphidte, tone of them passed thlhrough tlle solution of iodine. Still we see whatl a small effect is produtcled by their withdrawal. Thle actual proportion of luminous to obscure rays, as calculated from the above observations, may be thus expressed:.Dividi.ngi the'adia0t.on, fromt a plain.tbton ~wi'e i'ised to a dctzzingy whitteness by an elect'ic current into twenty;foutz cqtzal parts, one of those parts is hoinmizoUs, and twenty-tthlree ob(585) A brighlt; gas-flame was substituted for thile 1)ltitillnu spiral, tlhe top and bottonl of the flame being slhut off, and its most brilliant portion chosen as the source of rays. The restult of fotrty cxperimcnts witht this source may be thius expressed:.Dividing the radiation. frome the most brilEiatnt port'ion of a tflame of cotr-gas into twenetyli/ive eqtawl pamrts, one of those Jparts Is lumiot(ts an(l tlwentll/;/]' obscut:re. ))tIATIIRMtl: ANOY OF' 10))INE,. 399 (586) I ncxt examined thoe ratio of olscure to luminous rays in the clectric light. iA bttery of fifty cells \as cml)oycd, 1and the rock.-salt lens was used to render the rays from the coal-points lparallel. To p)revent tLhe deflection from reaehlilg antl illconlvenictt magnitudc, the parallel rays wcerce caused to pass throughl a circular aperture 0'1 of an inch in diamllcter, and wcere sent alternately throug] tie t ransls)alrent bisulphlide -4,4 ) \ iV, /;! i — w.; g -.4 X. i oo.. COs': 0 ft *", ~. _. _~ - -~.=..'<,>, 5 === 5.. OO- O, O- _ O O.'' — o -;r: e,! _* -, ": _ —-------— ~:~i-:.. F... c-, CnC -*, g C) a j- -3 tf':,:h~illl........!:0!'!'..4 > __,_) - O.: s o, - _ _' _ s -. C) - A~ - ECO ii — C C) 03 C) o n -7~"ozo 406 IXIIAT A$S A MOD1l01 OFt' MTIOON. was poured tlthroug'h t he oponingl andl fell like a caseade to tem bottom of the receiver. Thle paper seemed to burtn without incandescence. HcLrc walso a thttrmog'raph of the coal-points DFo. 102. was staInpled out,.'When black paper is placed at the focus, \iwhere the tlhermal imate is welll dc{lced it is atlways pierced inl two poilts, anlswerilo'g to tile illnages of tlte two carbons. The superior heat of tth positive ciarbon is shown by thel fact that its image first pierces the paper; it burns out a large space, and shows its pecutliar crater-like top, while the negkative carbon usually lpierces a small hole. (605) Palper reddened by the iodine of mercury had its color discharged at th f places on which the, invisible image of the coal-points fell upon it, thoughl not with the expected )rompl.tless, (006) ])isks of paper reduced to carbon by different p'rocesses were raised to brilliant incandescence, bothl ill the air an-d in tlhe exhausted receiver. (607) In theise earlier Cxlerimtenls all a)paratus was mployed which had been constructed for otrher lpurposes. The mirror, for example, was one detached from a )Duboseq's camera.; it was ftirt silvered a{t t{he back butt afterward silvered in front. The cell employed for tlhe iodine solution vwas also, that which usually accompanies D)uboseq's lamp, being ilntended by its maerilment is going on; and, to find the quantityr thuls lost, tlhe first and last experinents aro nelcdcd,,In order to obtain thle whole heating-powver of the stln we must add the quantlity lost during the time of exposnirc, aAnd this quantity is the mean of the first and last observations. Supposing thie letter l. to represent the laugmentation of temperature by five minutes' expos1ur to tthe stun, and that t aind t' represent the reduct.ions of temperature observed before and after, then tile whole force of the stiunll, wich Nwe may call t, wvould be tlts expressed: t +} tl T It:.1... (683) The area of the surface, on w-hich tlhe sun's rtays here, fall, is known; tihe quantity of mercury within the cylinlder is also known:vl; lhence, we canll express tlhe effect of the sun's hent upon a given area, by stating that it is competent, in five minute's, to raise so miluch morcry, or so m1ucht water, so mlanty degrees in temperature. WVater, indeed, instead of mercury, ywats used ill. VPouillet's )yrlelilomnlter. (684) The observations were made at differcntl hours of the day, alnd, consequently, tlrough different t hicknesses of the earth's atmosplhere; augmentling from tlie minimum thickncss at noon, up to the maximum at 0 p.. -, w\thich uwas thle timle of the latest observationl. it w\as found that the solar llnrgy (liminished, according to a certain 1law, as the thickness of the air crossed by thoe siunbelams increlased; Iand, friomll this law, MA. lotuillet was enabled to infer tlhat the absorption, if the rays were dircected downward to his instruiment from the zetithl, would be 25 per cent. of tihe wlole radiation, l)oubt ACOTION OF1 TIIF. EART!I'S ATM0OSPlEIRfE. 4443 less, this absorlption wouldd be chiefly exerted upoll the lollgel unduhltions emitted by the sunt; the aqueous vapor of our air, not the air itself, being the principal agent. Taking into account thle whole terrestrial hemisl)hre turned toward the sunll, the amoiunt intercepted by the atmosphe1ric envelop is fourtelths of the entire radliation. Thus, were the atmosp)here removed, the illumlinated hemislphelre of the earth would receive nearly twice the amlount of heat from the sXun tlmt now reaclhes it. The total amountt of solar heat received by the earth in a year, if distributed uniformly over the earth's surface, w\ould be suffllcint to liquefy a layer of ice 100 feet thick, and coverilng the whole earth. It would also heat an ocean of flresh water 6600 miles dep, from the temlperature of melting ice to t~he temlerature of ebwullition. (685) Knowing thus the allnual receipt of the earth, we can ealculate the entire quantity of heat emitted by the sun in a year. Conceive a hollow splhere to surround tle sun, its centre being tile sun's centre, and its surfitce at t;}e distance of the, earth from thie sun. The section of the earth cut by this s urfiace is, to thie whtole area of t;he }hollow sphere, as 1: 2,t300,000,000; hence, the quantity of solar heat intercepted by thie earth is only - of the total radiatiolln. (686)'Thec helt emlitted )by tilh sun, if used to melt a stratulll of ice applied to the sun's surfce, would liquelfy the ice at the rate of 2,400 feet an holr. It would boil, per hour, 700,000 millions of cubic miles of ice-cold water. 1Expressed in another form, the heat givel out by the sun, per hour, is equal to that which would be generated l)y tihe combustion of t layer of solid coal, tell feet thlick, entirely surrounding the sun; lience, the heat emitted in a year is equal to that whichl wvould be l)roducld by the combustion of a layer of coal seventeen miles in thliclnless, (687) This, then, is the sun's expenditure which has been going on for agcs, without ourt being able, in historic times, to detect t}le loss.'When tihe tollingl of a, bell is heard at a. distantce, te sollorous vibratfionls arle lqulickly wasteld, ald re 444 1IIiAT AS A 30DI) OF MO TION. newved strokes are nceOssary to maintain the sound. Like the bell-... "Die Soenne ti~nt nachl alter Weise." ]But how is its tone sustained? Iow is the pclerennial loss made good..? We are at to overlookl thle wonderful in tlhe common. Possibly to mnany of us — and even to solne of tile most enlightened among us-.-thlte slil appei)ars as a ilre, difflering fromn our terrestrial fires only in thle magnitudle and intensity of its combustion. But what is tlte burning matter whilch can thus maintainl itself? All that wCe klLow of cosmical phlenomena declares our brotlelrhood with the sun-a —alfih'ms that the same constituents enter into the COmiposition of his 11mass as those already knownI to chemistry,. But1 no eartbly sutlstanl \with whiih Cf e are acquainted —^110no siubstalne which the fall of meteors has landed on the earthl-. rwould be at all competent to maintain tile stiun's colmblusttion.'.he chelmical enlergy of suchl sutbstances would 1be too weak, and their dissi)pationl too speedy. Werrc' the sun a block of butinling coal, and were it supplied with oxygen suffllicient for tthe observed emlission, it would be tutterly consumed in 5,000 years. On tile other hand, to imagine it ta body originally eldowed wvith a store of heat-.......- a lhot globe now cooling. ncessitates tlhe ascription to it of qualities wholly (difterent from those possessed by terre'strial mattoer. If we knew the specific heat of the sutn, we could calculate its rate of cooling. Assumingi tlhe specific heat to be thle same as that of water —-t.le terrestrial substance which possesses tlhe highlest specific heat-.-at its presenit Irate of emission, the entire mass of thle sunl would cool down 15,000~ Fall'r. in 5,000 years. IIn short, if tile stun be formed of lmattct like our own, some meanus lmust exist of restorinlg to it its wastedi pow r. (088) Thle facts are so extraordinary, that tile soberest hypotlhesis regarding tlhem must appelarv wild. The sun we know, rQotates upon his axis once, in about twenlty-five days; and the notion has been entertained that tlie friction of the peripherytl SUSTElNAN(C OF1 TIlE SUN. 445 of this wteel tt aaist something il stlurrounding space produtccs tile light and heat. ]ht whvlat forms the brake, land by wlhat agency is it held, while it rubs against tlhe sun? Granting, mioreoverT the existence of tile brake, we calculsate the total tmnoutlnllt of heat whlich the slunl could generatq lby such friction. We kn1ow his mlass, we know his time of rotation; we ]now tihe mchanicatl equivalent of heat; and, from these data, we can deduce, with certainty, that the force of rotation, if tentirely converted into hleat, would cover less than two centuries of emission.* There is nothing hypothetical in this cacultation, (689) 1 have already alluded to another theory, whlich, however b)old it may at first sight alpear, (Ceserves our serious attentionl —-tthe Meteoric T'lleory of the Sun., Kepler's cclc1)ratcd statemenit, that " there are more comets in the heavens thIan fish in the ocean," implics that a small portion only of the total number of comets belonging' to our system are seen from the earth. Bltit, besides comets, and planets, aind moons, a nutlcrous class of bodies belongl to our system which, from their smallness, might be regardcd as cosmical attoms. TJike the planets and the comets, these smaller asteroidls obey the law of gravity, and revolve iln elliptic orbits round thle sun.l, It is they whlich, whenl tXhey colle withinl the earth1t1's atmosl})lere, and are fird )by friction, appecar to us as meteors and fallingl stars. (690) On a bright ni;ghlt, twenty minlltes rarely pass at any part of the earth's surface without the aplpearance of at least one lmetcor. LTwice ac year (onQ the 12th of August and IL4th of November) they appeal)l) in enormous numbers. I)uring nine hlours in Boston, when they w-ere dlescribed as failling as tl1ick as snow-flakes, 240,000 meteors \were observed. The number filling in a year might, perhaps, )e estinmated at. hltildreds or thousands of milliollns and even thse would colstitute but a small pOrtion of the total crow\ d of asteroids thlat circulate round the sutl,. From the )icnltomena of liglht -.and * Mayor, )ynamitk (des l1Iinmtlm, p. 10. 446:f1EAT AS A MODE OF?MOTION. heat, and by dlrectc observa.tionls on 11 E]eke~s comet, we learnl that thle luniverse is filled by a resis{ting medium, through trhe friction of which all the masses of our system are durawn gradually toward tlhe s-un. And th3oughl t.he larger planets showvin historic times, no diminution of their periods of revo1lution, it may be othcerwise wvith the smaller bodies. In the time required for the mean distance of the earth to alter'a single yard, a small asteroid may have aplroached thousands of miles nearer to t he sunl. (091) Following up l thcoe roftlcottiolns we tnhould be 1cd to theo cotclusion that, while ann immeasurable stream of ponderable meteoric matter moves unceasingly toward the sun, it mtust augment l * density as it iapl)poachs its centre of convCrgc nce. And here the conjecture naturally rises, whcther tha/t vast nebulous mass, the Zodiacal Light, which lembraces the sutn, Imay not be a crowd of meteors. It is at least proved thJat t~his lumintous p)lcnolnelnon larises from matter which cireulates in obedience to pl)anetary laws; hence thte entire mass of thIe zodiacal light; must be constantly approaching and iltncessanltly raining its substalnce (lownll upon t)he sun.t, (692) it is easy to calculate both the maximum and the minimumn velocit.y, imparted by tile sunt's att'raction to anIl asteroid circulatintg round him, Thile maxinum is glloenerated wlhen the body approaches the st,un friom an infinite distance; the ent'i-e p.dll of the sunt being then exerted utpon it. The mininium is that velocity whbich would barely enable the body to revolve round the sun close to his surface. The final vlocity of the formcr, just before stril in I fhI Iill -ehl.vt oCf the to.- 0 ot, n i; le tl0 ) polts o0 the cradle inl whlicll the mountailleelr, by Awater-power, Iocks his b)al to sleep. All the foris of mechanlical motion 1here indictated are simply the p)arcclling out of an amount of caloriiflc motion derived originailly from the sun; land, at each Point at which the mcchanlical motion is destroyed or diminished, it is the sun's iheat whicl is restored. (709) WVe have thus far dealt withf t he sensible motions and energies whicll the sutn lroduces land confers; blut there aire other motion3 and other energics whose lacitions are not so o)bvious. tc'ccs. dS vt tabk. 5s ow p ti upo t al rth mtld, twhent burned, they give, rice to heat, fom whichl imnmolse quanltities of mlechaical energy are derived.'What is the source of this energy? Sir John Hforschcl alswered this question in a general way; while l)Dr. Mayer and Professor HIelmholt.z fixed its exact relation to tlhe more gener al questiol of conservation. Let me try to )put their answers into p)llil wortds. Y.ou see this iron rustf, Ioduced by the falling togetherl of the atoms of iron and oxygen; but) though you cannot see tlhis transparent carbonic-acid gas, it is formed by ItoO tdliltll t O xltL'yglictltd OX~tt'C}'l.lt lwse ltoms tllts unite(l ;,NEIRG;Y OF WOOD AND COA.,, 459 reselnble a weight resting on the earthl; their mutual att-raction is satisficed. ]But, as I can mind iup the weight, and preplare it for an6tfier fall, cven so these atolls canl be wound tup, separated from eachl other, alnd thus enabled to repeat the proccss of combination. (710) In the building of plants, carbonic acid is the material from wlich the carblon of th e planIt is derived, while water is tile substance from which it obtains its hydrogeln.?'The solar beama winds utp the weighlt; it is the agent which sevters the atoms, setting the oxygen free, and allowing the carbon and the hydrogen to aggregate in woody fibre. If the sun's rays fall upon a surface of sand, the sand is heated, and finally radiates away as much heat cats it receivcs; but let the same bea3mCs fall up)on a forest; then the quantity of hteat givcen back is less tlhan that received, for a. lortion of tlhe sulnbclams is invested ill the building of the trees. AVc have alI-ready seten hIow hteat is consumed il forcing astlndcr the atoms of bodies; and lhow it real)pears,when the attraction of the separated atoms comes againl into play.* The precise considerations which we then applied to lheat, we have now to applly to lighlt, for it is alt tihe cxpenscI) of tihe solar light tlhat thle chlemical (lecoml)ositionl takcies pflace. Vithtout the stil, the 1cduction of the carbonic acid and water cannot be effectced; and, in tis act, an amount of solar energy is consumcd exact, ly equivalent to tile molecular work done. (711) Combulstion is the revcrsal.of this process of reductioll and all the energ, invested in a l)lant rea)ppcars as heat, whlen the plant is burned. I ignite this bit of cotton, it bursts into flame; the oxygen again unites with its carbon, andt an amount of heat is given outl, equal to tlhat originally sacrificed by the sun to form tihe bit of cotton. So also as regards the "deposits of dynamical efliciency " laid up il ourt coa.l-strata; they are simp)ly the sutll's rays in a potential form. Are dligt froml our pits, allmally, eighty-four millions of tons of coal, tlhe mechanical cquivalent of whieh'is of almost fabulous vast* Chapter V. 460 HIIFAT AS A t10)rE OF 5MOTION. ness. The combustion of a single pound of coal ill one minute is equal to tihe rolrk of tbhree hundred lhorses for the same time. lt would require one hundred and eight millions of horses, working iight; and day wvith unimpaired strength for a yea'r, to p)erform an amount of work eq(uivalent to tihe energy which the sun of tihe Carbonifterous epochl invcstled in one year's p)roduce of our coal-pits. (7:l.) The further we pur-sue this subjcot, tile more its intcrest and its wonder grow ullon us. You have learned how a sun may be produced by the mere, exercise of gravitlating force; that, by the collision of cold( dlarl', )lanctaay masses, the ligoht arnd heat of ourr central orb, and also of the fixed stars, lmay be obtaincd. ]But here we ftlnd tile )hysical powers, dcrived or derivable fr'om the action of gravity uplon dead matter, introducinog themselves at the very root of the question of vit'ality., We find iln solar light and heat. the very mainspring of vegeta})le life. (7:13) Nor ctani we halt at the \vegetable world, for it., 1mcdiately or immediately, is the source of all animal life. Some anlimals feed directly oiln lants5, others feed upon their herbivorous fellow-creatures; but all in the long;-run lerive life and energy from tlhe vegetable world; all, therefore, as TIlelmioltz has lrcmallked, may tlrace their lilteage to the sun. Tn the animal body the carbon anl hydrogen of the ivcgetalle are again brought into contact withl the oxygen from which they had b-een divorcedl, and which is now sup)plicd by the lungs. Recunion talces place, anid animal heat is tle result.. Satve as regards intensity, there is no difference between tile combustioll that thus goes on within us and that of an ordinary fire. lhe products of comlbustion are ill both cases't he sate, namely, carbonic acid and water. Lookinlg, then, at the- physics of tile questionl we see that tlhe formation of a vergetable is a process of winding tup, while the formation of 1an animal is a process of running down. This is the rhytihm of Nature as applied to animal and vegetable life. (71,4) ]ButI is fCiere nothing in the human body to liberate ANIMAL POWER. 461 it from tlhat chain of necessity whicl t he law of conservation coils around inorganic Nature? Look at two menl upoll a, l1mountain-side, with tequal health and )hysical strength; t1he one will sink and fail, while tile other, with determined energly, scales the summit. Hlas no t volition, in this case, a creative power? PIhysically considered, the lfaw that rules the operations of a stceam-engine rules the operations of the climber. or every )poundt raised I) y the former, an equivaltc t quantity of its heat disapplears; anld, for ctvry step the climber ascends, eflan amounlt of hlct, ecquivalent jointly to his own we\ight and thil heig-ht to which it is raisetl is lost to his body. T'ihto strolng will can draw largely upon the physical energy funrnished by the food; but it can create nothing. The ftnlction of the vill is to 3applty and didrect, not to create. (7156) I have just said thfat, as a climber ascends a milountain, heat disapl)ears from his body; lthe same statement apIlies to animals pecrforming work. I!t would app)ear to follow hrom this, thlat the body ought to groV w colder in the act of climbing or of wLorking, whereas universal experienclle rovcs it to grow w armer. The solution of this seeming contradiction is fond in tlhe fact that,, whenl1 thle muscles are exerted, augml entced res)ilnation atnd'increased chemical action set in. f'The bellows which urge oxygell into t.1he fire' within are more briskly blown, and thus, though heat actually disapp)ears as w' climb, tie loss is more thanl covered by the increased activity of the chemical )processes. (716) By iecans. of a modification of the thermo-clectric pile, AMMA. Becquerel and 3Breschlt proved that heeat is developed in a muscle when it contracts. I MMA. Billroth and Fick have also found that, ill the case of persons who die from tetanus, tlhe temperature of the.mliuscles is someti1es nearly eleven degrees Fahrenheit ill excess of the normal temperatumre. M. HelmXholtz hias showln that the muscles of dead fr'ogs in contract'ing pl'oduce hleat; anl an extremetly important rcsult as r1egardts the intluence of contraction has been obtainedl by Professor Ludwig, of Vienna, and his pupils. Arterial 4062 I9I.A'T AS A MODE), OF MOTION. blood, you know, is charged with oxygen: when this blood plasses through a muscle in an ordinary uncontlracted state, it is chalngd into venous blood whlicnh still retains about 7-1 per cent. of oxygen. But, if the arterial blood pass throughl a Ceontiated:tl uscle, it is almost wholly deprived of its oxygren, the quantity rei-maining amomlting, in some cases, to only 1-i30per cent. As a result of the augmnllted combustion withill the muscles when in a state of activity, w\e have ant increased amount of carboinic acidt expired from the lungs. l)r. Idward Smith has shown tlihat the qvanltity of this gas expirced during periods of great oexrtion mIay be five t imcs that expired in a state of repose. (7fl7) Now, whentt we augnient the teml)erlaturo of tlhe body by labor, a porItion only of the excess of miolecular motion generated is tapplied to tihe performanlce of the work., Suppose a certain amlount of food to be oxidized in the body of a man in a state of repose, tle quantity of heat piroduced in the process is exactly that whichl we should obtaitn from the direct combustion of the food in an ordinary fire. But supl)pOSO the oxidation of the food to talke place while t-he man is performing work, then thIe heat generated in the body falls short of that which could be obtained from direct comlbustion. Al attmount of heat is missing, equivalent to the wvorkl done. Sup)posing the work to consist in the development of heat by fiiction, then the amount of heat thus generated outside of the man's Ibody would be exactly tlhat which was wantingi within his boday to make the leat there generated equal to that prodluced by direct combustion. (718) It is, of course, easy to determine thle amount of lheat consumed by a mountaineer in lifting his otwn body to anly eleovation. When lightly clad, I weigh about 145 lbs.; what is the amounlt of heatt consumed, in ly a11se, ill clilbing from tihe sea-level to the top of Monet Blane? "The height of the mountainll is 15,774 feet; and, for every pound of my body raisedl to a height of 772 feet, a quanttity of heat is consumed sufficient to raise the temperature of at pound of water 1O MAN WORKING AN lRESTING. 463 ]?al'. Consequently, oin climbing to a elight of 15,774, or about 2O 1. times 772 feet, an amoulnt of helat is consumed sufficient to raise the tcmpcrattlure of 145 lbs. of'water 201 —0 ahr. If, on the other hand, I could perform a glisstade from the top of the mountain to the sea-level, tile quantity of heat generated during the descent would he precisely equal to that consumted in the ascent. Vour attention has betn more than once directed to the energy of molecular forces, and here the subject appe1iars once more. Measured by one's feelings, thcle amount of exertion nlcessary to reach the top of Miont B1lano is very great. Still the energy wYhich performs this feat wxould be derived from thle combustion of about two ounces of carbon. In the case of anI excellent steam-engine, aboit one-tenth of the heat employed is converted into work; the remaining nine-tenths being wvasted in tle air, the condenser, etc. Tn thle case of anll active mountaiticer, as mucell as one-fiftth of the ]heat due to tile oxidation of his food may be converted into olrk; hellnce, as a working-machine, tile animal body is much more pclrfect than the steam-engine. (719) Wec see, Iloiwever, t hat the cnginel and thle animal derivec, or0 may derive, these powers f'om tile self-same source. wtc cant work atl engine by the direct collmbustion of tile sublstances which we emnploy as food; and, if our stomachs wvere so constituted as to digest coal, we should, as Icemholtz llas rellarked, be able to derive our enerllgy fr'om tlis sublstance. Thle granld point p)ermanent throughout all thtse considerations is, tlhat nothingry is c'atedf. \We can 1tmake no nlovclment w'hich is not accounted for by the contempllorallncous extinction of solme other movement. And how complicated soever the motions of animals e;may be, \whatever elmay lbe thCe change whichl tihe mtolecules of our food undergo within our bodies, the whole energy of animal life consists in t lhe falling of tile atoms of carbon, and hydrogen, and nitrogen, from the highi level which they occupy in thle food to the low level whichl themy occupy when they quit tile body. ]3But what has cnabled the carbon * Phil. Mag. 1850, vol. ix. p. 610. 4i4 1t11hAT AS A MODE) 0F MOTIO N. and the hydrogen to fall? What, first raised theml to the level whlichl rendered the fall possible? Wae liave already learncd that it is the sun. It is at his cost tlhat animal heat is p)roduced, and animal motion accoml)lishc d. Not only, thetn, is the sun chilled, ftllat we may have our fires, but hie is likewise chilled tfhat we may have our )powers of locomotion. (720) The sulbject is of suchl srast importance, and is so suare to tinge the xwhole future colrse of p)hilosophlic thought., that I will dwell upon it at little longer, fland cndeavor, by reference to ailalOgical p)rocsses, to give you a clearer idea of the part p)hycd by t lhe sun in vital actions. Nre can raise water by mechltanic(al (action to a high level; and that water, in dcscending by its own gravity, may be mlade to assume a variety of forms, and to perform various kinds of mechanical work. It may be made to fall in cascades, rise in. fountalin8s, twvirl in cddies, or flow along a uniform bed. It may, moreover, be cmlployed to turnl wheels, lift hammers, grind lorui, or drive piles. Now, there is nlo t)oower created by the water during its descent. All the energy which it exhibits is merely;the 1arcelling out and distribution of the original encegy which raised it up onl high. Thus also as regards the complex motions of a clock or a watch; they are entirely derived from the energy of the thatnd which winds i: t up). Thus also the sitrilng of tle little Swiss bird ill the ilnternational Itxhibitiotn; thle quiverting of its artificia.l organs, tthe vibrations of the air which strike the car as rmelodr, tfhe flutter of its little wings5, and all other motions of the pretty automak:-ton, wtere simply derived from the force by which it was wound up. It; givYs out noth;ing that it has not received. In t his plrecise sense}, yol will perceive, is thle cnergy of man atl adnimals the parcelliing ou{t and distribution of an energy originally exertcet by the sun,. In the vegetable, as we have remarkcd, the a{ct of clevation, or of wintding up, is performed; atnd it is during' the descent;i, in the anim{l al, of the carbon, hydroglen, and nitrogeen, to thle level from which thcy started, that the powers of life appear. ATOMIC MACHIINERY. 465 (7t2) 3But thle question is not yet exhausted. The -water whic i we used ill our first illustration producc s all thel motion displayedl ill its descent, but the Jbrm of the motion depends on the character of the machinery interposed ill thle patll of the water. And thus the lprimary atction of the sun's rays is qualified by tle atoms and molccules among which their plower is distributed. Molecular forces determin e tie frm wthich tie solar e0nerg' will assume. In the one case this energy' is so conditiontd lby its atomic l machinerIly as to result in the formation of a cabbag'e; in another tase it is so conditioned as to result. in the formation of an obak. So also as regards the reIlion of the carb)on and the oxygen.-tle thebrmi of their retunion is detelrmlined by the molccular machinery throtugh which tlhe combining forc e acts. n one case the action may result in tlhe formation of at man, while in another it )mayt result; i}n the formation of a g,'tssho)tpper. (722) The mattert of outr, bodies is that of inorganic Nature. There is no stsubstance in the animal tissues whichl is not primarnly derived from the rocks, the water, and the air. Are tlec forces of organic matter, then, different in kind from those of inllorgnic? All the philosophy of the )present day tends to negative t he question; and to show that it is the (lirecting anll compounding, in the organlic wolld, of forces belonging equal1y to the inorgalnicl, tlat constitlte the mystery and thle miracle of vitality. (723) In discussing the material combinations which result in the formation of tie body and thle brain of man, it is impossiblc to avoid takilng side-glances at the p)hlnomena of conscioulsness and thought. Science has asked dari1ngt questions, and will, no doubt, conthille to ask, such. Problems will assuredly )rsenit themselves to menl of a future age, which, if enunciatcd now, would a)l)pcar to most people as the direct offspringl of insanity. Still, thltough the progress and develot)monCt of science maly seem to be unlimited, there is Ia region beyond her reach, a line with which she does not even tend to osicutlate. Given the masses and distances of the planets, we 4606 IEAT AS A MOD)E O1 MOTION. can infer the l)erturblationls consequent on their mutual attra-e. tions. Given tie nature of a disturbatlce ill water, air, or cther, w, canll infer from the properties of tlhe meditm how it-s particles Nwill be affected.:Int all this we deal wvith physical laws, and the mind runs fireely along the lince which connects the phlclnomelzlna from berrillinlg to ent. }But when we elnde.avor to pass, by a, similar pr0ocess, from the region of physics to t, hat of thought, we meet a problem not only beyond our present powers, blut transcending any conceivable expansion of the powers we now possess. WVer may thlink over the subject again and again, but it eludes all intellectual )rese1ntation. The origin of the mnaterial universe is equally inserlutalble. Thus, having exhausted science, and reached its very rim, lthe reatl mystery of existence still looms around us, and thus it will ever loom.il-..ever bevond the t)ourn of man's intellect — giving the poets of successive ages just occasion to declare that "We ae sasuch stuff As dreams are made of, and our little life Is rounded by a sleep." (724) Still, presenlted rightly to the mind, the discoveries.and generalizations of modern science constitute a poem more sublime than has ever yet addressed the Iullan imagination. The Iuatulral p)hilosopher of to-daty mlay dwell amid concel)tions which beggar those of AMiltol. Look at the integrated energies of our world...... -the stored power of our coal-fields; our winds and rivers; our fleets, armies, and guns. What are they? They are all generated by a portion of tIe sun1l's energgy) which does not atlount to -a-3-0o-t0-0-0 -- of tile whole. Thiis is the entire fraction of the suln's force intercepted by the earthtl, ad we convert but a small fraction of this fraction into mechanical energy. Multiplying all our po)wers by millions of millions, we do not reach the sun's expenditure. And still, notwithstfanding, this enormous drain, in the lapse of human history we are utle)l to detect a diminutionl of his store. M easured by our tlarges terrestrial standlards, such a reservoir ,1IMITS OF' SU UN. 467! of power is infinite; but it is our privilege to rise above these standards, and to regard the sun himself as a speck in infinite extension.-....a Melre drop in the universal sea. Wre analyze the sl)ace in which lie is immersed, and which is the vehicle of his power. We pass to ot.her systems and other suns, each pouring forth energy like our0 ownN, butt still wit hoult infriingemenlt of the law, twhlich reveals irmmutability in the midst of change, whvich recognizes incessant transference or conversion, but neither final gain nor loss. This law generalizes the aplIorism of Solomon, that thlre is nothing new under the sun, by teaching uts to detect everywhere, under its infinite variety of appeaIrances, the same l)rimeval force. The energy of Nature is a constant quantity, and the utmost man can do in the1 pursuit of physical truth, or in the applicattions of physical knowledge, is to shift the constituents of the never-varying total, sacrificing one if lie would lroduce another. The law of conservation rigii(dly excludes both creation and annihilation. Waves may change to rip)lcs, and ripples to waves....... -magnitude lmay be substituted for number, and number for magnitude.. asteroids may aggregate to suns, suns may invest their energy in flor'a and fatunte, antd floriu alnd fau may melt in air...... the lux of p)ower is eternally the same. It 1rolls in music through the ages, while the manifestations of physical life as well as the display of physical phenomena are )but the modulations of its hlytllm., 4168 1l~ET tI t AS A rODE 01: OF MOTION. CIIAPTI.'4, XV. AO1'ON O' }IW-;;t;RWAYE 0' OP SHlOR' P11llttl) UPON (AS. EOUS MA.'tLT'R —-OLOUDS }'OtME.D B3Y AO'INIS IO )ECOOSt't:IO~X —-COI.O PODUCOED BY $M MALT,,i...FL...POtABIAT'tOI OP',IOGHI' BY NIUtL.US MfA1Pt:RIIT —-E- WIXU}V O''11} SKY AND'ITM, POXLARIZATlION OP ITS I~tGH:T. (725) AQTOU h-ave had laid b)efore you an abstract of the,oi. principal researches which have occupicd m)y attenttion for the last teln yeart. It tiese investiat1ions, my chief aib was to rendler the longer waNves of the )prisllmatic spectrum interl)roters anid expositors of molecular conditioni, and wev wound up our 13th Chalter by filtering the waves of visible period from those of illvisible period, and then broke tu) the larger heat-waves so as to cnable them to )roduce all. t1he )phenoimena of light,. iUnike the l)eautiful researches of Melloni and Knoblauchl the investigations here referred to malde radianit h1lt a inmeans to an end. I endeavorcd to place before my mind such itnag:c1 s of molecules and thecir constituent atoms,as modern science crenld rs lrobable, and stucht images of the lumiiniferous cther and its mot,ionls as the undulatory theory of light enables uts to form, and to found upon t.hcse conceptions exl)erimental inquiries whichl should give us a more sure 1an1d certain ]hold of molecular constituttion. (726) One result, a' mong many now known to you, of these researches is, the sudden change of relations, betwccnl the ether of space and ordinary matter, whiclt accomt )allies theC act of combinat:ion. Prescrvingo' the quantit y and ultimate quality of the tmattcr traversed by the ethereal waves constanllt, vast CJfIEiIAXT1 ACTION OF SORT $ AW ES. A 9 olchanges in the amount of wave-motion interccplted may be produclC d by the act of chemical union. If Initrogn tand oxygen, for example, be mixed mechanically together in ttle 1)prol)ortion, by weight, of seven to four, radiant heat will pass through the mixtutre s through a vacuum. At all evnts, the (ulnantity of heat intcrcc)tcd is multiplied a. thousand-fold thle 10momenl1t the oxygenl and nit.rogen combine to form laughnllggas. So, in like minnoer, if nitlogen and hydrogen b)e mixed mechanlically in thle prol)ortio ln of fourteen to three, the amount; of radiant heat which they absorb in this condit ion is mult iplied by thous-ands- -it may be by millions-.- --—..thet moment they unite chemically to form ammonia. No single experiment shows the air Nwe bre1athe to )be a mechanical mixture, and not a chlemical compound, with the same conclusivelless as t!hat whlich proves it to be as p~ractically pervious as a vacutm to the rays of heat. (727) B3ut the molecules -whiclh, like those of ammonia and laug'hing-gas, can intercept the waves of cther, must be shakien by those wayelcs.. — — possibly sakLn, asulnder. That ordinary thelmometric heat: can produce chemical chlanges is one of tlhe *commonest facts. 1adiant heat also, if sufficiently intense, and if absorbed Nwith sufficient avidity, could )'roduce all the efteets of ordinary tlihermometric healt. Tle dalrk rays, for Cxamliple, which can make plati lln white-hot, could also, if absorbed, )produce t;he chemical effects of white-hot platinulllm. They could, for example, decompose wvater, as thefy can no w in a. moment boil wvater. But the decomposition in this case would be cffected tlhrough the virtual conversion of the radiant heat into thernmometric heat,. T'here would be nothing' ill tle act characteristic of radiation, or demanding it as an essentital element in the decomplosition. (728) The chemical actions for which t he radiant form scces essential are mainly produced by the least energetic rays of the spectruim.'t.lTus tilhe p)hotog'ra)hcr hals iis heatfocus in advance of his hemlical focus; which l attcr, though poteint for his special )utl')OSe, l)oSSCsses allmost, infilitely less 47 O II',AT AS A MO)DE] OF MOTION. mechanical energy than its neighbor. Thle mechanical energy depends upon tlhe amplitude or range of vibration of the individual particles that constitute a wave of other. Now, th:t heat-waves thave enonrnously greater amplitudes tfan the chemical wavecs; hence decomposition is less a matter of amplitude than of period of vibration. ThIe quicker motions of thfe shorte1 and wveaker waves are so related to the pcriods of vilbration possible to the atoms that, like thle timncd impullses of a boy in a i wing, thley accumulate so as finally to jerk the atoms astlnder; thus effecting what is called chemical tdecomp)ositionh. (729) It is this jerkinlg asunder of the constitluent atoms of molecules that we have to examtine during the coming ]hour. Our previous investigatlions dealt with tfhe action of thle long waves; this withl the action of the short waves upon gaseous matter. Vapors of various kinds were sent into a glass experilmenetal tube a yard in length, and about, three inches i'n diameter. As a general rule, the va-pors were perfectly transparent; the tube, when they were p)esenlt, appearing as eml)ty as wihen tlthey were absent. In two or three cases, however, a faint cloudiness shoAwed itself within the tube. This caused ne a momentary anxiety, for I did not know how ftr, in describing lly previous experiments, actions might fhave been ascribed to pure, cloudless vapor which were really due to those nIt\ly-observed nebulah. Intermittent (discomfort, how6ver, is the normal feeling of the investigator; for it driv\es himtl to closer scrutiny, to greater accuracy, and often, as a consequcelce, to new discovery. It was soon found that the nebule reveale(d by the beam were all gener aecd by thle beam, and the observation opened a nlew door into regions inaccessible to sense, but which emlbrace so much10 of tle intellectual life of the physical investigator. (730) What a,'e those vapors of which we lhave been speaking? They are aggregates of nzoletcles, or small mnasses of matter, and every molecule is itself an aggregate of smal:ler p)artt called atoms. A molecule of aquetous vapor, for exam Cl IlMXMIAlj ACTION OF SHIORT WAYES. t41 pie, consists of two atoms of hydrogen and one of oxygen. A molecule of ammonia consists of three atoms of hydrogen and one of nitro.'e:n, and so of otllher substances. Thus the molecules, thcmselvc s inconceivably small, are made up of distinct parts still smaller. When, therefore, a compound vapor is spoketl of-, the corresponding mental image is air.-aggregate of molecules set)art'tcd friom each olther, though c excedingly lnear, each of these being composed of a group of atoms'still nearer to eachl otrher. So much for the matter which enters into our conception of a vapor.* To this must now be added the idea of motiot,. The molecules have motions of their own tas wholes; their constituent atoms have also motions of theiir own:, whlich arc executed ideptendently of those of the mloleciles; jiust as the various movement-s of the earth's surface are executed indepcendently of tile orbital revolution of our llanet. (731) The vapor-molecules are kept asunder by forces whicl, virtually or actually, are forces of repulsion. Between thelse elastic forces annd the atmospheric pressure under which the vapor exists, equilibrium is csttablished as soon as the proper distances between the molecules have becn assumned. If, after tJhis, thoe molcciles be urged learer to each ot her by a momenitary force, they recoil as soon as the force is cxpecnded. If, by thl exercise of a similar force, they be separated more widely, when the force ceascs to act, they agaill approach each other. Tlte case is different as regards the const.itucent atoms. (732) And here let me retmark that wve are now upon the very outmost %verge of molecular physics; and that I aml attempting to familiarize your minds with conceptions which halve not yet obtainced universal currency oven amlong chem- Newton seemled to consider that thle molecules might b0 rendered visible by microscopes; bhut of atomS lie appears to have entrttained a difblrent opinion. lie finely remarks: " It scems impossihlle to see tle more secret and ntoble works of Nature withi tile corpuclcs, by rson of tlheir t;nspac('.tt... (1eirschel, "On ]Light," art.t 1145.) 94 12 11XUTlEA' A$ A MODE, OF MOTION. ists; whtich manyll chemists, moreover, might deem unctenable, ]ultt, tenable or untenable, it is of the highest: scientific importance to discuss telm. Let ius, then, look mlentally at our attoms grouped together to form a molecule. 1Vveiy atom is hlcld aplart from its ncighbors by a. force of repulsion; why, then, do not the mututally rep)llent mncembers of this group1 part company? The molecules 7do separate from each otlher,whcnll tfle xternal lprcssule is lessened or removed, but the atoms do not. The reason of this stability is, that two forces, the one attractive and the other repulsive, are' ill operCatioll betwccn1 every twVO katoms; anlld the position of every ato.ll.its distance from itlowss os-mns detcrmincl 1y the equilibration of these two forces. If the at0oms collme too near, relul8sion predominattes and drivces them ap)art; if too dist'ant, attrlaction prcdomi1nates and draws them together. The point at which ittraction and repulsion are equal to each other is the atol's position oj' e.tuilibrim. If Inot absolutely cold —.. and tlhere is 1no such thing as absolute coldness ill our corner of Naturclle — the atoms are always in a state of vibration, their vibrations being executed to and fro across their position2s oJ etcilibrintU (133) Into a. vapor thus constituted, Awe have lnowt to }polur a1 beam of light..; itlt. what., in the first instlance, is a betam of light?.You kn\ow it is a train of inlumerable -waves, excited in, and 1propagltce throughl, a alllost iillitCly-a ttenuatcd and elastic medium, which fills all space, and which we name the.1jthc,. You know that tihese wiaves of light are not all of the same size; that; some of them are much longer aind higher tftan others; thlat trhe short waves and the lonig ones move with the same rapidity through space, just as short and long waves of sound travel with the same rapidity throug11 the air, and that hence the shorter wNaves must follow achl other in quicker succession tlan the longter ones. You know thatt the diftfrent rapidities withi w1tich the wav lrc s of light impingee upont the reti lla, or 0optic 1cltvc, give0 rise ill consiousness to ditterenccs of color,; that ther are, howcvcrlu ntlulberlesss waves eimitted by tlhe sultl and other lutminos bodies Nwhiclt Cl[EMIOAIL ACTION OF SHORT WAVES, 473 reach the retina, but which'tare incomtpetent to xcxite the sensation of light; that, if the lengths of thie waves exceed a certaimn limit, or if they fill short of a certaint other limit, thtey cannot generLate vision. And, it is to )be particularly borlnc inl llind, thati the capacity to p)ro(lduc liflet does not depend so tmu1ch oin thle strength of Itle waves, as onl their period(s of re(734)'{The elements of all the comncetions withl which wo shall have subsequently to deal ar1e in ow in your possessiol. And you will olserve that;, thioughl we are speaking of things which lie clltirely beyond tlhe range of thle senses, the conce)ptions are as truly meelxanical as they would be if w\e were dealing with ordinary masses of matter, and with wNyaves of sensitble magnittude. (do not lthink that any really scicntific mind at thle piresent day will be disposed to drawl a. substantial distinctionl )betwcen chemlicat and elChitanical phentlomena rihey differ fronm each othller as regards thle magnitude of tloe masscs involved; but in this en se the plhenolnmet a of astrolnonly differ, also, from thlose of ordinary mechanics. Thl maiil bent of the natural philosophy of a futtture age will probably:be to chalstenl into order:, 1)t subljecting it to mechanical law s, tle cxisting chlaos of clhemi 1ical )lhenomena. (735)'Whecther we see rightly or wrongly -. —-l. whiether our notions b)e real or imlaginart -t — it is of thte ut1tost ilportlance in scietce to aim at perfect clearness in thte description of all thatt comls, or seems to colte, withini the ranuge of the iittellect,. F.or, if we ale right, cle'arncss of utterance forwards the cause of rigilt;; while, if we are w0rong, it inslurlcs tlhe speedy colrrection of error, In this spirit, antd withi the deterillnation at all cvents to speak plainly, let uts deal witht our conccl)tions of cther wa\tves and molecules. Suplposing a wavce or a train of waves, to implinge ptlort a molccule so tas to urge all its parts Nwith tlte same motion, thte molecule twotld ltmove bodiiy itts a whole, but, because thtey are animated )3' a 0comwzonr motio, there would be tno tentdency to its constituent atoms to separate frol each, othcrs. DDOt'rentitl motions 4'r4 IIA'T AS A )MOD1E1 0F MOTION. amlongo the atoms trhemselves would be ncccssary to cffect a separation, anld, if such motions be not introduced by t he shock of the waves, tfhere is n1o mechanical ground for the decomposition of the molecule. (736) It is, however, difficult to conceive the shock of a wave, or a train of waves, so dist.riibutcd among the atoms as to cause no strain among them. For atoms are of different; weig'hts, probably of dliffrent sizcs; at all events it is almost certain that the ratio of the malSs of the atom to the surface it presents to the action of the ether-waves is different in differclIt cases. f this be so, and I thinlk the probabilities are immensely in falvor of its being so, thelln every wave which passes over a molecule tends to tdecompose it. —tends to carry away firom their weightier and more sluggish' companiolns those atoms which, in relation to their 11mass, present the largcst resisting surfaces to the mot.ion of the waves. The case may be illustrated by reference to a man stanlding on the decek of a shilp. As long as both of them share ecqually the motions of the wind or of tihe se~a, there is no tendency to separation. In chemical language, they are in a state of coim bination. But a wave passing over it finds the ship less rapid in yielding to its motion than the man; the man is consequently carried away, and we lhave wlhat may be roughly regarded as dccomposition. (737) Thus the conception of the decomposition of cormoiund molecules by the waves of ether comes to us rccommcnlded by a priori ltrobability. /But a closer extmination of the question compels us to slpplement., if not materially to qualify, thlis conception. Itt is a most remarkable fact that theO waves which have thlus far been most effectual in shaklting asunder the atoms of compound molecules are those of Ieast mechanical p)owerI. Billows, to use a strltong comparison, are inlcoll)etent to produce effects which are readily prolduced by r'ipples. It is, for examlplc, the violet anld ultra-violet rays of the stun tthat tre most. cffecttual in producing these chemlical ldecomlpositions; and, complared vith the red an1d ult ra-red solar rays, the energy of these " chemical rays " is infinitesimal. OIEX1MICALT ACTION 1OF S1OUT WAYvES. 475 Thiis cnerg'y would )probably in some cases have been multipliced 1by millions to bring it up to that of tile ultra-red rays; and still the latter arc p)owtlCss where the smaller waves are potent. We here oblserve a remarkable similariity betfween the blehavior of chemical molecules and that of the human rctina. (738) MWhlence, then, the power of these smaller waves to unlock tfhe bonds of cmohical union? If it be not a result of their strenglth, it Ilust be, as inl the case of visionl, a resutlt of their periods of recurrence. But:how are we to figure tlis taction? I should say thus: the shock of a single wave produces no more than an infinlitesilmal effect upon an atom or a molecule. T'o produce a larger effect, the motion must accIumulate, and, for wave-imlI)ulses to accumulate, they must a'rivc in perio'ds identical with the periods of vibration of the atos oln whicl they impinge. In thlis case each successive twave finds the atom in a position tNwhich enables that wave to add its shock to the sum of the shoce llcctl inl tle hands of acht of you. \,Vell, this blue that I have been looking at turn.s out to be a bit of more perfect sky than the sky itselft. ooking across the illuminating b)eaml, as we look across the solar rays at; the sky, we olbtain not only partial polarizattion, but:pefect )olarization. lIn one position of the Nicol tiel blue light seems to P)ass unimpeded to the eye; in thle other it is absolutely) cut olf, the eXlperimentll tube ibcing rediuced to optical emptiness. Blehind the experimental tube it is wvell to place a black surface, in order to plrevent foreign light friom troubling tile eye. in one posit ion of tile Nicol this black surface is seen writhout softening or qualification; for the particles wittin tile tube are thesel es invl\isible, and tIle light wilich they reflect is qucnlecd. If tile light of tile sky were polarized with the salme perfection, on looking' properly toward it thlrough a Nicol, we should also meet, not the mild radiance of t;hte firmamentat butt tife unillumclde blacknuess of space. (764) The colsthruction of the Nicol is such tlant it pcrmits to pass through it vibrations which are xecuted int) a certain * Possibly a photographic impression might be taken long before t1he blute beicomes visiblec for the ultra-blue. r eas are first reflected, f For which I have to thank the obliging kindnetss of Dr. Mtaxwell Silmp fi )l, DI)RElCTION OF V1B]31ATION ROUNJ) BE1AM. 491 dcltrminato dircctionl) tanId these only. All vibrations executed at right angles to this direction are compltetly stopped; while compontcnts only of those executed oblitquely to it are transmtsitcted. It is easy, therefore, to see that, from the position l llin which the Nicol lmust be lheld to transmit or to quenlch the light of our ineip)ient cloud, Ae can infer the direction of the vilbrations of t hat light.i, Youl will be able to 1)icture those vibrat.ions without diflicultyr. SupposC a line drawn from any l)oint of thle "' cloudl" 1 )clrpendiculari to the illumning'ating beam. Along that line, the palticles of ether which carry the light from thle cloud to t eye yvibrate inl a direction pcrpclndiclular both to the line and to the beam. And if any numberlll of lines be drawn in the same way from tfhC cloud, like the sp)okes of a wheel, tihe particles of ether alongl all of themll oscillate in the same manner. VhYcrefore if a pltane siftce, be imagined cuttirng t he inlcipient cloud at right angles to its lengtlh, the perfeltly polarized vibrations discharged laterally will all be parallel to this surface. rThis is the lanoe of vibration of the polarized( light. Or you may suti)OSC a circle drawn round the expierimental tube, and a series of stzrings attachcl to various points of this circle. If tall the cords be stretched as p)erpe1ndiculars to the experimental tube, and caused to vibrate by a series of jerks imparted at rigiht;angtles both to them and the tube, the motions of the particles of the strings will then re)present those of the )particles of cther. A distinct itmage of those vibratiollns is nolw I hope0), ill your minds. (7965) Our incipienlt blue cloud is at virtual Nicol's prism, and between it lland the real Nicol we canll )roduce all the offccts obtainable between the lpolarizer and analyzer of a polariscope. When, for example, a thin plate of selenite, which is crystallized suliphate of lime, is placed between thle Nicol atndl the incipient cloudt, we obtalin the sllendid chromatic p)henomena of )olariz(ed light. The color of the gypsum-pllate, as nlanly of yolu know, depends upon its thickness. If this be 11unifor, the color is nifor11b. If, on the con'tra3y, the plate be wedge-shaped, thickenling grtadually tamld untiformnly froml( edge 4092 IltUTA A8 A MODE OF M3OTION. to back, we have brilliantL bands of color produced parallel to the crtge of thle w\edge. Perllips the )best form of plate for experiments of this character is that now ill my hand, vwhich vwas t)preplared for me some years ag'o by a man of genius in his vay), the late Mlr. I)arker, of Latmbtlh. It; consists of a pl)ate of selelite thin at the centre, anld gr1tadually thickening tovward the circumftrcence. Plaachig thlis film bet ween the Nicol tlnd the cloud, wve obtairn, instead of ta series of ttarallel bands, at systelm of c ored ri. e colors are ost vivid when the lncil)ient cloud is looked at t)ertlcdicularly. Precisely tlhe samec plhemnomelnla. are observed when ll w Ne look at the blue firmamenlt in a direction petrpendicular to tlhe solar rays. (766) WVe have thus failr illuminated our incipient; cloud witI ordilnarly lightl, and found the I}ortion of this light reflectcd laterally from the cloud in all d(lirections round it to be pelrfcctly )olarlizCed. We wrill tnowN examcsaine the flfects 1')roducctl when tlhe light whvich illuminates the cloud is itself polarized. In front of the elet1ic lanp), and betwxeen it and the experimental tube, is; J)laced this fine Nicol's prism, which is sulfl ciently large to emabrace awd to polarize thle entire beam. lThe prism is now p)laced so thalt thle l)alne of vib)ration of the light emlergent from it, and falling ulpon ttle cloud, is vertical. Hfow (does the cloud behatte toward this light? This fobrmless aggregatoe of infinitesimal particles, without defiite d structure, shows the two-sidclncss of the. ligliht in thle most st.riking manner. It is absolutely ilcoll)Ctc lt to refltect upward or down-'ward, ywhile it frlcely dischar ges the light horizontally, right and left., I turn the polarizinl; Nicol so as to render the l)lane of vibration hoirizontal; thle cloud now freccly refltcots tlhe light vertically upwarld andl downard, but it is absolutely incompetentl to shed a ray horizontally to the lrightt or left. (767) Sul)pose tihe atmllosphere of our p1lallet to be surrounded by an envelop imperviou s to li,'ht, witth an al)crtll, onl tle smlward side, through which a, solar beam could cniter. Surr(oundc d oni all sides b)y air not directly illutlinated, the tracl of the sunlifght would resemble thlat of thi electric be1aml1 NATURAL AND) ARTIFICIAL A/ZUJll COMlPARt'I). 493 in a dalrk space filled with our incipient cloud, The course of the stnbeamll would be blue, and it would disclarlge laterally, in all directions round it, light in precisely the same pol'arized condition tas that disceltrgced firom the inci)illnt cloud. In ftact, tte azure rtfcalcd by the sunit cn i wo\l(d be the azure of such Ia cloud. And if, instead of permitt. inl the ordinary light of the sun to enter the a)ertlure, a Nicol's Iprism were placed there, which should polarize the starlight on its enttrance ilto our atmosp)here, lte particlcs producing tihe color of the, sky Awould act p)reeisly like those of our incipient elotld. Int two, directiolns 3we sNhould hlve the solar litght reflected; inl tYwo others tunrefllected. In fact,: out of such a solitary beam, traNversing the unilluminated air, we should be able to extract every eftect shown by our incipient cloud. In the p)roductionl of such clouds we virtually carry bits of the sky into our laboratories, andl obtain with them all thte effects obtainable in {hle open firmament of heavcn. (768) A(nd here, had. not a sufficient strain been already imposed( upon your minds, I: might enter )upon the description of a series of extrlaordilmry ffel cts observed when the particles of our inci)ipient clouds are allowed to augment in size, so as to appro-ach the conlition of true cloudy matter. The selenite ring-system, already referred to, is ai most delicate reagent for the detection of polarized lightt. WVhen we look ior'wtmilly, or perpendicularly, at ttln iltcipient cloud, the colors of the rinlgs are most vividly developed, a, diminlltioni of tile color being immediatel)y atpll'irit when the incipientt cloud is reg'ardced oblictuelt. But; left us continue to look ti.hrouglt the Nicol and scelnite nort mally at the clouql: the particles a lugment in size the cloud becomes coatsel andl whiter, thle strength of the sclenite colors becomingi gradually feebler. A.; lengtlh the cloud ceases to discarge.polarized lightt along' tle nornmal, and the selenite colors e(ntirely (isiapipear. If now the cloudt be, regarded obilfquely, the colors are rcstored, very vividly, if not, withl their first vividness and clealrness. Thus the cloud th;Lt has ceased to dischtrge )polarized lighlt lt righlt angiles to 494 HIElAT, AS A 3MODEIl 01F IOTION. tile ilimllttinatilug beam, 1) pours out such light ol)tiously int oblique directions. The direction of maximum olttarizationt changes with the texttrtle of the Cloud. (769) B]t; this is not all; and, to understand, even l)artiatlly, whatl rcttain8s, word must be said regardingr thie appeartince of t he colors of our plate of selenitc. If, as before stated, thle )plate be of uniform thickness, its hue in white poliarized light; is unliform. Suppose, theln, that )by, arranging the Nicol thle color of the plate is raised to its maximum brilliallcy, and suppose tlhe color produced to be green; oit turning the iNicol roun(1d its axis the green!ecotres faiintcr. Wthcn the, angle of rotation amouilts to 45 degrees, t}he color disappears; vwe then pass what may be calldt e a ne utral point;, where tue selenito behllaes, noot as a crystal, but as a bit of amorplous glass. (1Continuinf toe rotation, a color reappears, tbutt it is ito longer green, bu)tt'ed. JThis attains its maximlum at a distance of 45 degrees from the nevutlral poiint, or, inl other -words, at a distance of 90 d(egrees from thte position whiichl showed tt green at its maximnum. At a furtlter distance of 45 degrccs front the position of lmaximuXt1 red, ttle color disappears a second time. VWec have tlhere a secottd ncutral point, beryond wltich the green colmes again into view, atlaining its nuaxintuin brill-:iancyr at thl end of a. rotation o(f 180 (degrees. 3 y thoe rotationl of tloe Nicol, therefore, througlh an angle of 90 degrees, AIwe p)roduce a color compleiCentary to titat wit;h which we started. (770) As imay')be ilfcrred from this result, the selenite ringsystem changc s its claracter wllen the Nicol is turneld. It is possible to ltave tthe cclltro of tloh circle dark, tle surr'ounding rings being vividly colored, TIhe turnting of itoe Nicol through niatn angle of 90 degrccs renderss the ccnttro bright, while every pointl occupied by at certain color in thte first instance is occupie2d by ttle comiplerent of that color in tLie seconld. 1tlt what am 1 aiming at in tlhese long lreliminilaryy statemenlts? I want to 1)0 able to stay, with full assuranee of being unttlerstood )by everybody )presenlt, titatt a cloud may so alter its textture as to produce upon at liglt an eftect equivalent to the rotation of ithe COLORS OF So"XElt.Xl' 490 Nicol through 90 degrees. lyv curious internal actions, not here to be described, the clouid ill our experimenltal tube sometimes divrides itself into sections of difflrent textures. Some scctions are coarsetr than others, while it often happens that some are iridescent to the naked eyc, and others not. Lookilug normally atsuch a. cloud through the selenite and Nicol, it often hal)pens that illn )assing from, scction to section tlhe Nwhole character of thle ring-system is changed. You start with a scction producing a darkl contre and a corrcsponding system of rings; to pass to another section through a* neutral point and find ill that scction the centre brig/it, and at the samel radial distances find each of the first rings displaced by one of tlhe complementary color. Somncetimes as many alt s four such revcrsions occur in the cloud of 1anl experimental tube a yard long. Now, the chtanges here indicated mean that in p)assingi, from section t-osectlion of the cloud the plalnle of vilration of the pol'arized light turns suddenly through' an angle of 90 (legrees; this chlange being cltirely due to ttle diflerent texttie, of thie two parts of tlle cloud. (771) You will now be able to understand, as far as it is capable of beling ut(ledrstood, a very beautiful effect which, under favora-ble circlmnstances, might be observed in oulr atmosl)here. This experimental tube contains an ilnch of the iodide of allyl val)or, the remaining 29 inches necesstary to fill the tube being air, which has bubbled through aquecols ht)drochl11l;ic acid. Besides, thcrefore, the rvapor of iodide of atlly, wt have those of water and of acid within the tube. the light has been acting on the mixture for some time, a beautiful blue color being rodtucceed. As before stated, tile " incipient cloud " is wholly dillerent in texture and optical l)ropcrtics from an ordinary cloud; l)ut it is impossible to precipitate the aqueous va)or wlitin this tube so as to cause it to formt a cloud sililar to those of our atmolsphere. This new and retal cloud will be precipitated inl the midst of th;le azure of the ini)ientt cloud. An exslhatusted vessel of ablott onei-tird of the ca)aelity of the experimental tube is now connected witli the 4906 HEAT AS A MODE, 01 MOTION. tube, ithe passage unIiting both being closed by a stop-cock(, O11 openingl this cock, the mixed air antd vapor will rush from thile expcrimental t. ube into tile empty vcssel;; and, in consequiecenof thle chilling due to rarefaction, thie tapor ill the experimental tube will ftall togetlher as a trute cloud. You a Inow prepared for thle experiment. I first loolk at this azutre, so as to obtain a vivid ringl-system withl a dark centre. lurning onl th1e cock, tile air' is r1'refied t1and thle cloud )precipitated. What is toire result? lInstantly the centre of the system of colored rings becolmes bright, and tlie whole series of colors correspondiing to definite; radial distancs, comelllcmlelltarl. AAWhile I continue to look at the cloud, it gradually melts away as an atmospheric cloud mightl do in the azure of tleaven. And tfhere is our azure also remaining behind. The coarser lo1ud seems drawn aside like a veil, the blue reappears, the fitrst ring-system with its dark cellntire and corrcsonllingly colored circles, being restored. (772) Tihe vision of an ob)jectl always implies a. dilffercltial action on itot retina of tile observerl. The object is distitngtuishled frolt surrounding space by ils excess or detfect of light in r0elation to that space. By alteritngl tlte illumination, citlher of th1e object itself or of its environment, wecX- alter the al)l)pearance of the object. Tl'ake thle case of clouds floatinil in thIe atosplhere withl patclhes of blute between them. kAny thling that changes itoe illumination of either alters thle ap)pearance of both, t;hat apptearance depeceding, as state(d, u1)0on diflbrentihal actioll. Now, the light of thle sky, being poltarizcdl, nmay, as you know, be in grcat p)art quenehed by a Nicol's prislm, while tthe lighlt of a cloud, being unpolarized, cannot lbe ttus extinguisheId. Ifence thoe possibility of veiry rcmarkablle variations, inot only ill tfhe a.spect of tom firmament;, which} is really clanged, but also in tihe aspect of the clou1(s which have thalt firmtament as al background, When a reddish clolud att sunset chances to tloat inl tlhe region of maxiIlnn n poltarizatioln, thte quenceling of tle sky behind it causes it to flaish with a brighlter crimson01. L1 43ast Est'er-eve the l)artmoor sky, wh\icll ASPEC0T'S OF T11E AIPS, 4907 had just 1)eell cleansed by a snow-stornm, wore a a very wild api)elltralc. J!oiund thel horizonl it was of stccly brillianco, while reddish cumuli and cirri floatel southlward. iWhen the skiy was qucttchcd bchiid them thl se floating masses sccmed lik(e dullt elb)ers suddenly blown upon, brig'htening into firc. In thte Alps we have the most ntagnificent exanmples of crimlson clouds t anl snow\s, so that the effects just referred to may be Cere studied,under thle best possible conditiolns. On thle 23d of Anugust, 1869, tle evening Aipcn-glow was very fine, ttlough it; did ot reachll its itmaximlum depth and splendor. Towvard sunset I walked iup the slopes to obtain a better view of tlhe ~Weisshorll. The side of the peak seen from thle B3l All), being turned froml tihe sun, was tinted mauve; but I wtished to see one of thle rose-colored buttresses of til moumtain. Such vwas visible from a point a few hundred feet above thie lhotel. T5'he'Matterhorn also, tlough for the most10 part in shade, h]ad a crimsonl projection, while a deep, ruddy red lingored along its western shoulder. IFour distinct lpeaks and buttresses of the D1)o,1 in addition to its dominant lhead -a.ll covered witlh pure snowt. —.'.wecre reddened by the lighlt of suntlscet;'Thoe shoulder of tle Alp)hubel was similarly colorcd, while tile grecat. 1mass1 of the l3etschorn l' as all aglow, and so -wtas tle slowy spilne of the Monte Leone. (773) L'ooking at tlhe Weisshorn through tlle Nicol, the glow of its protuberanc e was strong or weak according to thle positionl of the l)isl. I, Tle summit also undl rwent a change. In one positioll of the prism it exhibited a p)ale white against a dark background; in tle rectangulatr )osition it was a. dark matve against a light background. ttie red of tile Matterhlor chlanged in a similar mannerl; but tie whlole mnountati also plassed through strikilng clanges of definition. The air at the time was highly ofpalesceent —..tdllcd, i ct, with a silvery lazc, in whicell tile Matterhorn almlost (listnppearedl. Tlhis could be wiholly quenchled b t6 Nicol, and thllen thle mounltain splrang foirtll with tstonislling solidity and detachment from tile surrounllding air. Tle chlanges of tile l)om wcre still m1lOlro 498 IfIEATT AS A )MODE OF MOTIONT. wonderful. A. vast amount; of light could be removed from the sky blehind it, for it occupied the position of maximuml polarization,.lhen the sky wa8s quenched, the four mlinor peaks andt butltresses and tlh stmmit of the I)o0lm, together withx tl he shoulder of the AlPllubel, glowed tas if set suddenly ol tfir. This wtas itm1lntediately) dimmed bl y turning the Nicol through an angle of 900. It was 1not the stoppage of tlhe light of tihe sky alone which )produccd this startling effect; tile air between the B]el Alp) and the I)omn was, as lS have said, highly opalescent, and the quenching of tlhis intermdiate glare augmentcd'remarkably the distinctness of the mountain. (7741) Oni tihe morning of th(e 24th of Ai ugust similar eftfects were finoly shown. At 10 tx. t. all tlhree mountains,s, the )om, the Matterhorn, and thle Weisshlornl were )powcrfll l affected by tlhe Nicol. 13tt ill teils istance also t}he line drawn to tile Dom being accurvately p)eCelndicular to thCe direction of t-he solar sadtows, and consequently very nearly lpc) prndicullar to the solar lbeams, the effects on tbhis mountain were most striking. The gray sutmmit of the IMatterhorn at thle samte time could scarccely b distintguishlcd from the opalesccnt haze around it;' but, when the Nicol quenclied the haze, tlle summit becallo instanltly isolated, ntld stood out inl bold (lefllition. It is to be remembered that, il the producttion of these effcts, thle only things changed are the sky belhind andt the luminous ]haze ilt front of the mountains; that thes te ar lchanged becaus the light emitted fromn the sky and from tlhe haze is plane pokuirzewtd light; and thatl; tie lighlt from the snows and from tile mountains, beilng scensily unpoltarized, is not dirlectly afiected by the Nicol. It w.ill also be understood that it is not the ilntcrposition of the haze ais a - optaqte body that rcnders the mountains indistinlct but that it is:tfhe light of tle lhaze which dinms and bewilders the eye, and thus weakenC s the definition of objects seen t.lnhought it. (775) These results have a direct bearing upon what arlists call "i adrial lersp)ectite." As we look from the summit of the Aletschhorll, or from a lower elevation, art the serried JIANDSCAPL'N COLORS, AERIAl, PR$lSPECTIVE,' 499 crowtd of )peaks, especially if lte mounltains be darkly colored.covered wNvith )illes, for exanplc —eevery )ealk alnd ridge is Sclarated tfrom tlhe mountains behlind it by a thlin bitie lhaze which renders tlhe relations of the mountains as to distancle tlunmistakable. \When;) ts e is caze i8 realrded through the Nicol pierplcldicular to the suln's rays, it is in many cases wholly quenllchd, becIus tlhe light whtich it emits ill this dircction is wholly polarized. When this happenls, aiirial perspcctive is abolished, and lountains very differently distant tappear to rise i1n the satme vertical lplanle. Close to the t3cl All)p, for instance, is the gorge of tie Massa, a river produced by thle alblation of the M\ltsch Glacier, and beyond tile gorge is a hlig-l ridge darkcned by tilnes. This ridge, may be projected uponl the dark slopes at thle opposite side of tihe Rhone Valley, alndl bet;ween otl we halve the blue' haze referred to, throwing lthe distant mountains far away. ]3ut at certain hours of thie day thlis laze may bie quenchled, and tlhen tlle Mass-a Rlidge antttd tihe mlounltains beyolt d tle ER hone sccmlm alIlost equally distant from the eye. The one appears, as it were, a vertical continuation of tlte otler. The lhaze varies itll the tepll)erature and hIluidity of thie attlmosipliere. At certain timles and places it is almost. as blue as ti:e sky itself; but, to sce its color, tle attentionl must ibe withldrwn a ron tle mountains ald from tle treeccs which cover tihemi. In p)oillt of fact, tle 1laze is a piece of more or less perfect. sky; it is prIoducedi in tile sal mannlller, and is subtject to thle same laws, as the firianlet itself. We live 1t tlhe sky, not u1der it. (776) These points were further elucidated by tile deportment of the selenite plate. On somlle of the su1nny days of August the Ilaze in tile valley of the hlloone, as looked at from tile Bel All), was very remarkable. Toward evening tile sky above the mounltains opposite to 3mly place of observatioln yielded a serics of tIe most splendidly-colored is-rings; ltnt, onl lowering tile sclenite ultil it had tile daikness of thle pinele at the opposite side of the Rilone Valley, ilstead of thle darlrhe5ss of space as a. baclTIONS. MY friend Mr. Ingleby Ilas directed my attention to three articles published in tlhe Mfonthly Magazine for 18P20, vol. ii, pp. 33, 12,9, and 505 by a writer who signs hlimlself Coimon-Sense. The first a-rticle CONCLUDING ADI)lJTION. 613 is headcd l ]"Electricity and Galvanismt explained oin tlio Mect lanical Thelory of Mattelr andt Motion.x' The second is "1 On the Nature of iMotionl and the Laws and Phenomena -of its 1)ropagation." The third is entitled " New Views of thel Economy of Animal Nature in accordance with tlhe Theory of LMatter and nMotion." These titles indicate thle character of the writter's thoughts. With a good deal of unavoidable error, these articles disllay in many cases a power of penetration, and a truth of insight, altogether remarkable for theo tile. Take the following quotations as exasmples: " Butt in a certainl \variety of cases transft r of motion doees not prolduc ctlange of place; and this excelption gives rise to a new sot of ptllenot,ena. Thus, if two bodics moving in contrary d(ireetions iml)inge against one anotl0her in a line vwhlitch joins tlhe centres of their mastses, the disposition to change trhe place, in both is dest royed, and apl)atrcntlty ttheir motion.'The notion, how ever, in such case is not destroyed; but only changes its appearance, anld is ilparted to tle atoms of the body, which by the collision are thrown into active vib)rations, representiltng the previous motions of the bodies. Aggregaite motion is thus colnvertcd into attomic, m~otiols, and these give rise to many coniplicated and curious phenomena, as ill heat, light, and gas." 1'The italics are here the author's own. Until Mayer and Joule al)pearced, more than twenty years subsequently, nothing coamplarable aS regards l)recisioll and completeness to thle flregoing statement, to nmy knowledge, found utterance. Indeed, some of the l)lbrases employed by myself mightl fairly be regarded nas having bee copied from this anonymous correspondent of the onthlty Magazine. The second one of the articles above referred to is thus sumtmed u1): "'That all forco, all weight, and all power of bodies, are derived firom thle mlotio01, or motions, imparted to them or possessed by them. And that forcee, weight, -and motioln, are convertible ternms and p1ysical synonymes.'2. That every force, weight, awr motion, is generated locally by its own set of proximtate cattses or motions. "3. That, althougl motion constantly chanlges its fsubjects an11 its mode of exhibition, yet no mottion is either lost or created, but is ilt constant cirtculationl anld varied approp)lriation.'4(. That,motions of aggregates are convertible into motions or vibrations of atoms, and vie crsa,; the lmutual conversion producing mianlly classes of' 1tphenomena. 614 ~,,EAT AS A MODE 0 F MOTION. "' 5. That action and rac tion, inertia, resistance, and fi'iction, a so many phenomena of partting'with imotion, and of reciving, and dividing it with a moving body. ". That the mediumin l which a body in atomlic motion is sitItated, conveys away the atomic motfion, till the excitement exceeds its powers of transmission; when heat', evaporationl gaseous plroduction, liglht, and decolmposition, take place as variciies and accelorated degrees of atomic motion.'7A. That atormic motion is lheat; and, being parted with from the air in the act of respiration, creates anlimal heat and vital etion., " 8. That all local imXotions on the earth are derived fribm the deflection of the eartlh's mot:ions" j [ho missed the part played by the sun's rays}l, " and ar finally returned to the eartlh. "9. Motion ill all these inquiries andl dete'rminations is to )c con-t sidered as the secondary causo of the sublie agelncy of tEteral Omnipotence." IXn his article onl thle Elconolmy of 4Jnimal Nature, lie says:' JlAnimalls consist, therefore, of a ~basis of bones for strenlgth.- -of a continuity of muscles for motion —-..of a medullary system of brain and nerves for sens-ation, comparison, anld retention. —-of r' es.piratory organts bfor alpropriating gaseous atomic mtotion -.and of arteries and veins for circulating nutriment and excitation to thte wvhole. "'A steamnboat deriving its internal energies from an engine wrought by tihe altl0ernate i:tr'oduction iand fixation of aqueous gas, and put into motion by tihe reaction of wheels against water or land, is exactly coarsely analogous illn all its olerations to a locomlot-ive aninal, which derives its eternal energies friom the fixation of atlmos lpheric gas, and its locomotion froim the reactions of the feet or body ag,,inst thte earth." Thmus do great questions sintlmr before they receive complete ox)l'reSSion. PRODI3)JTION OF IlFRE 1, 13Y SAYAG]E:S. J:Wract from1 "Adventures namong the ])yakls of Bornello," by F. lBomY:. Publlshed by ]H[urst & Blackett., 1865, pp. 67, 68. " AmoJng some of the l)ygak tribes there is a manner of striking fire ttmulch more ext'fraordinary. The instttlenilt used is a slenlder CONCLUDING ADDITIONS. 515 cube of leIad which fits tightly in a case of bamboo. Thie top of the cube is hollowed into a Cupl), and, when fire is required, thli: cupt is filled with tinder, the leadenl piston is held upright in the left hand, the batlboo case is thlrust sharply down over it, as quickly with(drawn, and the tinder is found to be alighlt. T'hte natives say that 3no rmetal but lead 1wil l produce the effect. I must obServe that wo lnever saw this singular m1ethotd inll use, though the officers of the raja soeemed acquainted with itt." MORNING CHILL PROI)UCING SNOW IN A R1OM.;"A umIOf.us plhenomenon mlight also bo observed, at:Et'zroo'i, upon the door of one of the slubterranean stables being olen~, when., although tXhe day was cleatr and fine without, the warm ai r w ithin immedilately congealed with a little fall of snow; this might be scen in great perfection every morning onl the first opening of the outer door, when th]t house was warm fr'om its having been slhut u11 all nlight." 1' Tle prece;ding sentence is contained in a work by the Ilonorable,t Curzon, ntitled' Armenia: a Year at lrzerooin and onl the Frontier.s of Russia, Turkey, and Persia,' and is quotedl il the Athewe11mm1n, 8th April, 1854, 1p. 4131, all 1st coltlumn1, tfrom whuich it is 1hero transcribed." [The writer of these lines had heard mo give Diove's aceount of a fall of s$now in a Russian btt, 158, note. coolicint of expansion of, 69. Fluorescene of sulplihate of quiniune - absorbs those rays which it emits, in thle in isille spectrti, 813. 411. experimeints on tiieqrcscencc and - radiation from a luminlous jet of, calorcscellee, 608, d seq. 526. o-.signalling, app)licability of radi- Gaseous condition of matter, 60. tnt heat to, 646. Gases, coistittiiowof; 62 65. Foot-ounids, explalnation of, 38. -^ velocity of particles of; 64. Forbes Prof J. J 1), his viscous theory - expans}ion ot, by' heat, 60. of ice, 235. - specific heat of simple)C and coinF1orce ofC heat in expanding bodies, pound, 169, d seq. 105 -..collnductivity of, 291. at v i:up)poscd conservative action.firnt experiments on their absorpof, 20. tion of heat, 3q2. Forces, m0lecular', energy of, 99, 158..-.mode of expelilent implr ox ed, 38... -polar, heat required to over-..(diibrent )poiwer of ccepting me. come, 168. tiont from the ether, or difkrehnce in. colnertibtility of nlatural, remarks albsorption possssescd by, 397, 412. on, ). 503....diffierent powers of imparting moF1'ranklaud, D)r., his experimenits on tion to thle ther, or dilkience in Comb)ustion, 56. radiation possessed by, 413. Franklin l Dr), his experiment on col..- table of dynamicn radiation of, 455. 01o 63. -- chemieal action of slhorxt waIves of Fraunhotfr's lines, 6044. ether upon, 425, 438. Freezing eftect of, ol wVaterl - pipes, Gassiot, iron cylinders blurst by, 99. 100. G(auze witre', cause of its stopp)ing pas-.. point lowered by pressutre, 123. sage of flame, 288. of;water produced by its own (eIysevr, the great, of Iceland, descrip. evap)olatioll, 18, lion of, 142. togeether' of pieces of ice, 231. 1 Bunsen's theory of, 145. F'rictionl, genleration of heat by, 6. -p Iroduced inl lectulrexroom, 148. -. against space, heat developed by,.... its history, 151. 30. (lacirt, fotrmation of, 228. Frost, mieans of 1)reserving p)laints.mtotion of, deseribed, 229. frollm, 654. -...... point of swiftest motion shifts, 229. - cause of their preservation, 654... their dualy ito of mtotion, 229. Fusible alloy liqueftied by rotatiotu in - viscous theory of, 230. tiiagnetie Ftiheld, 360. - regelation dtitto, 232. Fusiolln, oit oft effect of p)resire on, ancient, evidencel of, inll valrious 121. places, 236, d seq..-.....- hypothesis to account for, 239, 1 AITVrANOM11'71'ER described, 3. et seq.'.... note ol the construction of, ---.cold alonte cannot l)roducec, 240. Appetndix to Chap). I. 1). 18. (laisher, his table of nocturnal radia)peculilarity of, in high deflections, tioi, 654. 348. Glass, wrhy cracked by hot water, Mellouni's method of calibrating, 105. Appelndix to (Ihalp. X. p. 295. - broken by a grain of quartz, 1006. INDEX, 523 The Numberis (except where p. is prefixed) refer to the Tlm,'rap ds, ahtnl not to thle Pages................... Ginsa, optacity of, to heat, 3.19. ieant imparted to a gas under constant. absorptiol of heat by diftlrent lrecssure, 69. thlicknlesses oft 366. a. —. —* nt constant volume, ire-screelns, use andt philoso)phy 1 3. of, 363. prodcced by stretchin g India-rub. Ginclin, his deflintion of heat?, 1. ber, 110. Gore), his experiments on revolving -.- direct conlversionl1 into mlechanical balls, 118. motion, l 117. Gravity, velocity imparted to a body --- dcvceloped by elctricity, 118, 261. by,,it. -. l)crtformance of work by, in steamGrease, phlilosopltic tuse of; oiln wieels etgine, 140. and axles, C9. M. llirn's expertlintts, 140b, 4 Guhlf Streaml, 223. seq. G.!ypsum, powdered, b)ad cotnductionl -.- w Xo'er of, ill expanding bodies, 168. of hteat by, 285. -.... two kinds of motion produced in bodies by, 15 9. AIl'AtiMONICA, chemical, 301. - interior work performed by, 160. I. lHeat and cold, opposite tfiects -..- -.consumcl, in forcing atoms asunulponl thermo-lcctric pile, 4I. der, 16I, 189. Hleat, generated by meclanical pro- - generated by atoms falling to. cesseus: 6. getler, 160. fiictiotl 6, 13. -. quantity yielded 14) by different. comp- O treCssiotn,'7. bodies ill cooling, 106................ prcus siottl 8. - speci/ic?, 166, et seq.. —...- - allintg of mercury or,water, -- auses chanmtge of state of agg'r8. gation inl bodies1, 1~6. Col conslmptio n of, in work, 16. -- latent, of water, steal, and aqute-- nature of, 1, 20, 61, otls vapor, 116, seq., 240. -- a motiont of ultimate particles, 20. ---- definitioll of, IlI. cotsidteretl tthus, by Locke, 20. -....- generated int passing from liquid..............aco.. ot.'an, 20. to so(id state, 186... consideted a motion. of ultima;te - caluse of more equal distributlion particles by Rtuilnlord, 13, 23. of, 216, 223..........- Davy, 23. -.-. coyvcel6on of; 221..... dleveloped when air comprcssed -......- ncessary for the production of 26. glancics, 24 i, -. —- -..- motion of air stoppedt, 16,....... distinction betwelen it and ordinary 2'?. mnotiont, 242..by rotation in magitaetie fiehl, -- coadtuctitonr of, defitled andl illus36. itated, 2.43. u.eelt.ehanical equivalent of, 38, /56,...- -. — -. not equal in all substances, 9J1, 244. proportional to height througlt - *-lmethod of determniitg tile conwhich a. body falls, 4t0. diuctibility of bodies fort, 24?. - relation of, to velocity, 42, 46. - -— 1and electricity, relationiship of, a. —.. anualtagonist to cohesion, 69. 260. — of f'ictiotn, tll tuford's essay on thte - motioll, of, itterrferes wsith tile mosource of; Appenldix to Chapl l. 11. l)- tion of elctrlicity, 262, 651. ~ — ctonverstion of, inito potential etier-(x- J:Ylpf siont of gases by), 6'. gy, 266.....-.. liquids by, (. *.- ( ifltrenee of cotductivity of, in..- -A.:- solids by, 103. crystals andl wood, 268, 4t seq. 524t INDEX.''toe Numllbe rs (excSpt. \ero ). is profixed) re,'r to thle Plaragaphll, an not to tle I'aes,. tat, tIrainsitnission of, through wood, I Ifeat, amoumnt of, equivalent to tthe 92556 26,9. rotationl of the smull, and the revo. influenced. — intltlcncd aby the eclhanl- 1l1tion of the ll)1mets,'t02. Cal state of the beotdY, 285. - rays, intvisible, experiment's on,. dotubltfild contdtltion of, by hydro- 614, gen1 gas, 2091 -...-. polarizationl of, p. 611,.. — its passage through a vacuum, 297. Hcight, iflluence of, on combustion, a nalodoy oft to sound, 302.. -..... to wlhat lmotionl of, il)nl tcd, 300. cIlinholtz, his ealculation of tihe. —. adimtt, 30)t lhea t that wouldt e deveilopeld by.r. rlays beyvold visible spectrum, 311. stoppage of earth's motionll, 40... obepys the siame la ws as light, 323.. — remarlks- on til exhaustion of action ofo on xygen anld hydro- the mechaniceal force of our s'sgelt, 330. ter)n,'Ct. lawns ofitsvcersesquartcs ajppliedtl to, rIfels, toinatic, action of their 337. odors on axdinllt heat, 439..tia.tran8svesal \Iundulation of waves lterschcl, Si XWillialn, his discovery of, 339. of tile obsclulre ralys of thile spcc-...-..quit of, 360, trill m 31, 560....... transllisin sn of, through opaque -. — Sirt J1ohn note on rock-salt, 3'13. )bodiesa, $07. -. — measulerclnts of solar ra...ef ct of on ice, 300. a liation, 081..h... (bsxriplJt of, bY l ascs, flret'mode imirn, M., hins experimentss oin tlhe of experimentclt, 3712, steamengine, 1410b, dt seq...Mne. -..- I.ens of detecting 1 Itfnboldt, oni tle coal of Central mniinte amount of, 386. Asia, )192. abtsorption of, iby ases, iiproved tluyghlcis, his theory of light, 303. appml'atus for reseiarches 0n, tde- lydrogen, collision of atoms with scribed, 38'. oxygenll, 910..-lfeu pass-age of, through (dry aire, -.vlocity of its particlets, 64. oxygen, lydrohoen, and nitrogen,.-...- nollunt of Ihefat generated by comi 411). b)ilnlr witht oxygen, to formt water, - tables of alsorpltion of, biy gases 1/9. atnd vapois, 14 it, 418 21, 433, 43, -4 - cooling efti ct of, oin heated bodie-s, 439, 522. 293. s-,pettrumi, detached fromln ti --. l fo"0 power of aibsorption posscssedt tiots, 432. by, 392, t12. -- absorption and radiation of a gas -..- tflame, radiation fronm, through lioi Vapor decermlined without ex- qulid andt vapors, 530, 6512, 517. turnal heat, 1449. -- --- character of its radiation, 047..'l. absorlpition l oft by aqueous vapor,.... --.exper ilments on its radiation, 14(0, et scq. 5i48, e seq. c —. ltetrurll rladiation of, thte cattse of dew, o65ie2,. (EI liqufited by fr'ictioto 2, and:-. altlioutt of, generated iby collision I. Appendix to (hap. 1f. ). 88. of tmeteors x:ithl the still, 69. -.-.. tli it six tis oi watfer, 101 --- bdvlotled >y friction of tidal -..- liquefied byi presire', 19,5. wave, 006. ---- strcletture and lbeauty of, 126........ soutie of tflis hneat 6907. -- cdissectetd by l heat, 1;2. Sir Wiliam Tihomson's statemenet —.o towets, 127, e seq of the chemiical actiotn necesssary - caIrrier, or cryophorls, 187. to produce the stin's heat, 700. V- viscouls theory of, 230, INDEX. 525'Tle Numberslt (except vwher p. is pt-fixed) refir to tlhe laragnps, and nft to the Pges. ee, retclation thieory of, 232. - A NP-LAJ(,A, anioimalous deport - mouhtllr1d by vte)Sttsce 234, _ iJ minlt of, 427. t its absorption of heat, 351, 365. ~- radiation of heat through, 367,...... artitficial fitrmation of, )by noctti- 42$8. tal radiationl, 655. r -.fi1'11, 524. tli- theoty suppilet entcdt 655,..Land and sea breezes, how iprodtuced, — a.. oumnt melted ) inul Illtellt by 212. solar radtliation, lerschcl anld Pot- Landscape colon;, siftings of light illet's ieas;urlllcients, 681 i, i 7t... atomoli0tt imelted pet hour by total Latent heat of water, 26, 1!'l, 210t) ct1ission of suti, 686. - - - mechanical valuec of, 181. Iceland, geysers of, 112. -. - - - liquids, 182. Idtlia-rubbcr, stretclhi of, produces -. —.-.. -.of.vaporls, 186. heat), 1 0. Lead ball heated by collision, 40..contllactionl of, by hitt, 119,. -...... curiotls effect of expansion of, Ingclhausz his experiments on thle 10. conduction of lheat, 2416. leidenfost, first. observerl of the Intentsity of light anld heat, to what s)phlteoidal state of 4lituids, 199. ditel, o38. Lesie's cttle, radiation folr, 340, Interior work, dillerent kinds of, 168. 521..pet rrmcd by heat, 160. -- a:thlioseope, 493. Iodinle dissolved in istliide of car-.- obseivation-s explained, 494f. )QOi, diatlhertlnacy of, 430, 582, 4 Light prodtceed by frictionl of quartz, Seq. 11.. di athtcrianctlyln of solvenits of, 692. of lamps, to what due, t9. Ireland:l mtore irail on we-st side thaln of gas destroyed when mixed with on east; 219. air, )52. t. aces of 0aticient glaciers in, 238. -- law of dimimttison with distance, Iron bottle burst by freezing watev, 99. 335. - x- xspan:mioti of, by heatt, 104. - theories of, 303...... presnee of i suitn, proved, 583. analog y of somnd to, 303. Isotlihermal tlue i ns: north and s outh tt -.- propagation andt senswation of, 305. int EPlaudt, d223. reflection of, 322. Ivory, bad condtuctivity of, 27'6. a- ction of, on chlotine and hydrogei, 329. JOUEII,,) Dr., his experiments- on undulatios of ft1ansversal, 329. tIe nlclttanical equivalent of a- a:pacity to )odtuce,'133. ]heat 13, 84t el aq. - sifting of a beam o f, 1t4,7l............. >...- at and work, 31 (... s-.s t...su laces not hitherto kn1own to...-.. magtletoeletticity, be chelically sutseet)tible) to, r719, 84. - polaizationll of, explailned, 35. -..........- - thle shortening of in- -- cthemnical value of sky-lightl, 7l80. dit-rubber by heat, Ill. liilne-light, dtark rats, experiilellts on, r explaitis heat of nmeteorites, 12. 626. - his experiments ot the cold Liquerfactionl of ice by frictionl, 22. produced by stretehingl wires, 109. -...... presstlre, 125. liquid conditioni of matter, 59. JfNOIIgAU(II, explanation of liquids, chlanging to solidls prodtuccs IV sotme of his results, 648. hea t, 185. (opp, Professor, his determination. sexpains;ion of, by heat, 97. of thIe cubic coefficiet:s of expan- t tIe spheroidal state of, 193. sion, Appendix to Chap. IIl. p. 86. conducttivity of, 291. 526 INDEX. t'!he ullbers (except wherel p. is prefixed) rerur to the Paraoaplis, andh n ot to the fafees. Liquids, caloi ic trallsmissioln of, Melt. diminution of heat as thte square loni's table, 353. of tihe distance, 338..'app ar ttts ifor dctenl.lmnilng their Melfoni, his researchets on radiant ablsorption of lteat at different hlat, 3s50. thlickesses, 500. - his table of thte transmis.sion of t.. t:ble of absorption of heat by, iheat through solids, 351.i 506. -. table of the tralsmission of -. nd their vaporS, order of their hl eat ttriougnht liquttids, 353. absortption of lheat, 510, 51t,. -. source of error in his experiments loyd, J)r., hlis tables of railfildl i3n on1 tranlsmission of theat through Ireland, 211. lHquids, 499. Locke, tis view of heat, 20. -- his thteory of screin, 495. lun\inous and obscure radiationl,.- texplanation of some of his results, 3O. 551...... his additionl to the theory of dtw, nlAGO NETIC fiel, apparent viscos. 600. it) of, 32. -. expeilmeits on the waimth of Magnus111, Professor, his experiments the lItunar rays, 661. onil gIaseous contluction, t991. Merury, low speciftic }eat of, 163. A _ _v.th..- -.- tile conductivity of -- frozent by soidt car1bolnic acid, 192. hydrogen, 294. -..... il rel-hlot crucible, 203. Material theory of 1heat, 14. hMetals, good conductors of heat, Matter, liquid condition of, 59. 24t5. -.. gaseous dtitto, 60. - bad radiators, 840. Maximuml deltlsity of waterl, 98. -- alsorbel s, 3:15. NMayer, Dr., compares locomotive - -.effect of their bad radiation, 6053. iroceC to distillation, 9. -:.band-s SCeen in spectrat of tlheir va--- --- t.l.tciates tle rl'elationship be. pots, 004.'.xcen heat and wrork, 34. -..t.presence of terrestrial, in suln, calculationl of the heat that p)rove(], 619. be plroducltte by stoppage of Meteorology, absorption of heat by.n's motion, 16. aqueous vapor arppllied to phltenom-..l.mechallnical equivalent of elta of, d484, t sceq. at, 82. tMetcolxs, zodiacal light- stt)uposed to..-... essay ont celestiid dynamics, be, 1'1, 091, 099. reerrted to, 8,, 698. -- unb)er of, seCh inl JOFston, 690. meteolic theory of sun'tlls heat, -. m.aollt of' henat generated by cot698, lision of, with sun, 692.. - elected to the French Academy, —. tl sul's liglt andl ieat possil.y kept p. 233, stolk, up by, 6t3. Mechanical processes, getler.atioll of Miller, 1r, tr. l rays of burning hydrotleat by, 5. gernl, 54i. -.- work, cotstuttptiolt of hteat itl 16. Mitscherlichlt Irof, his expermlitenlts -- theory of heat, 20. on thle exp'a1so80n of cry.stals, 108.... equivalent of heat, 38. 3Molectla tr motion, lIeat deitled as, 20,. M..........._ tayer's determilation, 58S 2 P,. 3'o, 81. e. -. vilab t ionl of a body more intense Jole's determil atioll, wh en hea ted, 058. 3'1, 84........ forees irresCistible) 99 Meidilter, M., lhis experiilentts ott..-.potsr of, 1 58. ozone,,144t, 4ote. st —o le euleulatced, 180. Melloni, his, mode of p)roviug the.action in wood, efflect of, 2/1. INDEX. 2FX'itoe Numbllllr (ex'ept wheri p. is prefixed) reftr to the 1'arlatgphs, and not to the ages. Molecules, theitr motions as whol0es, 0 lESOUItEIf 114hlAT, rays of, obey 130. s s.5ame laws as light, 823. Mongolfier, equivalence of hlcat andw -.- -^ ratio of, to luminolus rays frTom -vwork, maintainedt by, 37. diffrlent sources, 310. Moon-blindness, cau11se of, 0354. Ocean, influelnce of, ont temperattire, -beams, cause of their putrvefting 1t16. powevr, tI54. Olefiant. gas, athermancy oft 393, -. experimenlits on warmlth of, -165. 421. -ob —. oscutre heat of, cut off by our.-..- table of absorption by, at difteratmosphere, t1.5 5.lt pressues, "3, 5630. Moraines of ancient glaciers, cedars -...- b*. y varlions meastres, of Lebanlon grow ol, 238. 402. Mtosely, tRev. Canonl, curious effiet of ---- radiation of, 4141 expa:llsion noted by, 107. - -.. dynlamic radiation of, 455. Mlotion, heat considered to be, by..- -- varnlishli ng metal by, 457. Bacon and Locke, 20; by Rumford, Optic tlerve eftct of heat rtays 0on, 13; by l)avy, 13, 23. 6 012a.... transfircncle of, from matss to mo10e- - efftect of invisible rays on, 0643. 1ules, 23, 58. Organic motion, (xtiacts fiom a paper - point of maximum, il a glacier, by Mayer oit, 8o3. 229. - structures table of conductivity of, 3o01tainsll, their action as codenlsers, 275. explained, -190. Oxygen, collision of atoms of, with Moving force, amount of iheat pro. carbon, 18; with hydrogen, 180. duced by destruction of, 40. - velocity of particles of, 61. - ) )1produteed l)y steam, 1410.......- small absorption of heat by, 392, or dynamic enertgy, defined, 421. 15. Ozolne, action of, on radiant heat, TATU RA'tIA t philosolpher, his voca-. increase of, lby radiation ii; I tion, 102. electrodes, I44 3.,i Naturel, adaptation of means to ends --- Irobable constitution of, 44f;. in., 10.2. N6v6, the feeler of tile glacier, I)AIIABO IAO iirrors, rclict: 228. 1. light and heat fr'om, 328, elt.sq. NKeciton, his opinion of the diamond, Particles, impact of, causes sensation 48. of heat, 05..- theoly' of light,t 8303. --- ullt:imate, motion of, plroduces heat, Nitrogen, velocity of particles of, 20. 6t4. 1Percussionl, hleCt generated b)y, 8. Nitrous oxide, absorption and radia. Perilrles, how propagated, 6 2. tion of, 414t. table of absorption of ]heat by, - —. dynamllic radiation of, 4165. 413 aci gas, bands produced by spec- Plerio, heat and light liffri only it, trati of, 671. 617. Nocturtnal radiation, artificial formal- ---.influl nce of, on absorption, i1'7. tiont of ice by, 66o. - determines the ftuality of heat - experitments; otn, by Wells, emitted by bodies, 618. Glaislher, and otherst, 650, G51. Periods, vibratig, of fo'lrmic alnd Nov\tt Organttm, extract ftoln 2d sulphuric ethert, 524. b)ook of, Appendix to Chap. 11. p. -. - - ofa hydrogen-flame, 54'. 60 --.shortellinl of, 54,'. 528 INDEl"X. The Nitmln'lrs (except where 1). is pieflix'rt) refer to tho l'largralphs, and not. to tile Pages. Pterslpiration, use of, in hot climates, Radiant heat, ieflectionl and conver2o6.b gence of rays of, 326. I'hotosplier of sun, action of on solar law of inve1se squares applied ia's, 68.. to, 338, ct se. PIlysical analysis of the human breath, -.-. its origill and p'oplagatio,tt 339. t0610..-...-..aptpaiatlus for resiearuchies on, lite'thl-mo- cclectric, constrtiction described 3'?2 —391. and utse of, 3, antd Appendix to s-.d- - palsorption of, by gases, 418. Chap. I..p 16. -. pois, tS33, 522. Pitch of nlot, upon what dependent, a...- -. —. ction of perfilmes on, 43'. 316.:-. — ol)ject of rtesearches on, 49,. P)lanets,, orbital velocity of tlhe inte- Radiating body and air, difterence be1iol, 12. twceen constant, 4159. —. heat th-at wonild be tdeveloped by Radiation, eflet of color on, 34l1. their fallitng into stll, or by resist- Radiation and absoription, reciprocity in1 rotation of,'102. of, 3413, 351, 415, 4159. 1itnimnlln lamp, describedl, 503. of metals, 310. Polafr forees, lheat required. to over- - helat by solids, 34t0, 354. come it68, 3......:.'-'-. gases, 413, 455. Polarization of heat, p, 603.. -. - - vapors,'t1 4465. of hlhtt, cxplailued,'?35. --- ant absorption of a gas or vapor P'otential or possible lenergy defined, dctcrmied without external heat, 1Sf.t 4419. Polillet, M., hi os expertimnnts on tthe........dyamii, table of gases, 455. tempelsratutr ofnir alld swamnt'is-downl: -- dew...all effect of chilling by, 652. 6O 59. - nocturnal, (Glashir's table of chill- - netastu'eement of solar radia- bing by, 05?l. tioll, 081; of its partial absorption...... - artificial formation of ice by, by our atl-osphelre, 8i. 55.......... pyvlteliomceter, 681Q. — throug tlohe eartlh's atmliosplhere,'Cessuil'c, llating to lheating of gases, Appendix to Chalip. XI, p. 344, 69'?t.I -- ltlumilnous anld obsceure, 3'0. ef- efet of, omr point of fil}on, 121. Rain, caulse of thle torrlcts of, ill the..r.. s.- -..st of elrth, 122. tropics:, 214. liqueltection of ice ly, 125....... fall, greater on west than onl east P1revost's theory of exllaniiges, 319. coast of Irteland, 21'1. yrhlelomltcr, use and ldescriptiotn D-. l)r. Lloyd's table of, in Ireland, of, 081. 21'?. P'ylometellC, 10. --- place where the greatest occli, 220, a2te. Q'I.... mipint wohati lt depcliellet, 220. clearll'' dn sl filroly, tand)S- smokye,4cctioi, e llling elftct of, 2'?t.. it, mir equal atmounts of hl.at, 351. 1. e o Quality of r-adiant het, defiition of, ill not itself lower me 360, 513, 531 pciAttuc, c8,. Rectilinear motion, atoms of gases inoove nith, 60. ) Ai1 IANTl heat, (lefinition of, 30. Reflection of light and heat obey _.l - - -- d..... and light, analogy between, same la\ws, 323. 306. Rtefrig ration by expansion of a gias, em- -- elitted by all t)odies, 319. 95..- -. laws the same as those of ligtht, Rcgetlation, discovery of, by Faraday, 323. 232, INDEX. 529 The tuntbers (excTept whetre p. is prefixed) refer to thle 1'argmaphs, and not to the Pages. R1llnul his plastic theory of ice, 235. Scwnvart, z his observationt of sound lResistlance, heiat of electric current p1oductled by cooling Silver, 113. ptrol)oItional to, 261. Se"a wvualmer after a sstormnl, 8. Retina of the eye, question whlether breeze, how l)roduced, 212, thie invisible ra1ys elnitted by oltni- S6geuin, equivalence of heat antl work nous sources reach the, 643. developed by, 34. Revolving balls, Gore's expereltlts tIenite, absorption of heat by differon, 118. cnt thick nesses of, 358. l-iltle-bal, aioiunt of heat geTnerated o- coiors of, 069. by stoppage of its m1otion, 45. Senarmont, his experiments oln thle Rocker used in the Trevelyan instru- conduction of heat by crystals, iment, 114. 268. Rock-talt, tra:sparency of, to heat, Serein, Metloni's theory of, 496. S49, 361. Sh0ooting-stars., theory of, 11. use of, in experiments on absorp. ilica, water of Geysers contains and tion of heat by gases, 3'3. depos its, 1 48....t.'ygrostopi character of;, 4I1. *-* as crystal, highi conductive p)olver -...... deposition of moisttture on, avoided, of, 2:'3.,I42. I- as powder, low ditto, 286..-. cell, desribed, 600. Singing ilanmes, 301. Roseo, l'iof., his experiment'si on p-.aper on,) Appendix to Chap. sky-lifght, /80. VIII., pp. 231 —24t4. 0Rum1ford, Coulnt, his experiments onl Sky', color of, 496. Ileat produeedl by friction, 13, and e- - causes of, 1'63u,'/61, 162. Ap)ttendix to Chai). 11, p. 51. -- -- nlatlral and artiftcial azur % or'n-.. oveltlhrow of the material piatred'104. theory of hleat 20. --- chetical value of sky-light, 4 80. a* - —. abstract of his essay on heat, Slow, slhower of, roduced by issuing Appendix to Chap). IT. p. 51. of comlpressed air, 29. - -- Itis cstimnationi of tihe ealorifie - -.- —. by chill inl a room,0 p. 6S. power of a body, 1t9. 1- carbonic acid, 189.....- -.. ex-periments onl the condue- -.crystals, 225. tivity of clotllinig, 283. -. line, tle, 226.......... —-.. -... -.-... liquids and. folrmation of glacieis firom, 22'!. gases, 291. — ball, cause of cohlerence of, 233..-................ ttransmissionl of - bridges, how crossed, 233. helat through a vacuum, 29v. squeezed to ice, 234. u"pert's (drops, 100. Sodiium, yellow banilds emlitted and ab. sorbed b)y vapor of, 643, 6'6., AFYETY LAMP, explanation and Solar speectrtuml, cause of dark lilles tuse of; 290. in, 0'7, 8Salt and sugar, dissolvinig of, pro- rays, (lark, discovery of, 6554, dutes cold, 183. 560. conmmon, yellow bands emittled -it -- visible and invisible, 669. and absorbcd by valpor of, 413.. — - ray.filters, 646. Scents, action of, oni radiant heat, -—..thermal image rendered l ulli. 435. otts, 689. Schafhotnselt, Count, htis pl, )al)e- on.-...... colmbustion andl incandescence acoustic extpcrmentts, App)endix to iy dlark solar rays, 692. (Chap). V1II.. 2t,1. Solidification accompanlied by expan1. S8chemnitz, mtachille for' comlltlssio sion, 18. of air at, 29, i - -. —- contitaction, 120. 23 530 INDEX.''liho N't111umbers (except weroi- p. i.s tpre.fix.ed) rwfii to the Parlagmi-s,tii anld not to tie XPages. Solids, expansion of, by heat, 103. Sulphate of soda, heat. produced by. — caloific trastlmission of, Mtlonti's crystallizingv, 186. table, 351. Sulphuric acid usetl d for dryiltlg gaset s Sounld, converssion of heat into, 114. 390. Sound, mode of its transmission — ether, abssorption of iheat by vathroullh a'ir, 300, 315. 1)or o') 0 t, 133,.i. )rodtuced by flame, 301. Suli, prolable cause of conitutance -- udulation of waves of, longituldi- of heat and 1light. of, 4'7, 693. Ial, 339. - - )roduction of winds by heat. of, -- analogy off heat and light to, 305. 21)6. Sounltis, inaudible, 318. -- does not heat Idry air senllsibly, 361. n.. tutsical, produced by gasIlamne --- discovery of tite dark1' rayi of tile, inl tulbes, 301, and Appenldix to 61t0. Chap. AVlIl. p. 237, — g- genlerationl anid tintensification of Specific heat of bodie.s holw deter- rays visible and invisible, 663. mihned, 165. rise of intensity with temperola. e..-..letcment:ary bodies, 165. tiurc, 563, 564...-... -- simple atnid comipotlund gases, -- combutstion by (lark rays, 592. 169, ct seq. -.- fluorescence and caloresccnc e of -.of water thle fighost, cot se- the s8tl, 08, 61'. quenccs attendtting, 14. -.Sir tV. thotnson's statelment of.mll...askitng tlhe conductive power the chemllical action ncceessary to of a body, 2,9. produce ttte sutt's heat, t00. Spectra of zintt, copper, etc., 6600. -.....- constitutioit of, V7'. Spcectrnt, heat of ot-lut linous,... flame atmosphere surtrotmdintg, proved, 311, o68. tonlmittnilous, obtained, 32. — and plattts, supposed common.-. solnar, llerschel and Miller's cx- origili of, 6"/9. perime'its on tlhe distribution of -.- heating po'er of, measurlements hea:t. it thle, 601, 602. bIy Ifershell and Pouillet, 081. - - cause of dark lines itn, 674. -m mnode of determtititlig tihe radia. - -- of incandescent earlbot, 064. tiont frot, 682. —. of solids, similar to, G66. atm- tospheric absorptiont of heat Spheroid, floating of, on its vapor, of, t8:1. 195. -- total amoiuntt of iheat emitted by, not ill contact with vessel, proved, 685. 197 -- all orgatlic andl intorgallic energy Spheroidal state of liquids, 103, (t seq. referrced to, 07', 42t1. -condition, first observer of, 199. -- small fractionl of its heat that Spiral of platinum wire heated by protduces all terrestrial energy, elect.ric current, r1adiation from, 12 1t. 522, 5t4'. Switzerlland, evidences of anlcicent glaSprings, boilitng, of Iceland, describetl, clefs inl 236. 142. Steamt, how produced, 137, r l llMPEIIATUEIi), absolute tzeo of, - elastic force of, increrased by lteat. 1. 96. ing, 140 i..- - itilluetlce of, on conduction of - latent heat of, 181. leectricity, 252. Storms produtt ed by- heated air, 2 05.. l.....i how eh nttdured, 20.6 Strokmti',d lhe imitatiotn of, 149..*...dew caulsed by lowering of, 650. Su1p11)ate of sodatl, cold produced byt -.- infltence of, oil the qultality of dis;olvitlg, 185. heat crtitted by a body, tS18. INDEX. 531 Tlhc Nulmbers (except where p. is -prlfixed) rcfelr to thle Paragraphs,l and nt to thllcnPlges. Temperature, influence of, on trans- A'fAC(CUM in centre of ice-llowCers Insi;sion, 622. Y 128. i- dillculties in aseccrtailitg t ue tftre, *- p)assage of heat throughlt, 2972. 063. -.....dry air similar to, with regard to l'teneriffi, Peakl of, two currents blow radilant teat, 3092. on, 215. Vaporous condition of matter, 60. Thermal effectsl proldued by stoppage Vapor of water condelnsed by rareof motion, 24t, 39. faction of air, 28. T her1to.e1lectric pile, 3. *- -...- its action or iradiallt theat, -..... o - -- nlote onl the construction o f; 4210, 4t92. Appendlix to Cha). I. p. 1. -....... condensation promoted by,..- -:. —...1.used in researches on,ra- 191. diant heat, 302/. -. p)rotuection of, consumes )teat, 180, lluhmkorff's, 568. 2.40.'thermogralph of coal-points, 601t. - slppoltting of spheroid by, 19f1. Thlernmomleter, conlstructionl of Ap - of imet-als, Sl)ctrum of, 66t4. plendix to Chap. 111. ). 86. -.)absorlbs those rays which it emnits,'thickness, influlence of, on ab)sor1). 32. tion of heat,:365. Vapors and liquids, their absorptio n lhomlson, l'rofessor Sir William, on of leat comparedtl, 610, 6514. earth's; crust, 101, 122. -—. tables of absorption of hcat by, h.is suggestion that India- 4133, 43/3, 159, 522, 621. rtubber woull shorten by heat, - - 1 —-- 11 t-lamtic radiation anid abM11. sorption of, 4659, 466...- —.. —. meteoric theory of thie -- tter entering into our coneep. Sktl), 4't. tion of a vl)apor, 430... tables of enlergy, 2 ^02-.... chemical absorption b)y liquid and -Pr l'rofessor.James, on the influence vaplor, 7242. of pressure on fitsion, 124. Varnishling a metal or feeble gas by'idal wave, velocity of earth's rota- a powerflti one, 452. liot diminished by, 697. Velocity of t lalnets and ia6rolites, 12.'trade-'(linds, upper and lower, 206..- relation of ]teat to, 42.'Tfllansmtission of heat through solids, V'iblration of' heated metal, 113. Melloni's table, 351.. -.-. -- sountding disks, 290........... *. lif quids, ditto, 365) 3. Viscous tlleory of ice, 2830.........influence of temperature, of ViSion, a differential efh'ct,,22. sourIce onl, 622, 524, 64t. Vital force, supplosed conservative Transparency, of bodies, cause of, action of, 205. 346, 651. Volcanic eruptions showing ul)pler n-.ot a test for diathermancy 351, currents of aim, 209. 367. --- eruption of M0orte G'rou, 210. T''revelyatt Mr. A., his instrttment, Volume of a gas augmentetld y heat, 113, 660 t stq. -~ —- I...caus- caulse of vibrations of, 116. Volumes of vapor p)roplortional to tropics, flow \of air froom and to, liquid, table, 512. 211., - te region of calans or rains, 214. ITAtE It boiled by1 frietionl, 13, and..*- cause of the torrents of rain in, }I Appendix to Chal)p. I,. p. 61. 4188....... expanded by ]heat, 08... c-otI, 98. |TUNDUtA1'TION T'PIIIOlY, 30.1. - maxilmum density of, 98. -.. -. pilCpS, why lmrst, 100, 532 NFI)sX. The Nummbers (except vlwhere 1). is prefixed) refer to the Paragraphls,l and not to tile 1Xages. Water, cothesion of, increased by re- Waves of soa mdr}s, 316. moving air from, 131, el seq. light, 314..-. hammlrlle 131. - - heat and solund, (lifterence be-- effects of, whenl in a highly-cohe- twcen, 339. sive condition, 132. Wells, l)r., lhis theory of dew, 618, et.. formerly rcga(rded as incompres. seq. sible, 1. - many curious effects explained - Bacon's experimentl on tile com- by, 654. pressioll of, 164 note. WWicdemann anld Frlanz, their table of a- nmount of heat yielded:by, in con ductivities, 241. cooling 1V, 163. \1Winds, extinction of light, of gas by,....... has the higlhest specific heat, 66. 165. --.rod.tl1rced by sun, 206. a- mount of work equal to heating - trade, 2,06. of 1, 106. — direction of, influenced by earCth's -elfkeet of high specific lheat of, rotation, 207. 1it,.\Winds, lesser, cause of, 212, l- atetnt heat of, 1474. Wollaston, )Dr., hhis cryophorus, 184/. -- mcchallical value of combilattion, l- -. ies in solar seectrlumn obcondensation, and colnlelation of, served by, 687. 181. \Wood, bad conductivity of, 265. ev\aporation of, l)roduces cold, - difference of' conductivity in, 269, 186. 269. firozen by its own evaporation, 18. - apparatus for asceertaining ealo- spheroidal state oft 193, ct seq. rific conductivity of, 260. -frozelln il rced.hot crucible, 202. - threce axes of conduct1ive Ipower I- oiacity of, to heat, 348, 607. in, 21. - tdistilted, color of, 356. Woollen textures, imperfect. conduceltlects of its energy as a radiant tion of, 283. in all its states, 4191. Work, constant prlopoltion i betweenl..absorbb s samne rays when solid, it and heat, 16, 384, 156. liquid, or vapor, 365, 496, 64t. - interior, 160, 167. - n- amount of, woild be boiled by the total tlemissioln of sun, 686. r. limos, establisnent can..se of its hardness, 286. 1 of thle undulatory theory, 13, - - - -f {t'ltransparency to lighlt, 642. 804.,.-..-. opacity to heat, 6t2. -absorption of heat from hydro- /fMl(O, absolute, of temperature, O6. gern-flame at different thicknesses,.. Zinc, bands seel in spectrum of 64'2. vapor of, 66. Watet'rston, his meteoric theory of the Zodiacal light, probable cause of, 4', sun, 44, 699. 691, 699. 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