1 
 
GIFT OF 
 . W.B. 
 
5 z 
 
 A 6, < 
 
LESSONS 
 
 IN 
 
 C 
 
 HEMICAL HILOSOPHY. 
 
 BY 
 
 JOHN HOWARD APPLETON, A.M., 
 
 Professor of Chemistry in Brown University, 
 AUTHOR OF 
 
 " BEGINNER'S HANDBOOK OF CHEMISTRY," " THE YOUNG CHEMIST," 
 " QUALITATIVE CHEMICAL ANALYSIS," " QUANTI- 
 TATIVE CHEMICAL ANALYSIS." 
 
 SECOND 'EDiTiQN. 
 
 SILVER, BURDETT & CO., PUBLISHERS, 
 
 NEW YORK . . . BOSTON . . . CHICAGO. 
 
 1890. 
 
PROFESSOR APPLETON'S WORKS ON CHEMISTRY. 
 
 I. THE BEGINNER'S HANDBOOK OF CHEMISTRY: Price $1.00. 
 This is an introduction to the study of Chemistry, suitable for general readers. It treats 
 chiefly the non-metals, these being generally found to furnish the best material for an ele- 
 mentary course, and to best illustrate the fundamental facts and principles of the science. 
 
 The book is written in attractive style, and has had a very large sale. It is profusely 
 illustrated with engravings, and has, in addition, fourteen colored plates. 
 
 II. THE YOUNG CHEMIST: Price 75 Cents. A book of chemical experi- 
 ments for beginners in Chemistry. This is designed for use in schools and colleges. It 
 is composed almost -entirely of experiments, those being chosen that may be performed 
 with very simple apparatus. The book is arranged in a clear, systematic, and instructive 
 manner. 
 
 III. QUALITATIVE ANALYSIS : Price 75 Cents. A briei but thorough manual 
 for laboratory use. 
 
 It gives full explanations, and many chemical equations. The processes of analysis 
 are clearly stated, and the whole subject is handled in a manner that has been highly 
 commended by a multitude of successful teachers of this branch. 
 
 IV. QUANTITATIVE ANALYSIS: Price $1.25. A text-book for school and 
 college laboratories. 
 
 The treatment of the subject is such that the pupil gains an acquaintance with the best 
 methods of* determining all the principal elements, as well as with the most important 
 type-processes, both of gravimetric and volumetric analysis. 
 
 THE EXPLANATIONS ARE DIRECT AND CLEAR, so that the pupil is enabled to work intelli- 
 gently even 'without the constant guidance of the teacher. By this means the book is 
 adapted for self-instruction of teachers and others who require this kind of help to enable 
 them to advance beyond their present attainments. 
 
 V. CHEMICAL PHILOSOPHY : Price $1.40. A text-book for schools and 
 colleges. 
 
 It deals with certain general principles of chemical science, such as the constitution ot 
 matter; atoms, molecules, and masses; the three states of matter and radiant matter; 
 the change of state from one form of matter to another. It also presents such topics as 
 Boyle's and Mariotte's law, Charles's law, and the other general laws of matter. It dis- 
 cusses from a chemical standpoint certain forms of energy, such as heat, light, electricity. 
 It treats of the nature of chemical affinity; the chemical work of micro-organisms; the 
 modes of chemical action; thermochemistry; and those attractions of substances which 
 are partly physical afldpajtty chemical.* Jt^lsp Jjreents a lull study of atomic weights, 
 the methods leading to a 5rsf adopticvn/oirtKeni", nd;then to the grounds sustaining cer- 
 tain numbers selected. Trie periocTic system is 'of course discussed. 
 
 The work is fully iilt*str?.te,d.' r ' , " ; 
 
 Copies senf 'by ih'aii, ''postpaid; by r tire' Publishers, upon receipt of the 
 advertised price. 
 
 COPYRIGHT, 1890, 
 BY JOHN HOWARD APPLETON. 
 
 TYPOGRAPHY BY J. S. CUSHING & Co., BOSTON. 
 PRESSWORK BY BERWICK & SMITH, BOSTON. 
 
PREFACE. 
 
 THIS book is a formal presentation of certain subjects 
 which the author has been in the habit of offering to his 
 classes in the form of lectures. It is intended to explain 
 to beginners, or even tolerably advanced students in chem- 
 istry, certain of the general laws of the science, and that in 
 a compact and easily handled form. To a certain extent 
 theories are given. While these have their important use, 
 they must not be relied upon too strongly. " Theories," 
 says Dumas, " are like crutches. To find out their value 
 we must try to walk with them." On the other hand, 
 where theories have been presented in this work, effort has 
 been made to show distinctly the basis upon which they 
 rest. Particularly in the chapters relating to atomic weight 
 the attempt has been made to lead the pupil to formally 
 distinguish between facts and inferences. 
 
 The efforts of investigators in chemistry as in other 
 natural sciences are, as Berthelot remarks, to transform a 
 mere descriptive science into a truly physical and mechan- 
 ical, i.e. a mathematical, one. This thought has not been 
 forgotten in arranging the work presented herewith. At the 
 same time the attempt has been made to avoid making the 
 book mathematical. The mathematical treatment, valuable 
 
 iii 
 
 237484 
 
IV PREFACE. 
 
 as it is for highly advanced students, is apt to be repellent 
 to beginners. 
 
 The author takes this opportunity to thank the teachers 
 of the United States for the kind reception they have given 
 to his earlier text-books in chemistry. He cherishes the 
 hope that they may find this one of service to them in 
 their studies and their teaching. 
 
 BROWN UNIVERSITY, PROVIDENCE, R.I., 
 September, 1890. 
 
CONTENTS. 
 
 PAGE 
 
 CHAPTER I. The branches of natural science. The place of chem- 
 istry i 
 
 CHAPTER II. The constitution of matter. The atom. The molecule. 
 (Elements and compounds.) The mass. The modern atomic 
 theory 3 
 
 CHAPTER III. Is matter indeed molecular and atomic? General 
 discussion upon atoms and molecules. Ordinary observation. 
 More searching examination. Evidences of heterogeneity found 
 in mechanical and physical relations of substances; in their rela- 
 tions to heat, to light, and to the electric current; and in their 
 chemical properties. Compound radicles. General conclusions. 
 The atoms of the chemist viewed as composite. The genesis of 
 atoms. Shapes of atoms. Movements of atoms. Positions of 
 atoms in space 15 
 
 CHAPTER IV. The three states of matter. Solids, liquids, and gases. 
 
 Importance of the study of gases. Radiant matter . .35 
 
 CHAPTER V. Change from one state of matter to another. Influence 
 
 of addition and withdrawal of heat 42 
 
 CHAPTER VI. Changes incident to addition of heat. Addition of 
 heat to a solid. Rise of temperature according to specific heat. 
 Melting. Latent heat of liquefaction. Special forms of liquefac- 
 tion. Dissociation. Addition of heat to a liquid. Vaporization. 
 Ice machines. Changes incident to withdrawal of heat. With- 
 drawal of heat from a gas. Withdrawal of heat from a liquid. 
 Solidification of homogeneous and of mixed liquids . . -45 
 
 CHAPTER VII. Certain general laws of matter. Boyle's or Mariotte's 
 law of the pressure of gases. Charles's law of the expansion of 
 gases by heat. Graham's two laws of gaseous diffusion. The law 
 of Henry and of Dalton of the relation of pressure to the solu- 
 bility of a gas in water. The law of definite proportions. The 
 
 V 
 
VI CONTENTS. 
 
 PAGE 
 
 two laws of multiple proportions. Gay-Lussac's three laws of 
 combination of gases. Avogadro's and Ampere's hypothesis of 
 the size of gaseous molecules ....... 63 
 
 CHAPTER VIII. Certain forms of energy closely connected with 
 chemical change. Heat; temperature; expansion; change of 
 state; chemical combination and decomposition; light (spec- 
 trum analysis) ; work. Electricity : its sources and effects . .82 
 
 CHAPTER IX. The attractions of masses. Gravitation . . .99 
 
 CHAPTER X. The attractions of molecules. I. Cohesion. In solids; 
 in gases; in liquids. Polarity; crystallization; cleavage. Crystal- 
 line systems. The process of crystallization . . . . 101 
 
 CHAPTER XI. The attractions of molecules (continued}. II. Adhe- 
 sion. (A} Adhesion between solids and solids. (B} Adhesion 
 between solids and liquids; moistening; capillary attraction; 
 spheroidal state; solution (deliquescence, freezing mixtures) . 114 
 
 CHAPTER XII. The attractions of molecules (continued}. II. Ad- 
 hesion. The separation of a solid from a liquid. (C) Adhesion 
 between solids and gases. (D} Adhesion between liquids and 
 liquids. (E} Adhesion between liquids and gases. (F} Adhe- 
 sion between gases and gases (the terrestrial atmosphere) . .124 
 
 CHAPTER XIII. The attraction of atoms. Chemical affinity. Con- 
 ditions favoring chemical change: the liquid condition; heat 
 (thermolysis and dissociation) ; light; electricity; vital processes 
 of higher and lower living beings (organic and inorganic com- 
 pounds) . . . . . . . . . . .141 
 
 CHAPTER XIV. The attraction of atoms (continued}. The chem- 
 ical work of micro-organisms. Microbes : their conditions of 
 growth; results of their action; their usefulness . . . 155 
 
 CHAPTER XV. The attraction of atoms (continued}. Modes of 
 chemical action. Sphere of chemical action. Criteria of chemical 
 action. Results of chemical action. General laws of chemical action, 168 
 
 CHAPTER XVI. The attraction of atoms (continued}. Thermo- 
 chemistry : its laws and units. Calorimeters and the difficulties 
 they have to meet. Range of thermo-chemistry. Results . . 175 
 
 CHAPTER XVII. The attraction of atoms (continued}. Theories 
 
 of the nature of chemical attraction 187 
 
 CHAPTER XVIII. Atomic weight: method of work and method of 
 
 description . . . . . . . . . . . 193 
 
CONTENTS. Vll 
 
 PAGE 
 
 CHAPTER XIX. Atomic weight (continued}. First step: A unit 
 adopted. Second step : Selection of the compounds and the 
 processes to be employed 199 
 
 CHAPTER XX. Atomic weight {continued}. Third step: Experi- 
 mental work for securing a few atomic weights. Study of chlo- 
 rine, bromine, and iodine; sodium, potassium, and silver . . 204 
 
 CHAPTER XXI. Atomic weight (continued}. Fourth step: The 
 choice of a particular atomic weight from several combining num- 
 bers. Density of elementary gases. Volume composition of com- 
 pound gases. Vapor density of compound substances. Specific 
 heats of elements and of compounds. Atomic heats. Specific 
 heats of chlorine, bromine, iodine, potassium, sodium, silver. A 
 study of oxygen and of sulphur 209 
 
 CHAPTER XXII. Atomic weight (continued}. Fifth step: Con- 
 firmation of the atomic weights chosen. Molecular formula sup- 
 ported by volume composition, by chemical substitution, by melt- 
 ing-points and boiling-points, by crystalline form, by molecular 
 stability, by relationship, by results of decomposition, by excep- 
 tional compounds, by special properties of substances . . . 225 
 
 CHAPTER XXIII. Atomic weight {contimted}. Sixth step: Bring 
 all the atomic weights into one table. (The periodic law.) The 
 work of Newlands, Mendeleeff, and Carnelley. Prout's hypothesis, 243 
 
 CHAPTER XXIV. Atomic weight (continued}. Elementary sub- 
 stances as molecular 249 
 
CHAPTER I. 
 
 THE BRANCHES OF NATURAL SCIENCE. 
 
 THE PLACE OF CHEMISTRY. 
 
 THERE may be as many sciences as there are kinds of 
 subject-matter for scientific treatment. 
 
 The scientific treatment of a subject demands exact 
 observation, precise description with fixed nomenclature, 
 classified arrangement, rational explanation. 
 
 The term natural science is usually applied to the 
 classified knowledge of external material nature and 
 certain of its forces. 
 
 In the ordinary every-day use of language, then, the 
 general term science is often applied to what is here in- 
 cluded in the term natural science. But it must not be for- 
 gotten that there may be a science of the human mind, for 
 example, as well as sciences of external forms of matter. 
 
 Divisions of Natural Science. One grand division of 
 natural science is that called Natural History. In this 
 are included 
 
 Geology, a history of the inanimate matter of the earth, 
 and embracing physical geography and meteorology ; 
 
 Zoology, a history of animals ; 
 
 Botany, a history of vegetable beings. 
 
 Natural history is mainly descriptive. 
 
 Another grand division of natural science is that called 
 
; BRANCHES , QF NATURAL SCIENCE, 
 v 
 
 Natural Philosophy, or Physical Science. In this are 
 included 
 
 Mechanics, which treats of masses of matter ; 
 
 Physics proper, which treats of the motions of mole- 
 cules, and the molecular forces such as light, heat, and 
 electricity ; 
 
 Chemistry, which treats of atoms, the constitution and 
 properties of molecules, and the laws of chemical change. 
 
 Physical science is mainly explanatory. 
 
 Defects of the Foregoing Classification. Even a very 
 hasty consideration of this brief classification shows that, 
 especially as respects exact lines of demarcation, it is 
 inadequate. Thus the individuals discussed under each 
 department of natural history involve in their histories 
 the processes of natural philosophy; for the animal, 
 the plant, and the rock are formed, or live, or grow, or 
 merely exist in one place, as the case may be, under condi- 
 tions involving mechanical, physical, and chemical forces. 
 
 Again, it will be noted that certain branches of study 
 evidently belonging to natural science astronomy, for 
 example are not specifically mentioned. 
 
 But the defects of classifications of this sort are refer- 
 able to difficulties that nature itself places in our way, 
 for the multitude of natural phenomena are not in them- 
 selves characterized by strongly marked division lines, 
 but are mostly intimately interwoven. 
 
 Finally, it is not intended, in this chapter, to offer a 
 perfect classification of the subjects of study afforded 
 by natural objects and forces ; it is merely proposed to 
 place before the reader the general relations of chemistry 
 to other natural sciences. 
 
CHAPTER II. 
 THE CONSTITUTION OF MATTER. 
 
 THE ATOM ; THE MOLECULE ; THE MASS. 
 
 MATTER is believed to be capable of existing in por- 
 tions of three different grades of magnitude. These are 
 called respectively the atom, the molecule, the mass. 
 
 The Atom. An atom is the smallest unit of matter 
 now recognized as existing. About seventy different 
 kinds of atoms are now known. 
 
 Each atom of matter is viewed as possessing the fol- 
 lowing characteristics, in addition to many others : 
 
 It is extremely small (but not infinitely small). 
 
 It is indivisible, and, indeed, in itself unchangeable. 
 
 It possesses a definite weight, which may be determined relatively and 
 absolutely. 
 
 (The atomic weight is different for different kinds of atoms, but practi- 
 cally the same for atoms of the same kind. Thus each atom of hydrogen 
 weighs I microcrith ; each atom of oxygen weighs about 16 microcriths. 
 See p. 199.) 
 
 It is capable of manifesting an attractive force, called chemical affinity. 
 
 It almost invariably exists in a group, of which the component atoms 
 may be alike or may be unlike. In a very few cases an atom may exist 
 singly. 
 
 Examples of single atoms are : 
 
 C Barium (Ba), 
 ) Cadmium (Cd), 
 i Mercury (Hg) {Hydrargyrum), 
 Zinc (Zn). 
 
4 THE CONSTITUTION OF MATTER. 
 
 The Molecule. A molecule is the smallest particle 
 of any substance that manifests the chemical properties 
 of that particular substance. Thus : 
 
 H 4 C represents one molecule of marsh gas. 
 
 H 3 N " " " " ammonia gas. 
 
 H 2 O " " " " water. 
 
 H 2 " " " " hydrogen. 
 
 O 3 " " " " ozone. 
 
 O 2 " " " " ordinary oxygen. 
 
 A molecule is believed to be capable of possessing 
 most of the following characteristics, in addition to 
 many others : 
 
 (#) It is extremely small. From recent physical investigations, " it may 
 be concluded with a high degree of probability that in ordinary liquids or 
 solids the diameter of the molecule," that is, the distance between the 
 centres of contiguous molecules, is between the one two-hundred-and-fifty- 
 
 millionth (. in L^ u .) and the one five-thousand-millionth (,,^^1) of an 
 
 \ &>,.), 000,000 / \o,uuu,ooo,uuo/ 
 
 inch. 
 
 (^) It is not completely nor absolutely in contact with its neighboring 
 molecules, but is separated from them by relatively large spaces. 
 
 (V) When in the state of gas a molecule demands the same amount of 
 space as every other molecule in the gaseous state (under the same condi- 
 tions of temperature and pressure). 
 
 (</) A molecule usually consists of a group of atoms. These atoms are 
 bound together by chemical affinity and into a union of exceedingly inti- 
 mate relationship. In fact, in most cases the molecule cannot be divided 
 without absolutely changing the identity of the substance. This result might 
 be expected in the case of molecules composed of different kinds of atoms. 
 
 Thus water has molecules, each expressible by the formula H 2 O. When 
 water is decomposed, the change takes place as follows : 
 
 2 HO. when decomposed, yield 2 H 2 + O 2 
 
 Two molecules of Two molecules of One molecule of 
 
 Water, Hydrogen gas, Oxygen gas, 
 
 36 4 32 
 
 parts by weight. parts by weight. parts by weight. 
 
THE CONSTITUTION OF MATTER. 5 
 
 But though at first unexpected, it is easily seen that a change of iden- 
 tity may result from the decomposition or rearrangement even of molecules 
 whose individual atoms are of precisely the same kind. Thus ozone has 
 molecules, each expressible by the formula O.j. When ozone is decomposed, 
 the change takes place as follows : 
 
 2 O 3 yield by decomposition 3 O 2 
 
 Two molecules of Three molecules of 
 
 Ozone, Ordinary Oxygen, 
 
 96 96 
 
 parts by weight. parts by weight. 
 
 Now it is an observed fact that the properties and powers of ordinary 
 oxygen are very different from those of ozone. 
 
 (^) The term molecule, usually reserved for groups of atoms, is ex- 
 tended to single atoms in the four exceptional cases, already cited, in which 
 the single atom is capable of existing apart. Thus a molecule of mercury 
 means also a single atom of mercury. 
 
 Elements and Compounds. Matter is called simple, or 
 elementary, when its molecules are composed of atoms 
 of the same kind. 
 
 Most of the elementary gases are believed to have two 
 atoms in the molecule. 
 
 Thus, the hydrogen molecule is expressed 
 
 H - H or H 2 , 
 and the oxygen molecule, 
 
 O = O or O 2 . 
 
 Matter is called compound when its molecules are 
 made up of different kinds of atoms. The number of 
 atoms in many compound molecules is but two ; in 
 others it is greater, sometimes reaching to several 
 hundreds. 
 
 Examples : Starch, C r ,H 10 O 5 , 
 
 21 atoms. 
 
 Protagon, C^H^PO^, 
 
 434 atoms. 
 
6 THE CONSTITUTION OF MATTER. 
 
 The Mass. A mass of matter is a collection of mole- 
 cules. Portions of matter appreciable by the senses 
 are, in most cases, masses. 
 
 FIG. i. Antoine Laurent Lavoisier. Born in Paris, August 26, 1743; died on the 
 scaffold in Paris, May 8, 1794. 
 
 The Modern Atomic Theory. The views of the con- 
 stitution of matter set forth in the foregoing paragraphs 
 may be said to have their origin in the atomic theory of 
 Dr. John Dalton, an English mathematician, who began 
 to develop his theory in the year 1803. 
 
 In some ancient metaphysical speculations matter 
 
THE CONSTITUTION OF MATTER. 7 
 
 was held to be infinitely divisible, while in others the 
 contrary view was maintained. 
 
 The ancient philosophers who constructed atomic 
 theories felt sure, upon general grounds, that matter is 
 susceptible of division to a degree far beyond that which 
 
 FIG. 2. Chemical balance. (A portion is shown cut away so as to 
 display the tripod.) 
 
 their appliances effected. But when the question arose, 
 " Is not matter then infinitely divisible ? " no valid answer 
 could be given. An unconquerable difference of opinion 
 existed. 
 
 The ancient views were defective because they were 
 too largely speculative and were not based on a suffi- 
 ciently large number of facts especially such facts as 
 can be learned only by carefully devised and conducted 
 experiments. 
 
8 
 
 THE CONSTITUTION OF MATTER. 
 
 The modern chemist declares that in fact there exist 
 at present limits to the divisibility of matter. More- 
 over, Dalton's theory, as well as other modern views of 
 the constitution of matter, are based not upon specula- 
 
 FIG. 3. Simple form of barometer, called Torricelli's. The tube A is closed at the 
 top. The mercury does not fall because of the atmospheric pressure exerted upon the 
 surface of mercury at MN. 
 
 tion, but upon discoveries reached by accurate experi- 
 ments with carefully devised appliances. 
 
 The most important and suggestive facts were col- 
 lected by the chemists of the last part of the eighteenth 
 century, and especially Lavoisier. He led the way by 
 
FIG. 4. Special barometer used for measuring the heights of mountains. The tripod 
 is constructed so that it may be closed, and thus protect the more delicate parts of the 
 barometer. Upon ascending a mountain the diminished atmospheric pressure is mani- 
 fested by the fall of mercury in the long tube of the barometer. 
 
FIG. 5. Water barometer in the Tour Saint- Jacques, Paris. Of course the tube is 
 much longer than a mercurial barometer. On the other hand, the variations in the height 
 of the barometer are much more marked. A variation of atmospheric pressure represent- 
 ing an inch on the mercurial barometer produces a variation of about 13.6 inches in the 
 water barometer. 
 
Fig. 6. Larger view of the base of the water barometer shown in section in Figure 5. 
 The self-registering apparatus containing clock-work and paper cylinder is shown at the 
 right. 
 
12 THE CONSTITUTION OF MATTER. 
 
 his rigid use of the balance in chemical investigation. 
 He maintained what was not perceived before that 
 weight is an important function of matter, and a safe 
 guide in chemical reasoning. His teaching showed the 
 value of those quantitative methods which have not only 
 afforded a sure basis for modern theories of matter, but 
 have been a most important aid in the general modern 
 progress of chemistry. 
 
 The first notions of the modern atomic theory appear 
 to have been suggested to Dalton by following La- 
 voisier's methods ; i.e. by the use of the quantitative 
 processes of the time. Dalton's work led also to the 
 enunciation of the important laws of definite proportions 
 and of multiple proportions, laws which are still the 
 chief support of that theory. 
 
 Modern instruments of precision, notably the balance, 
 the barometer, the graduated eudiometer, 1 and the ther- 
 mometer have afforded large contributions toward new 
 and exact knowledge of matter and its forces ; they have 
 also aided to dispel many old and false impressions. 
 
 " The vicissitudes in the fortunes of a truly scientific idea are aptly illus- 
 trated by the history of the atomic theory. After a period of dormancy of 
 more than 2000 years the atomic theory was revived and rendered definite 
 by Dalton, was firmly established on an experimental basis by Berzelius, 
 was almost abandoned by the school founded by the same chemist, was 
 rehabilitated and again nearly despaired of by Dumas, was largely advanced 
 by Avogadro, was subdivided and its parts clearly distinguished by Gerhardt 
 and Laurent, and is now the foundation-stone of a great and ever-increasing 
 edifice." 2 
 
 Atoms and molecules are now considered to be real existences as truly 
 as are planets and fixed stars; and they are as truly susceptible of measure- 
 
 1 Eudiometer: an instrument or vessel for exact measurement of the volume 
 of a gas. 
 
 2 Muir, Principles of Chemistry, p. 24. 
 
1 709 
 
 FIG. 7. Portion of a barometer, show- 
 ing al the top a part of the long glass tube 
 containing mercury. The reservoir of mer- 
 cury in the tube may be protected from 
 undue agitation by advancing the screw Q 
 so that its plug may close the lower aper- 
 ture of the long glass tube. 
 
 FIG. 8. Top of mercury column of a 
 barometer. The rack and pinion move- 
 ment is for the purpose of moving the 
 vernier in order to afford more accurate 
 reading. 
 
14 THE CONSTITUTION OF MATTER. 
 
 ment. Their motions, though of a different kind, are as much matters of 
 fact. 
 
 It must be admitted that even to-day some persons question the exist- 
 ence of the chemical atoms. The difficulty is probably the outcome of 
 ambiguity in the word atom. The atom of the Greek atomists may or may 
 not be the atom of the modern chemist. The Greek atom is something 
 that by its very nature cannot ever be divided. The atom described by 
 modern scientists is merely a very minute portion of matter that has not 
 yet been divided. The Greek atom is a metaphysical creation; the atom 
 of modern science is a real thing. 
 
CHAPTER III. 
 IS MATTER INDEED MOLECULAR AND ATOMIC? 
 
 EVIDENCE POINTING TO CERTAIN DEGREES OF HETERO- 
 GENEITY. 
 
 General Discussion upon Atoms and Molecules. Proba- 
 bly many persons and especially those whose reason- 
 ing powers are but imperfectly developed would have 
 more confidence in the existence of atoms and molecules 
 if they were visible. 
 
 But after all, the senses are by no means infallible 
 guides. The sense of sight often deceives ; even in the 
 commonest affairs of life reliance is often placed upon 
 the reasoning powers in opposition to the direct evi- 
 dence of the eye. Especially when the eye is diseased, 
 it gives false ideas of color, form, and the like. Even 
 excellent eyes may not be able to see the vibration of a 
 certain piano-string while it is singing ; but by applica- 
 tion of processes of reasoning to experimental results 
 proof can be obtained, to the satisfaction of every one, of 
 the existence of such vibration. It is only by processes 
 of reasoning that the distances of the heavenly bodies 
 are measured, but no one doubts the data that astrono- 
 mers furnish. 
 
 However, single atoms and single molecules are not 
 visible, and, moreover, it does not seem likely that opti- 
 cal appliances will ever extend the range of man's visual 
 
 '5 
 
16 IS MATTER INDEED MOLECULAR AND ATOMIC ? 
 
 powers to such minute objects. But neither these diffi- 
 culties nor any others need deter the 'investigator from 
 efforts to solve the problem of the constitution of matter. 
 He may with propriety continue to ask such general 
 questions as the following : Is matter constructed on 
 the type of an imperforate mass, or is it more truly a 
 network of some sort ? Does one minute portion of a 
 given kind of substance come into absolute contact with 
 its neighboring portions, so that no pointed implement, 
 however fine and sharp, can discover places of more and 
 of less resistance, or are there minute avenues of some 
 sort affording more easy passage in some places than in 
 others ? Is matter indeed atomic and molecular, as has 
 been declared ? 
 
 The questions come to this : Is matter heterogeneous ? 
 If the answer is yes, the next questions are as to the 
 character and extent of this heterogeneity. 
 
 These questions do indeed receive decisive answers from physical and 
 chemical facts. By proper reasoning from natural phenomena there have 
 been secured, at first approximate, and later highly exact, answers. A 
 blind man may discover the construction of a gate that bars his path. By 
 applying to such an obstacle instruments of various shapes and sizes he 
 may be able to state whether it has the apparent continuity of sheet-iron, 
 or has horizontal or vertical bars, or is of netted gauze, or even has perfo- 
 rations like fine card-board. In like manner scientific observations may be 
 made as to the action of matter in response to certain general experiments. 
 Then proceeding on and on, there may be secured a tolerably certain 
 knowledge of its constitution. 
 
 Ordinary Observation upon this Subject. Some forms 
 of matter show plainly to the eye that they are made up 
 of more than one kind of substance that they are 
 heterogeneous. Other kinds may appear to the eye to be 
 homogeneous, while a fuller knowledge of them makes it 
 
IS MATTER INDEED MOLECULAR AND ATOMIC? \"J 
 
 absolutely certain that they are not so. As a very sim- 
 ple example consider sugar. It appears at first consid- 
 eration to be alike all through ; it is very easy to prove, 
 however, that it contains at least carbon something 
 very different from sugar. 
 
 More Searching Examination. There is presented 
 below a very brief resume of certain observed properties 
 or actions of matter that point first to general hetero- 
 geneity or grained structure of some sort, and next 
 to distinct molecular, and finally to distinct atomic 
 composition. 
 
 The facts stated here are by no means all that apply. 
 The whole body of physical and chemical knowledge 
 points in this direction. 
 
 FIRST. Evidences of heterogeneity found in certain 
 mechanical properties of substances. 
 
 (a) The diminution in volume of solids, of liquids, 
 and in a yet more marked degree of gases, by 
 mechanical pressure, leads to the suggestion that there 
 are interior spaces which are diminished by that 
 action. 
 
 (b) In similar manner peculiarities of internal struc- 
 ture are revealed by certain changes of the outline of 
 bodies. The various degrees of -elasticity, hardness, mal- 
 leability, and ductility displayed by various solids show 
 various kinds of heterogeneity. 
 
 The changes of thickness of soap-bubble films, before they break, have 
 been carefully studied. The relations of these films to light show their 
 thickness at different times. The thinnest films appear to be commen- 
 surable with the diameters of a molecule, as learned by other methods. 
 
1 8 IS MATTER INDEED MOLECULAR AND ATOMIC? 
 
 (c) Crystals possess cleavage and certain other prop- 
 erties which offer most marked suggestions of internal 
 peculiarities of structure. 
 
 The foregoing classes of facts point, therefore, to 
 general heterogeneity of substances in their internal 
 make-up. 
 
 SECOND. Evidences of heterogeneity found in certain 
 physical properties of substances. 
 
 (a) Gases diffuse readily into other gases. Liquids 
 readily absorb gases. Certain solids swallow large 
 quantities of gases without corresponding increase of 
 bulk. (Palladium and platinum occlude hydrogen in an 
 especially noteworthy degree.) 
 
 (b) Liquids diffuse themselves into other liquids with 
 great rapidity, sometimes (as in case of alcohol and 
 water) with diminution of volume. 
 
 (c) Solids dissolve quickly in liquids without unusual 
 external influence, diffusing themselves throughout the 
 solvents. (Dialysis is a specialized form of the motion 
 of solids into liquids.) 
 
 Some solids seem to dissolve in gases, rising up into 
 those gases under certain conditions, at temperatures far 
 below those at which these solids ordinarily volatilize. 1 
 
 These classes of facts point to internal places of feeble 
 resistance in substances. Thus they point to molecular 
 grouping of the firmer portions. 
 
 THIRD. Evidences of heterogeneity found in the rela- 
 tions of certain substances to heat. 
 
 1 Hannay and Hogarth, 
 
IS MATTER INDEED MOLECULAR AND ATOMIC? 1 9 
 
 (a) Solids, liquids, and gases expand upon addition of 
 heat and contract upon its withdrawal. The details of 
 these operations need not be presented here. But the 
 fact that different gases expand equally with heat is a 
 
 FIG. 9. The flask contains a dark-colored liquid, which does not allow the light to 
 pass. It is diathermanous, however, for heat rays go through it easily, as may be demon- 
 strated by concentrating them upon a piece of phosphorus or other combustible material. 
 
 special case under this head, and it points to similarity 
 of arrangement of particles in gases. 
 
 The fact that some crystals expand more in certain directions than in 
 others adds force to the argument; so do the facts connected with changes 
 of substances in what may be called an upward direction from solids to 
 
2O IS MATTER INDEED MOLECULAR AND ATOMIC ? 
 
 liquids and gases, and downward from gases to liquids and solids, respec- 
 tively by addition or withdrawal of heat. 
 
 The continuity of the three states of matter as shown by Thomas 
 Andrews furnishes additional general evidence of molecular structure in 
 matter. (See p. 35.) 
 
 (b] When certain substances are highly heated, they 
 acquire anomalous vapor densities. These point to dis- 
 
 FIG. 10. Arrangement for showing the refraction of light by water. A beam of 
 light, coming through the aperture, penetrates the water in the jar, but is bent out of its 
 original course. 
 
 sociation of molecules (and thence to the existence of 
 molecules). 
 
 (c) All substances when highly heated give out light. 
 In most cases a given metal, for example, gives out light 
 made up of waves having many different rates of vibra- 
 tion. This fact is proved by the spectroscope. 
 
IS MATTER INDEED MOLECULAR AND ATOMIC? 21 
 
 Moreover, the same substance may give different 
 colors (and so different spectra) at different high tem- 
 peratures. 
 
 These facts seem to prove that an apparently homo- 
 geneous substance may possess different internal vibrat- 
 ing parts. (See p. 86.) 
 
 (d} Certain bodies manifest in a remarkable degree 
 the phenomena associated with specific heat and latent 
 heat. (See pp. 45 and 47.) Again, as respects radiant 
 
 FIG. ii. Crystal of Iceland spar, showing double refraction; the single line is made to 
 appear double. 
 
 dark heat, some substances are distinctly diatherma- 
 nous, while others are athermanous. These peculiari- 
 ties cannot be discussed at length here. But they are 
 explicable only upon the theory that the substances dis- 
 playing them are molecular. Thus they support the 
 proposition under review. 
 
 FOURTH. Evidences of heterogeneity found in the 
 relation of certain substances to light. 
 
22 IS MATTER INDEED MOLECULAR AND ATOMIC ? 
 
 The optical properties of substances afford some of 
 the most clear and decisive evidences of grainedness of 
 structure in bodies. The rays of light appear to be 
 capable of use as a penetrating agency of a most dis- 
 criminating character. 
 
 It is not practicable here, however, to do more than allude to the classes 
 of optical facts referred to. 
 
 FIG. 12. Method of displaying phenomena of double refraction. A single beam of 
 light passed through a double-refracting crystal appears on the screen as two beams 
 of light. 
 
 The opacity, the translucency, or the transparency of a given body; the 
 phenomena of ordinary and of double refraction ; the polarization of light, 
 and the passage of the polarized ray through crystals in certain positions 
 and not in others; the peculiar passage of the polarized ray also through 
 solids under pressure as compared with the same at normal conditions; 
 the rotation of the polarized ray by sugar solutions and the like, all con- 
 tribute greatly to substantiate the molecular theory. (In case of certain 
 substances, as cane sugar, the greater the quantity of the substance, the 
 greater the rotation. This suggests some greater interference of a greater 
 number of molecular particles.) 
 
IS MATTER INDEED MOLECULAR AND ATOMIC ? 23 
 
 FIFTH. Evidences of heterogeneity found in the rela- 
 tions of certain substances to the electric ctirrent. 
 
 (a) In some cases elongation takes place when sub- 
 stances are rendered magnetic by an electric current. 
 Thus iron bars and steel bars are so elongated. 
 
 (b) A certain substance (boro-silicate of lead) trans- 
 mits polarized light when electrified, otherwise not. 
 
 FIG. 13. Polarizing saccharimeter. A ray of polarized light passing through a solu- 
 tion of cane sugar in water is rotated in such a way as to enable the observer to determine 
 the percentage of sugar in the sample. 
 
 (c) Certain rarified gases and vapors show peculiar 
 stratification when made luminous by the electric dis- 
 charge of the Ruhmkorff coil. The stratification is dif- 
 ferent for different gases. This indicates the existence 
 of internal portions in these substances. 
 
 (d) A given portion of oxygen gas is changed by 
 the silent electric discharge into ozone without loss of 
 weight, but with diminution of volume. 
 
24 IS MATTER INDEED MOLECULAR AND ATOMIC 
 
 (e) The phenomena of electrolysis are very striking. 
 A given battery current passing simultaneously through 
 several compounds in different vessels may decompose 
 them all at once. The weights of certain elements thus 
 separated are very different. The following examples 
 may be given : 
 
 FIG. 14. A portion of gas appears at first homogeneous, but is shown in disconnected 
 strata when under the influence of an electric spark furnished by a Ruhmkorff coil. The 
 coil is connected with a single cell of a bichromate battery. The glass tube containing 
 the gas is called a Geissler tube. 
 
 Hydrogen 
 Copper . 
 Zinc 
 
 2.0 parts by weight. 
 63.2 " " 
 64.9 " " 
 
 The amounts are closely proportioned to the amounts 
 separated by similar chemical operations. These and 
 other similar numbers represent a tendency to subdi- 
 vision by different but definite quantities. Thus they 
 
IS MATTER INDEED MOLECULAR AND ATOMIC? 25 
 
 show definite internal structure in masses of the sub- 
 stances mentioned. 
 
 General Comment on the Foregoing Evidences. Heat, and 
 especially light and electricity, are agencies possessing an extraordinary 
 power of penetrating bodies. So far as they penetrate them with greater 
 or less ease in certain directions or under special conditions, they cannot 
 fail to raise the suggestion that the bodies in question are not perfectly 
 homogeneous, but possess spaces of greater or less resistance. But this is 
 what was to be demonstrated by the foregoing paragraphs. 
 
 FIG. 15. Geissler tube, showing that a gas appearing at first to be homogeneous 
 manifests stratification when illuminated by an electric current from a Ruhmkorff coil. 
 
 SIXTH. Evidences of heterogeneity found in the chem- 
 ical relations of substances. 
 
 The chemical suggestions of molecular structure are 
 far more distinct and precise than the mechanical and 
 physical. These may now be discussed. Of course 
 only a few will be presented. If completeness were 
 sought, the entire body of chemical knowledge would 
 have to be adduced. The whole of modern chemistry 
 must be referred to as the argument in this part of 
 the case. 
 
26 IS MATTER INDEED MOLECULAR AND ATOMIC ? 
 
 Illustration. Chemical analysis shows that most sub- 
 stances are made up of more than one kind of matter. 
 With the exception of about seventy substances called 
 elements, everything known has internal complexity 
 instead of internal uniformity throughout. 
 
 Thus the purest cooking salt, apparently homoge- 
 neous, is really made up of two very different sub- 
 stances or kinds of matter, chlorine and sodium. 
 
 The chlorine has certain properties distinctly its 
 own of which may be mentioned its gaseous form, 
 its green color, and its bleaching power. And simi- 
 larly, the sodium has certain properties distinctly its 
 own of which may be mentioned its metallic lustre, its 
 affinity for oxygen, and its power to decompose water 
 in a certain way. 
 
 By analysis pure salt may be subdivided into portions 
 of these two very different kinds of matter. 
 
 And by synthesis chlorine and sodium may be made 
 to combine, and when they do so combine, they form a 
 new product, which is precisely common salt and noth- 
 ing else. 
 
 But the new product is not a mere mixture of its constituents. 
 
 (a) The constituents are more thoroughly intermingled or interdiffused 
 than is possible in mere mixtures. For the minutest portions that can be 
 recognized as containing true salt contain both of the factors, and not a 
 single one. 
 
 () The constituents are associated in fixed and definite proportions by 
 weight (always 23 parts of sodium to 35.4 parts of chlorine), facts of 
 which mere mixtures are independent. 
 
 (c) The constituents cannot be separated by any mechanical processes 
 nor by any processes other than chemical or physico-chemical. 
 
 (</) The new product, the common salt, has no one of the six proper- 
 ties heretofore enumerated as possessed by the constituents (nor yet has it 
 the average of these properties) : instead, it has a new set of properties 
 entirely its own. 
 
IS MATTER INDEED MOLECULAR AND ATOMIC ? 2/ 
 
 It is plain that a mass of common salt is made up of 
 small portions of salt, each small portion containing a 
 definite amount of chlorine and of sodium. These small 
 portions are the units already called molecules. Each 
 of these molecules is made up of heterogeneous parts. 
 But this is the point that was to be demonstrated. 
 
 Practically the same method of reasoning may be 
 applied to every other substance known, except the 
 seventy elementary ones. But further, a careful study 
 of the elementary substances has shown that these are 
 composed of the little groups called molecules, only in 
 their cases the heterogeneity of parts is far less marked 
 in quantity and quality, though apparently equally 
 marked in a reality of distinct portions. 
 
 Compound Radicles. Sometimes a grotfp of atoms is 
 recognized as capable of transfer as a whole from one 
 complex molecule to another, and yet not capable of 
 existing alone. To such a group the name compound 
 radicle is applied. A simple example is found in the 
 hypothetical metal ammonium (NH 4 ). Thus the com- 
 pound called sal-ammoniac (ammonic chloride, NH 4 C1), 
 contains the group NH 4 , which is capable of continued 
 transfer, as a whole, from one molecule to another ; it 
 acts very much like an element only, if liberated, it 
 at once decomposes. Chemists know a large number 
 of other compound radicles of similar type ; indeed, a 
 whole class of bodies called compound ammonias has 
 been studied very thoroughly by Hofmann and others. 
 
 Conclusions Reached. There can be no reasonable 
 doubt then that there exist molecules in the general 
 
28 IS MATTER INDEED MOLECULAR AND ATOMIC ? 
 
 sensn of small portions of matter that preserve an integ- 
 rity tJirough a series of operations. They are in fact 
 units of a certain order. 
 
 Further, there can be no doubt that the molecular 
 
 FIG. 16. August Wilhelm Hofmann, Professor of Chemistry in the University 
 of Berlin. 
 
 units are made up of units of another order, the 
 so-called elementary atoms. These units are also inte- 
 gral parts, and they preserve their unbroken composition 
 whatever it may be in spite of any processes as yet 
 applied to them. Whether or not they are real atoms, 
 
IS MATTER INDEED MOLECULAR AND ATOMIC ? 2Q 
 
 in the sense of absolutely indivisible things, is not posi- 
 tively known. There are certain grounds for believing 
 tJiat they are not. That they are units of a most impor- 
 tant character must be true. Their recognition is the 
 work of modern investigators. Their discovery is a most 
 valuable and fruitful one, whatever theory of their ulti- 
 mate composition any one may hold, or whatever new 
 facts relating to them may be discovered hereafter. 
 
 Are the Atoms of the Chemist Composite ? The no- 
 tion has often been presented that the seventy substan- 
 ces called elementary are in fact very stable compounds 
 not yet decomposed. Chemists, physicists, and philoso- 
 phers have explicitly declared their belief in this view. 
 Dalton, Faraday, Gladstone, Brodie, Graham, Mills, 
 Stokes, Lockyer, and other experimenters have dis- 
 tinctly held it. From another side, thinkers like Her- 
 bert Spencer have recognized the necessity of it. 
 
 Four general grounds for believing the elements to be composite may 
 be stated here : 
 
 FIRST. Lockyer considers that the fact that a given element affords a 
 multitude of spectrum lines at once suggests that these lines are somehow 
 connected with different vibratory powers of different portions of the atom. 
 In other words, the atom is composite. 
 
 SECOND. Dr. Carnelley offers a strong argument from the analogies of 
 certain acknowledged compounds. It is this: There are recognized, in 
 the group of hydrocarbons, a continuous series of compounds advancing in 
 molecular weights by regular numerical steps, and having, with these, a 
 regularly advancing sequence of physical and chemical properties. So the 
 elements, as arranged by Newlands, Mendeleeff, Lothar Meyer, and others, 
 constitute a series of substances whose members show a regular advance in 
 atomic weights, and with it a regular progress of physical and chemical 
 properties. There is no doubt that the hydrocarbons are compounds; 
 there is then a high probability that the elements are so also. Carnelley 
 goes so far as to offer the theory that the seventy elements are composed 
 by union, in different ways, of three kinds of ultimate*. 
 
3O IS MATTER INDEED MOLECULAR AND ATOMIC ? 
 
 THIRD. Professor Crookes considers that he has in certain cases 
 actually decomposed certain elements. He says that the substances he 
 has obtained from so-called "old yttria" "are not impurities in yttrium. 
 They constitute a veritable splitting up of the yttrium molecule into its 
 constituents." 
 
 FOURTH. Certain so-called elements, as tellurium and didymium, have 
 been considered compounds on grounds based on their general chemical 
 properties. 1 
 
 The Genesis of Atoms. Professor Crookes proposes 
 the theory that matter, as we now know it, has been pro- 
 duced by a kind of evolution from an antecedent sub- 
 stance or fire-mist which he calls protyle. He considers 
 that by a series of large falls of temperature alternating 
 with periods of approximately stationary temperature, 
 the protyle has become condensed or combined into a 
 series of substances our elements. 
 
 " The first-born element would, in its simplicity, be most nearly allied to 
 protyle. This is hydrogen, of all known bodies the simplest in structure 
 and of the lowest atomic weight. 
 
 " Any well-defined element may be likened to a platform of stability, 
 connected by ladders of unstable bodies. In the first coalescence of the 
 primitive stuff there would be formed the smallest atoms; these would then 
 unite, forming larger groups; the gaps between the several stages would 
 gradually be bridged over, and the stable element appropriate to that stage 
 would absorb, so to speak, the unstable rungs of the ladder which led up 
 to it. 
 
 " In this genesis of the elements the longer the time taken up in the 
 cooling- down process, during which the hardening of protyle into atoms 
 takes place, the more sharply defined would be the resulting elements; 
 whilst the more rapid and the more irregular the cooling, the more closely 
 the resulting bodies would fade into each other by almost imperceptible 
 degrees. Thus we may conceive that the succession of events which gave 
 rise to such groups as platinum, osmium, and iridium palladium, ruthe- 
 nium, and rhodium iron, nickel, and cobalt might have produced only 
 one element in each of these three groups if the process had been greatly 
 
 1 Brauner, J. Ch. Soc., 1889, p. 382. 
 
IS MATTER INDEED MOLECULAR AND ATOMIC? 3! 
 
 prolonged. And conversely, had the rate of cooling been much more rapid, 
 elements might have originated still more nearly identical than are nickel 
 and cobalt. Thus may have arisen the closely allied elements of the cerium, 
 yttrium, and similar groups. In fact, we may regard the collocation of the 
 
 FIG 17. Sir William Thomson, distinguished English electrician and physicist. 
 
 minerals of the class of samarskite and gadolinite as a kind of cosmical 
 lumber-room, where elements in a state of arrested development the un- 
 connected missing links of inorganic Darwinism are gathered together." 
 
 Shapes of Atoms. - - Sir William Thomson has pro- 
 pounded the theory that atoms possess the shape of 
 
32 IS MATTER INDEED MOLECULAR AND ATOMIC ? 
 
 rings of various kinds. In this view each ring pos- 
 sesses a vortex motion, something like that produced 
 by the spontaneous ignition of bubbles of phosphuretted 
 hydrogen. 
 
 Movements of Atoms. It is at present unknown 
 whether there is any constant motion of atoms within 
 the molecule unattended by decomposition of it. Gen- 
 eral analogy and certain facts strongly favor the affirma- 
 tive view. There is plenty of room in gaseous molecules 
 at least. 
 
 Of course when a molecule is decomposed there is 
 generally necessity of movement of its atoms ; but this 
 kind of motion may be only temporary, dependent upon 
 the duration of the particular exciting cause. 
 
 Relative Position of Atoms in Space. The brilliant 
 studies of Kekule upon organic compounds have given 
 rise to pretty general adoption of a certain expression 
 for the substance benzol (C 6 H 6 ). This takes the form 
 of a hexagon called the benzol hexagon or the benzol 
 ring, as represented by the diagram below : 
 
 I 
 
 C 
 
 // \ 
 -C C- 
 
 I II 
 
 -C C- 
 
 ^ / 
 
 C 
 
 In this expression the six atoms of carbon are viewed 
 as forming a closed chain, of which each atom is attached 
 
34 IS MATTER INDEED MOLECULAR AND ATOMIC ? 
 
 to its neighboring atoms, on the one hand by two points 
 and on the other hand by one point of attraction. 
 
 Recent studies of van 't Hoff, Pasteur, Wislicenus, and others have car- 
 ried the subject still further. Attempting to explain certain remarkable 
 peculiarities of carbon compounds, the theory has now been presented that 
 the four points of attraction of a single atom of carbon are ordinarily dis- 
 tributed about its central point somewhat as are the four apexes of the 
 regular tetrahedron. Again, when the series of carbon atoms are attached 
 in circuit, it is supposed that in some cases one carbon atom is related to 
 its neighbor as if the apex of a tetrahedron were presented to the apex of 
 another tetrahedron. Again, in another case, a carbon atom may be 
 attached to its neighbor as if an edge of a tetrahedron were presented to 
 the edge of another tetrahedron. In the former case it is supposed that 
 the one carbon atom is capable of a certain amount of rotation about the 
 other carbon atom. In the latter case such rotation would naturally be 
 impracticable. It is supposed that this theory contributes something 
 toward explanation of asymetric carbon compounds; such, for instance, 
 as the different varieties of tartaric acid. 
 
CHAPTER IV. 
 THE THREE STATES OF MATTER. 
 
 SOLIDS, LIQUIDS, AND GASES. 
 
 THE various kinds of matter known are capable of 
 arrangement in a series representing many stages of 
 tenuity. Between the most rigid solids on the one 
 hand, and the most rarified gases on the other, there 
 exist substances which possess many degrees of den- 
 sity, the progress from one extreme to the other being 
 exceedingly gradual. 
 
 There are even no well-defined lines of demarcation 
 between a solid and its own liquid, and between a li- 
 quid and its own vapor. Indeed, Thomas Andrews has 
 shown that in case of the same substance there may be 
 even a continuity of its liquid and gaseous form. Thus 
 water may exist in a closed vessel under such condi- 
 tions, at once of high temperature and pressure, that 
 the upper portions shall be gaseous and the lower por- 
 tions liquid, and yet the latter may show no top surface 
 or other evidence of a distinct place where liquid ends 
 and vapor begins. 
 
 For purposes of convenience, however, chemists admit 
 the existence of three, and perhaps four, states of mat- 
 ter. They are called respectively the solid, the liquid, 
 the gaseous, and the radiant ftfrms. 
 
 From what has just been said, however, it is evident 
 that it is not easy to define the four terms used. 
 
 35 
 
36 THE THREE STATES OF MATTER. 
 
 A Solid Defined. In general, a solid is a form of 
 matter possessing such rigidity that a given portion of 
 it resists with considerable vigor any forces that tend 
 to change its shape. Of course, then, a given solid does 
 not unless under very great pressure assume the 
 shape of a vessel in which it is placed, and except, of 
 course, the one was contrived to fit the other, or the 
 solid is in the form of powder. (Possibly the latter 
 qualification is unnecessary.) 
 
 A Liquid Defined. In general, a liquid is a fluid form 
 of matter having such internal mobility as enables it to 
 assume the form of at least the lower portions of its 
 containing vessel, whatever the shape, and provided 
 only that the liquid shall be in sufficient quantity. 
 Further, an untrammelled liquid has usually a hori- 
 zontal top surface. 
 
 A Gas Defined. In general, a gas is a fluid form of 
 matter such as readily (under ordinarily prevailing con- 
 ditions) assumes the shape of the containing walls of 
 any vessel whatsoever. Far from having any distinct 
 top surface of its own, a gas has sufficient expansive 
 force to lead it to fill entirely any containing vessel 
 whatever its size. 
 
 One of the most characteristic features of bodies when in the gaseous 
 state is the great change in volume which they may be easily made to 
 undergo. Their volumes are particularly sensitive to change of pressure 
 and of temperature. When the gas is heated, great expansion takes place 
 if the walls of the vessel are such as permit it. (If expansion is fully re- 
 sisted, however, there is still an increase of tension or expansive force im- 
 parted to the gas. Hence addition of heat may produce increase cf volume 
 of a gas when pressure is maintained constant, or it may produce increase 
 of tension when volume is maintained constant.) 
 
THE THREE STATES OF MATTER. 37 
 
 Andrews recommends that the term gas be reserved for a substance 
 when it is at a temperature higher than its critical point. He recommends 
 that the term vapor\)Z applied to a substance, in the gaseous condition, 
 but existing at a temperature below its critical point. 
 
 The term vapor is applied, in a general way, to those gases which are 
 easily condensible to the liquid form. 
 
 The modern scientist accepts the hypothesis that even 
 a given small portion of gaseous matter is made up of 
 millions of millions of molecules, and that these are in 
 rapid motion in all directions. In all ordinary gases the 
 length of the mean free path of a molecule is exceed- 
 ingly small as compared with the dimensions of the 
 confining vessel ; in fact, in every second of time each 
 molecule has millions of collisions with other molecules. 
 Indeed, many of the well-recognized properties of gas- 
 eous matter are referable to these constant encounters. 
 
 Importance of the Study of Gases. Several of the gen- 
 eral laws of matter relate to it while in its gaseous condi- 
 tion. They show that bodies of the most diverse chemical 
 constitution and properties, possess many close physical 
 correspondences when in the gaseous form, while the 
 solid and liquid forms possess no equivalent resemblances. 
 
 These facts create the belief that there is some funda- 
 mental similarity and simplicity of molecular condition 
 characterizing the gaseous state. Hence this condition 
 has been very carefully studied as that affording to 
 molecular science the best common ground for the com- 
 parison of bodies. The hypothesis of Avogadro and 
 Ampere is a result of this view. (See p. 78.) 
 
 It is probable, however, that a given substance may possess in the solid 
 condition and in the liquid condition molecules containing a greater num- 
 ber of atoms than it has while in the gaseous condition. 
 
38 THE THREE STATES OF MATTER. 
 
 Thus it has been shown by Raoult and by Thomsen, from very dis- 
 similar points of view, that the molecular formula of liquid -water is proba- 
 bly as great as H 4 O 2 . (This does not invalidate the statement of the formula 
 of water vapor as H 2 O.) 
 
 It is probable that the systems of molecules in liquids and solids are not 
 only very different from those of gases, but that they vary greatly among 
 themselves. These molecular aggregations, too, are probably not perma- 
 nent, but are continually breaking up, their constituents changing partners. 
 
 FIG. 19. Crookes's radiant matter bulb. At b is a screen. When the electric cur- 
 rent traverses the radiant matter in the bulb, the molecules fall against the screen b\ 
 thereupon they are intercepted, and a dark shadow is thrown at d. 
 
 Radiant Matter. The radiant form of matter is much 
 more tenuous than even the gaseous, which it most resem- 
 bles. In the radiant form the molecules are very far apart 
 compared with their distances in the gaseous form. It 
 is this circumstance which leads them to display physical 
 properties different from those of the same bodies in the 
 gaseous condition. As a result, also, the mean free 
 path is rendered so long that the molecular hits in a 
 given time may be disregarded in comparison with the 
 misses ; thus each molecule is allowed more fully to 
 obey its own laws without interference. When the free 
 
THE THREE STATES OF MATTER. 
 
 39 
 
 path becomes so long as to be comparable with the 
 dimensions of the vessel, the properties which charac- 
 terize the gaseous condition are reduced to a minimum, 
 
 FIG. 20. Appearance of the illuminated screen and the dark image formed in 
 Crookes's bulb, shown in Figure 13. 
 
 and the matter becomes exalted to the ultra-gaseous 
 state called the radiant condition. 
 
 The study of radiant matter has been conducted of 
 late years with great earnestness and success by Pro- 
 
 FIG. 21. Crookes's bulb. The bulb contains a small amount of gas in the radiant 
 form. When the electric current is passed through the tube, the molecules of gas strike 
 against the paddles, setting the wheel in motion and making it traverse the track from 
 one end of the bulb to the other. 
 
 fessor William Crookes of London. His experiments 
 have been conducted in glass tubes or bulbs from whose 
 interiors the major portion of gas originally within them 
 
4O THE THREE STATES OF MATTER. 
 
 has been removed. The minute portions remaining are 
 then in the radiant form. Professor Crookes has investi- 
 gated the properties of radiant matter by the use of 
 these tubes and certain ingenious mechanical appliances 
 constructed within them. He seems to have demon- 
 
 FIG. 22. Crookes's radiometer. The bulb contains a very small amount of gas in 
 the radiant form. Under the influence of a very minute amount of heat, the radiant 
 matter causes the vanes of the mill to rotate rapidly. 
 
 strated a number of propositions like the following : 
 
 (1) The length of the mean free path of molecules of 
 matter in the radiant condition can be measured ; 
 
 (2) radiant matter exerts powerful phosphorogenic ac- 
 tion where it strikes ; (3) radiant matter proceeds in 
 straight lines it will not turn a corner ; (4) radiant 
 
THE THREE STATES OF MATTER. 4! 
 
 matter when intercepted by solid matter casts a shadow ; 
 (5) radiant matter exerts strong mechanical action where 
 it strikes this is proved by various forms of the well- 
 
 FIG. 23. Crookes's bulb. The bulb contains a small amount of gas in the radiant 
 form. Under ordinary conditions, even when the electric current is flowing through the 
 bulb, the gas is intercepted by the screen cd. When, however, the magnet^- is placed as 
 indicated, the molecules are attracted so that they flow over the screen. They strike the 
 paddles of the mill-wheel, setting it in motion. 
 
 known apparatus called Crookes's radiometer ; (6) radi- 
 ant matter is deflected by a magnet ; (7) radiant matter 
 produces heat when its motion is arrested. 
 
CHAPTER V. 
 CHANGE FROM ONE STATE TO ANOTHER. 
 
 INFLUENCE OF ADDITION AND OF WITHDRAWAL OF HEAT. 
 
 Introduction. Of the seventy chemical elements, five 
 are gaseous at ordinary temperatures, hydrogen, nitro- 
 gen, oxygen, fluorine, and chlorine. Two are liquids, 
 bromine and mercury. The others are solids. 
 
 The terrestrial globe (with its oceans and atmos- 
 phere) contains a very large number of compound sub- 
 stances. Under the conditions of temperature pre- 
 vailing, the majority of them are solid ; a considerable 
 number exist in the atmosphere in the gaseous form ; 
 there is one widespread liquid, water. (The water of 
 the ocean, it is true, holds in solution a good many sub- 
 stances which may be considered to exist for the time 
 being in the liquid form.) 
 
 Chemists have recognized an enormous number of 
 compound substances other than those existing in na- 
 ture. Of these, some are solid at ordinary temperatures, 
 some are liquid at ordinary temperatures, some are gas- 
 eous at ordinary temperatures. 
 
 Influence of Heat. In general, the condition of a 
 substance, whether solid, liquid, or gaseous, is mainly 
 dependent upon the amount of heat it contains. Of 
 
 course the foregoing discussion has dealt with sub- 
 42 
 
CHANGE FROM ONE STATE TO ANOTHER. 
 
 43 
 
 stances under ordinary conditions of temperature and 
 under the possession by each individual substance of 
 such an amount of heat as they may take up from the 
 terrestrial system of earth, water, and air. 
 
 But chemists are able to add to or subtract from substances a very large 
 amount of heat as compared with what the substances absorb from their 
 natural surrounding medium. Incidentally the temperature of the sub- 
 
 FIG. 24. Disposition of apparatus for determining the melting-point of a solid. At 
 the moment the solid liquefies electric communication is established through the wires 
 and a bell is rung. At the moment of ringing the temperature is read off from the 
 thermometer. 
 
 stance is raised or lowered by addition or withdrawal of heat respectively, 
 although not in exact proportion to the amount added or withdrawn. The 
 actual change of temperature is modified somewhat by certain natural 
 properties of the substance, which give rise to the phenomena of specific 
 heat. (See p. 45.) 
 
 The addition of heat to a body tends also to advance it from the solid to 
 the liquid, and even to the gaseous state. And likewise the withdrawal of 
 heat from a body tends to reduce it from the gaseous to the liquid and 
 even to the solid state. The word tends is used here with the admission 
 that, while in some cases the tendency is realized and the result sought is 
 
/j/| CHANGE FROM ONE STATE TO ANOTHER. 
 
 attained, in others it is not. In a few of the latter cases the requisite 
 amounts of heating or cooling influences are not obtained by means at 
 present at the command of the chemist. In many cases dissociation inter- 
 feres. (See p. 49.) 
 
 A Double Change of State. Certain well-known substances may 
 be made at will, by addition or subtraction of moderate quantities of heat, 
 to occupy either the solid, the liquid, or a gaseous state. Sulphur and 
 phosphorus are examples of such elementary substances, and water is an 
 example of such a compound substance. 
 
 It is not easy, however, to give many familiar examples of this sort. 
 
 A Single Change of State. There are, however, many well-known 
 cases of substances ordinarily solid that may be changed to the liquid state. 
 
 Thus, probably every one of the chemical elements, solid at ordinary 
 temperatures, may be liquefied (carbon, perhaps, excepted). Again, the 
 two liquid elements, bromine and mercury, may be vaporized. Further, 
 the gaseous elements may all be liquefied and probably even solidified. 
 
 Most compound substances, whether natural or artificial, which are solid 
 at ordinary temperatures of the earth, may be liquefied; and in most cases 
 these liquids and also compound substances liquid at ordinary tempera- 
 tures may be vaporized. An important exception must of course be 
 made of that large class of .organic substances (especially the organized 
 ones), which, ordinarily solid, undergo dissociation and that without 
 liquefying by addition of a moderate amount of heat. The same state- 
 ment applies also to certain substances which may become liquid, but dis- 
 sociate upon heating, before changing to the state of vapor. 
 
 In general, however, the chemist assumes that there are conditions 
 perhaps not always easy to secure under which every substance, with the 
 general exceptions noted, may be made to assume the solid, the liquid, or 
 the gaseous state. 
 
CHAPTER VI. 
 CHANGES INCIDENT TO ADDITION OF HEAT. 
 
 ADDITION OF HEAT TO A SOLID. 
 
 WHEN heat is added to a solid, a series of effects may 
 be produced in more or less orderly sequence, somewhat 
 as follows : (i) rise of temperature according to the 
 specific heat of the substance ; (2) melting, associated 
 with disappearance of heat under the conditions called 
 latent heat ; (3) a series of effects upon the liquid as, 
 for example, rise of temperature according to the speci- 
 fic heat of the liquid ; (4) partial vaporization followed 
 by boiling of the liquid; (5) complete change of a liquid 
 to the form of vapor ; (6) incidental to many of these 
 stages is expansion, whether of the solid, the liquid, or 
 the gas ; (7) dissociation may take place at different 
 points of temperature according to the nature of the 
 substance, i.e. in some cases a solid may dissociate, in 
 other cases a liquid may dissociate, and in yet others 
 dissociation may only take place after the substance has 
 attained the gaseous form and the gas has been sub- 
 jected to an extremely large addition of heat ; (8) fin- 
 ally evolution of light may occur. 
 
 Rise of Temperature according to Specific Heat. The 
 
 specific heat of a substance is the special amount of 
 heat involved in its rise or fall of temperature. To 
 
 45 
 
4 6 
 
 CHANGES INCIDENT TO ADDITION OF HEAT. 
 
 change a pound of water one degree upward in temper- 
 ature, a large amount of heat must be added to it. To 
 change a pound of water one degree downward in tem- 
 perature, a large amount of heat must be subtracted 
 from it. Most other substances have lower specific 
 heats than water; i.e. a smaller amount of heat, added or 
 subtracted, changes the temperature of a pound of any 
 substance other than water by one degree upward or 
 downward respectively. 
 
 TABLE OF A FEW SPECIFIC HEATS. 
 
 
 ATOMIC WEIGHT. 
 Hydrogen = i. 
 
 SPECIFIC HEAT. 
 Water = i. 
 
 Sodium 
 
 23- 
 
 293 
 
 Potassium 
 
 39- 
 
 .166 
 
 Iron 
 
 CCO 
 
 112 
 
 Copper 
 
 6l2 
 
 
 
 
 y j 
 
 Bromine (solid) 
 
 7Q.8 
 
 .084. 
 
 Silver 
 
 IO7 7 
 
 OC7 
 
 Iodine . . . 
 
 1266 
 
 OC4. 
 
 Platinum 
 
 IQ4. 4 
 
 O72 
 
 Mercury (liquid) .... 
 
 199.7 
 
 033 
 
 Mercury (solid) 
 
 199.7 
 
 .032 
 
 Uranium 
 
 238.5 
 
 .028 
 
 Gradual Melting. In some cases of addition of heat 
 to a solid, liquefaction seems to be a gradual process, 
 and the temperature of the solid steadily rises. Sealing- 
 wax illustrates this case. It becomes softer and softer 
 until distinctly liquid. This kind of melting is most 
 common in substances of decidedly heterogeneous com- 
 position. 
 
CHANGES INCIDENT TO ADDITION OF HEAT. 4/ 
 
 Partial Melting. In other cases addition of heat fully 
 melts a portion of the mass of solid, leaving the rest 
 hard. Little by little the whole liquefies. 
 
 This kind of melting is most common in bodies of 
 uniform structure and marked crystalline tendencies. 
 
 Disappearance of Heat during Liquefaction (Latent Heat of 
 Liquefaction) . A most important feature in these latter cases is the 
 fact that, from the beginning of liquefaction to its completion, there is no 
 rise of temperature. Large amounts of heat may thus be rendered latent 
 without effect upon the temperature of the mass. Ice is an example of 
 this process. 
 
 When ice of the temperature of o C. receives such an addition of heat 
 as will but just liquefy it, the water so produced does not rise in tempera- 
 ture, but still remains at o. In fact, one kilogramme of ice absorbs, while 
 melting, 79 units of heat without any rise of temperature at all. The heat 
 so absorbed is called latent heat. That it has merely undergone some 
 temporary modification appears from the fact that when water is solidified 
 into ice it always gives out 79 units of heat; in other words, the precise 
 quantity it previously disposed of. Moreover, the water makes this latter 
 change without any fall of its temperature. 
 
 In cases like the foregoing, the heat which does not increase the tem- 
 perature is supposed to do some kind of internal work in changing the 
 structure or positions of molecules. 
 
 In general, a definite amount of heat becomes latent when any solid is 
 changed to the liquid form, and it reappears when the liquid changes back 
 again to the solid form. 
 
 That solids in changing to the liquid form do indeed absorb heat is 
 easily proved in cases like that of snow or ice melting in the hand. 
 
 Exceptional Cases. There are certain exceptional 
 cases in which a solid, when heated, turns so quickly to 
 the gaseous form that it seems as if it did not exist in 
 the liquid form at all. Solid iodine is an illustration. 
 
 Experiment. Heat about one gramme of iodine crystals in a large 
 flask, with the intention of liquefying it if possible. It may be noted that 
 the substance turns quickly to gas, apparently without liquefying at all. 
 
48 CHANGES INCIDENT TO ADDITION OF HEAT. 
 
 Observe also that the iodine vapor condenses in the upper part of the flask 
 immediately to the solid form. If it condensed to the liquid form, it might 
 be expected to form in drops or run down in streams, as many other con- 
 densing gases and vapors do. 
 
 NOTE. Remove the iodine from the flask as follows : Introduce a few 
 c.c. of water and about one-half gramme of solid potassic iodide. Let 
 the solution flow to different parts of the flask so as to dissolve the 
 iodine. 
 
 Spontaneous Melting in the Air. Certain solids melt 
 upon exposure to air of the ordinary temperature. Ice is 
 an excellent example. It melts by the addition of heat 
 from the atmosphere or from any surrounding solid 
 or liquid bodies. The surrounding substances are of 
 course cooled by the process. Many cooling operations 
 in the arts depend upon this principle. It must be 
 observed, however, that the heat lost by the cooled 
 objects is absorbed by the melting ice as latent heat. 
 
 Special Forms of Liquefaction. In each of the cases, 
 previously referred to, a single individual substance has 
 been assumed to liquefy by the addition of heat. There 
 are many cases in which two substances together form 
 a single liquid. Thus two solids, when properly inter- 
 mingled, may change to a liquid. For example, solid 
 ice and solid salt, when pulverized together, turn into a 
 liquid. Then the one substance dissolves in the other ; 
 i.e. the solid salt dissolves in the liquid water. 
 
 In some cases a solid becomes liquid by solution in 
 some liquid. Thus liquid water and solid common salt, 
 when intermingled, may form a single liquid. Of course 
 a solid salt has dissolved in the liquid water. 
 
 Thus ammonic nitrate, ammonic acetate, potassic 
 iodide, and many other solids, when placed in a small 
 
CHANGES INCIDENT TO ADDITION OF HEAT. 
 
 49 
 
 amount of water, dissolve very rapidly ; at the same 
 time they absorb heat to a most marked degree. 
 
 In all such cases heat is absorbed. It may be called 
 latent heat of solution as distinguished from latent heat 
 of liquefaction already referred to. 
 
 Other Special Cases. Many 
 special cases are known in which a mix- 
 ture of two or more solids melts at a 
 lower temperature than that at which 
 the easiest-melting one melts alone. 
 
 Thus certain alloys of bismuth, tin, 
 lead, and cadmium, called fusible al- 
 loys, melt at the temperature of about 
 1 60 Fahrenheit; that is, fifty degrees 
 below the boiling-point of water. But 
 the melting-point of tin alone, the 
 easiest melting, is 451 F. 1 
 
 So also certain mixtures of chemi- 
 cal salts, called salt alloys, illustrate 
 the same principle by melting at a 
 lower point than either salt singly. 
 
 These and other somewhat similar 
 phenomena are classed under the term 
 eutexia. An eutectic mixture of sub- 
 stances is that mixture which pos- 
 sesses a lower liquefying-point than 
 that of either constituent separately 
 or any other mixture (not chemical compound) of them. 
 
 These phenomena must probably be referred to some obscure chemical 
 combination of the substances involved. (See p. 172.) 
 
 Dissociation. In case of certain substances dissocia- 
 tion takes place at a temperature far below that at which 
 
 1 Melting-point of tin alone 451 F. 
 
 " bismuth alone 515 F. 
 
 " cadmium alone 608 F. 
 
 " lead alone 619 F. 
 
 FIG. 25. A rod of fusible alloy is 
 suspended in the steam rising from boil- 
 ing water. The rod melts and falls in 
 liquid drops. 
 
5O CHANGES INCIDENT TO ADDITION OF HEAT. 
 
 the substances become luminous. The familiar case of 
 mercuric oxide affords an example. On the other hand, 
 some substances seem never to be decomposed by mere 
 accession of any such quantity of heat as we can add to 
 them. Silicic oxide, sand, is an example. 
 
 In a third class of cases dissociation occurs, but it is 
 merely temporary ; that is, if immediately after dissocia- 
 tion the temperature of the substances falls slightly, the 
 elements recombine so that the original compound reap- 
 pears as if it had never been decomposed at all. For 
 this reason many of the facts of dissociation long escaped 
 notice. Of late, however, new methods of experimenting 
 have made it possible to prove dissociation in cases in 
 which it was at first unsuspected, and later even denied. 
 
 When dissociation does occur, it may be supposed that the accession of 
 heat has become so great as to produce not only motion of the molecules 
 as wholes, but, further, to produce a motion of the atoms within the mole- 
 cules; in its earlier stages producing some of the phenomena of specific 
 heat, in its later ones the heat seems to increase the atomic motion until 
 the constituent atoms move through such distances as throw them outside 
 of the range of the force of chemical affinity. Then occurs dissociation. 
 
 It may be added parenthetically that in the view of certain eminent 
 investigators this line of research suggests the probability that what are 
 commonly regarded as ultimate atoms may themselves be dissociated by a 
 still greater heat. In this view the number of true elementary substances 
 is much smaller than is now admitted. (See p. 29.) 
 
 ADDITION OF HEAT TO A LIQUID. 
 
 When heat is added to a liquid, a series of effects 
 may be produced in more or less orderly sequence ; they 
 correspond tolerably well to those associated with the 
 addition of heat to a solid. They are somewhat as fol- 
 lows : (i) rise of temperature ; (2) expansion ; (3) vapor- 
 
CHANGES INCIDENT TO ADDITION OF HEAT. 5 I 
 
 ization, associated with disappearance of heat as latent 
 heat of vaporization ; (4) boiling ; (5) dissociation may 
 or may not occur ; (6) if the addition of heat is contin- 
 ued, evolution of light may be produced. 
 
 FIG. 26. One form of Carre's ice machine. The purpose is to freeze the water in the 
 carafe D '. When the air-pump is set in motion, the air, and at the same time the water 
 vapor, is withdrawn from the flask D. The water vapor is absorbed by concentrated 
 sulphuric acid placed in the reservoir B. This absorption is rendered the more prompt 
 because the sulphuric acid is agitated by a plunger in C. The very rapid evaporation 
 from the surface of the water in D absorbs so much latent heat from the water as to 
 freeze it. 
 
 Rise of Temperature. When heat is added to a liq- 
 uid, the temperature rises, according to the relations of 
 the liquid to specific heat, until the boiling temperature 
 
52 CHANGES INCIDENT TO ADDITION OF HEAT. 
 
 is reached. The boiling-point varies not only for differ- 
 ent liquids, but varies for the same liquid according to 
 the pressure at the time prevailing. 
 
 Vaporization of Liquids. At all ordinary tempera- 
 tures a liquid gives off vapor into the space above it. 
 The amount of this vapor depends on the nature of 
 
 FIG. 27. Small form of Carre's ice machine. Solution of ammonia in water is con- 
 tained in the receiver A. Under the influence of heat, ammonia gas is driven into the 
 jacket in B. The ammonia gas in this jacket is liquefied by reason of its. own pressure. 
 In the second stage of the operation the heat is withdrawn from A, whereupon the liquid 
 ammonia in the jacket volatilizes rapidly. It returns to the water in A. By reason of 
 its rapid evaporation, it cools that portion of water in the inner cylinder in B. The water 
 is thereby frozen. 
 
 the liquid and the temperature prevailing at the time 
 of the experiment. The vapor exerts a pressure called 
 vapor pressure. At first vapor comes from the surface 
 only. As the temperature rises, more and more vapor 
 rises. At length the entire mass of liquid reaches a 
 point at which bubbles of its vapor may form in parts 
 of its interior, and may even acquire sufficient vapor 
 
CHANGES INCIDENT TO ADDITION OF HEAT. 
 
 53 
 
 pressure to come to the surface of the liquid without 
 condensation. Then boiling begins. 
 
 During the continuance of boiling, large amounts of heat may be added 
 to the liquid without raising its temperature above the boiling-point for the 
 particular pressure prevailing. Such heat is called latent heat of vapor- 
 ization. It is definite in amount. It is believed to do some kind of 
 
 FIG. 28. Machine for producing artificial ice by the rapid evaporation of liquefied 
 ammonia gas (NH 3 ). 
 
 internal work on the molecules of liquid in changing their structure or 
 positions. 
 
 A marked concealment of heat takes place when water at 
 
 100 C. is 
 
 changed into steam of 100 C. In this case also an amount of heat, equal 
 to that made latent, reappears when the reverse change of state takes 
 place. That is, one kilogramme of water of 100 C. in changing to vapor 
 of 100 C. absorbs 536 units of heat (without any rise of temperature). 
 Again, one kilogramme of steam at ico C. upon condensing to one kilo- 
 gramme of water at 100 C. evolves 536 units of heat. 
 
54 
 
 CHANGES INCIDENT TO ADDITION OF HEAT. 
 
 In general, a definite amount of heat becomes latent when any liquid 
 changes to the gaseous form, and it reappears when the gas changes back 
 again to the liquid form. 
 
 FIG. 29. Method of employing currents of cold brine in circulating tubes for the 
 purpose of freezing the surface of an artificial lake. The brine is cooled below the 
 freezing-point of water by an ammonia ice machine. 
 
 Experimental Demonstration of Absorption of Heat during 
 Vaporization. That liquids do in fact absorb heat in vaporization is 
 easily proved in case of such liquids as water, petroleum naphtha, ethyl 
 ether, etc. 
 
 Other examples are found in the evolution of a gas from its dissolving 
 liquid, as when ammonia gas escapes from a concentrated water solution 
 of ammonia. 
 
 Spontaneous Evaporation. Certain substances evap- 
 orate very readily when exposed in open vessels to the 
 ordinary temperature of the atmosphere. In other 
 
56 CHANGES INCIDENT TO ADDITION OF HEAT. 
 
 words, they have low boiling-points. Ethyl ether (also 
 called sulphuric ether) is a good example of a liquid 
 of this sort. Of course in this, as in all cases, heat is 
 rendered latent. Heat is absorbed from neighboring 
 objects, and, as the evaporation proceeds rapidly, rapid 
 absorption of heat is associated with it. The ordinary 
 statement is that such evaporation precedes cooling. 
 
 
 FIG. 31. Three forms of receivers for collecting sulphur dioxide changed from the 
 gaseous form to the liquid by cold and pressure. The second and third forms are so con- 
 structed that a small amount of liquid may be transferred from one bulb to another, and 
 then may be withdrawn for experiment. 
 
 Ice Machines. The ice machines in common use depend upon the 
 principles already stated. The substance oftenest used is ammonia gas. 
 In one part of the machine ammonia gas is condensed to a liquid by the 
 aid of pressure and a considerable amount of cool water circulating about 
 the vessel in which the ammonia gas is confined. 
 
 Next the liquid ammonia is transferred to another portion of the appa- 
 ratus. Here its vessel is in indirect contact with the water to be frozen. 
 Upon opening a stopcock, the liquid ammonia rushes into vapor, instantly 
 absorbs an immense amount of heat from the water, and thereupon solidi- 
 ties it. 
 
CHANGES INCIDENT TO WITHDRAWAL OF HEAT. 5/ 
 
 CHANGES INCIDENT TO WITHDRAWAL OF HEAT. 
 
 WITHDRAWAL OF HEAT FROM A GAS. 
 
 When heat is withdrawn from a gas or vapor, the sub- 
 stance manifests various phenomena, similar to those 
 already referred to under the head of addition of heat, 
 only, of course, in the reverse order. 
 
 They are, among others, the following : (i) Cessation 
 of the evolution of light ; (2) recombination (in some 
 cases of dissociation) ; (3) fall of temperature ; (4) dim- 
 inution of volume ; (5) change to the liquid form ; 
 (6) evolution of latent heat. 
 
 If the liquid formed is subjected to further withdrawal 
 of heat, other changes follow. (See p. 59.) 
 
 Liquefaction of Gases. Latent Heat. In accordance 
 with the general principles already enunciated, it may 
 be expected that liquefaction of gases should be attended 
 with evolution of heat. This is indeed the case. In 
 changing from the state of gas to the state of liquid, 
 heat is given out. The heat is absorbed by any suitable 
 body. If the body is cold, it becomes warm thereby ; 
 if the body is warm, it becomes warmer still. 
 
 These two statements, simple as they appear, are worthy of further ex- 
 amination. A cold body condenses vapors, the cold body thereby becom- 
 ing slightly warmer, of course. Almost the only commonly-known case 
 of a moderately warm body condensing vapor is where some warm body 
 condenses steam; i.e. water-vapor. If steam falls upon a person's hand, 
 the steam condenses to drops of water. At the same time latent heat is 
 evolved, and the hand may be severely burned. Again, in steam-heating 
 appliances the steam, condensing in the suitable pipes, evolves its latent 
 heat by the act of condensation, and thus the air of the building may be 
 warmed. 
 
58 CHANGES INCIDENT TO WITHDRAWAL OF HEAT. 
 
 Influence of Pressure. Of course the liquefaction of gases is 
 easier under increase of pressure; but, after all, pressure is only a second- 
 ary consideration. 
 
 The English scientist, Thomas Andrews, discovered that however great 
 the mechanical pressure upon a gas, this pressure cannot reduce the gas 
 
 FIG. 32. Deleuil's apparatus for the liquefaction of carbon dioxide gas. The gas is 
 generated in the right-hand vessel. The inner tube, containing sulphuric acid, is broken 
 by forcing the tube against the pin at the bottom of the receiver. The acid acts upon 
 carbonate of soda, liberating carbon dioxide gas. The latter passes into the left-hand re- 
 ceiver, and is there condensed into the liquid form by influence of the great pressure of gas. 
 
 to the form of liquid, except when tJie gas is at or below a certain point of 
 temperature. This point differs for different gases. It is called in each 
 case the critical point. 
 
CHANGES INCIDENT TO WITHDRAWAL OF HEAT. 59 
 
 The same facts may be stated, in a sort of reversed form, as follows : 
 for each liquid there is a point of temperature at which this liquid will 
 assume the gaseous form, no matter how great the mechanical pressure 
 then opposing it. This is another definition of the critical point. 
 
 WITHDRAWAL OF HEAT FROM A LIQUID. 
 
 c When heat is withdrawn from 
 
 a liquid, a series of effects are 
 manifested as already indicated, 
 n They are in the reversed order of 
 those noted when heat is added 
 to a solid. 
 
 FIG. 33. Tube showing liquid 
 ammonia (NH 3 ) at A. Part of 
 the ammonia gas in the tube is liq- 
 uefied by great pressure and cold. 
 
 If the withdrawal of heat continues, the 
 liquid begins to fall in temperature accord- 
 ing to its specific heat. In due time the liquid may change to a solid. 
 For the moment no fall of temperature 
 takes place, but the heat withdrawn from 
 the liquid is supplied by the liberation of 
 latent heat. As soon as solidification 
 has become complete the evolution of 
 latent heat ceases, and the solid begins 
 to fall in temperature in accordance with 
 its specific heat. At the same time, of 
 course, it contracts in volume. The gen- 
 eral statement of the series of phenom- 
 ena associated with withdrawal of heat 
 is now complete. 
 
 Solidification of Homogene- 
 
 FIG. 34. Disposition of apparatus 
 for liquefying ammonia gas. In the 
 heated water-bath, solution of ammo- 
 nia gas in water is decomposed. The 
 
 ous Liquids. A liquid con- gas passes into . the other branch of 
 
 the tube, which is surrounded by ice. 
 Of a Single elementary Under the influence of the cold, and 
 
 substance, or a liquid consist- the great pressue of th a nia 
 
 fk . 
 
 gas, a part of the latter is liquefied. 
 
 ing of a single compound sub- 
 
 stance, may be called, in a general way, homogeneous. 
 
 (See p. 17.) When such liquids solidify, they may, 
 
6O CHANGES INCIDENT TO WITHDRAWAL OF HEAT. 
 
 under certain conditions, crystallize ; under others, not. 
 The size of the crystals, and in some cases the partic- 
 ular form, depends upon the circumstances under which 
 the solidification has taken place. 
 
 Solidification of Mixed Liquids. There are many 
 cases known in which two distinct solids, liquefied by 
 natural or artificial high temperature, are capable of 
 
 FIG. 35. Apparatus for liquefying sulphur dioxide gas. The gas generated in A is 
 purified in the wash-bottle b and the drying-tube c. It then condenses to a liquid in the 
 flask d, which is surrounded by a freezing mixture. 
 
 very complete liquid interdiffusion. They thus form 
 what maybe regarded as a single liquid so long as 
 the temperature does not fall the constituents do not 
 separate from each other. Of course, however, a suffi- 
 cient withdrawal of heat produces solidification. It has 
 been observed, further, that when the cooling proceeds 
 very gradually, a series of different substances separate 
 progressively from the liquid. The usual course is as 
 follows : first, one of the component solids separates ; 
 
62 CHANGES INCIDENT TO WITHDRAWAL OF HEAT. 
 
 next, some well-defined compound of the components 
 crystallizes out ; sometimes the operation terminates by 
 
 FIG. 37. Part of Cailletet's apparatus, showing its position when the tube P 
 is filling with the gas to be experimented upon. 
 
 the solidification of a nearly pure mass of the other 
 component. 
 
CHAPTER VII. 
 CERTAIN GENERAL LAWS OF MATTER. 
 
 SIGNIFICANCE OF THE GASEOUS CONDITION. 
 
 DALTON'S atomic theory, as now received, may be 
 said to have its basis in certain fundamental laws of 
 matter. The following arc some of the most important 
 of them : 
 
 Boyle's or Mariotte's Law of the Pressure of Gases : 
 
 % 
 
 The volume of any gas, when confined under a constant 
 and relatively high temperature^ varies inversely as the 
 pressure to which it is exposed. 
 
 The law is easy of comprehension. 
 
 It states certain facts that are capable of direct experi- 
 mental demonstration. 
 
 It is of wide application, for it applies to any. gas 
 whatsoever. 
 
 It has but one limitation. That is, a high tempera- 
 ture is necessary. What, then, is high temperature ? 
 Evidently the term is relative, and its application must 
 be interpreted according to the special properties of the 
 gas in question. 
 
 The guide to this interpretation must be found in what is called the 
 critical point for a gas. (See p. 58.) 
 
 When a gas is subjected to varying pressure at temperatures far above 
 its critical point, it conforms closely to the law as stated. 
 
 63 
 
6 4 
 
 CERTAIN GENERAL LAWS OF MATTER. 
 
 At temperatures below its critical point a gas is ready 
 to change to a liquid by a slight increase of pressure. 
 But this change brings the substance to a condition in 
 
 FIG. 38. Robert Boyle. Born in 1626; died in 1691. 
 
 which it obeys the laws of liquids, and not laws like that 
 of Boyle and Mariotte, which relates to gases exclusively. 
 At temperatures slightly above the critical point a gas 
 is found to manifest the beginnings of the influences of 
 the laws of liquids, and so it does not conform closely 
 to the laws of gases. 
 
CERTAIN GENERAL LAWS OF MATTER. 
 
 To these explanations of the law the following state- 
 ment may be added : 
 
 If a given volume of any gas whatever is 
 subjected to the single change of having the 
 external pressure upon it doubled, the volume 
 of the gas is thereby reduced to one-half ; and 
 the converse is likewise true that is, if any 
 gas is subjected to the single change of having 
 the external pressure upon it reduced to one- 
 half, then its volume thereby becomes double. 
 
 But a given gas strictly conforms to this law 
 only when it is at temperatures far above that 
 of its liquefying-point, wherever that may be. 
 
 Charles's Law of Expan- 
 sion of Gases by Heat : 
 
 The volume of a given 
 portion of any gas, under 
 a constant pressure, varies 
 directly as the temperature 
 expressed in absolute centi- 
 grade degrees. 
 
 Charles's law is a formal 
 statement of facts that are 
 capable of direct demon- 
 stration ; it is one that is 
 much used in the study of 
 gases. 
 
 The ordinary centigrade 
 
 thermometer reads o at the temperature of freezing 
 water, and it reads 100 at the temperature of boiling 
 water. 
 
 FIG. 39. Apparatus for showing that 
 different gases in the tubes / and G when 
 subjected to the same amount of pressure, 
 under condition of constant temperature, 
 suffer the same amount of reduction of 
 volume. 
 
FIG. 40. Apparatus for demonstrating the truth of the law of Charles. A portion 
 of gas from the globe A may be transferred to the tube BC. Here it may be subjected 
 to varying temperatures, from that of freezing water to that of boiling water. The 
 change of volume of the gas may be read off either from the graduations on the tube BC 
 or else by use of a cathetometer. At the same time the gas may be subjected to a given 
 pressure by adding mercury to the tube E, or by withdrawing it from the bottom of the 
 apparatus. 
 
CERTAIN GENERAL LAWS OF MATTER. 
 
 The absolute centigrade thermom- 
 eter reads 273 at the temperature of 
 freezing water, and it reads 373 at 
 the temperature of boiling water. 
 
 Now the absolute centigrade scale 
 is graded with reference to the fact 
 that the coefficient of expansion of 
 gases by heat is the same for all of 
 them ; namely, for an increase of one 
 centigrade degree the expansion is 
 2fj of the volume the gas would 
 occupy at the temperature of freez- 
 ing water. 
 
 Since, then, the absolute scale is 
 graduated at 273 at the freezing- 
 point of water, the volume of any 
 gas increases under the influence of 
 added heat at the same rate as the 
 numbers on the scale do. 
 
 Rule. Add 273 to any ordinary 
 centigrade degree, and the sum gives 
 the corresponding absolute centigrade 
 degree. 
 
 Graham's Two Laws of Gaseous Dif- 
 fusion : 
 
 1 . The diffusion-rate of gases of the 
 same density is the same, whatever 
 tJieir chemical composition. 
 
 2 . The relative diffusion-rates of two 
 gases of different densities are inversely 
 as the square roots of these densities. 
 
 FIG. 41. Apparatus 
 for experimental proof of 
 the law of Boyle and of 
 Mariotte. Upon pouring 
 additional mercury into 
 the tube at C, additional 
 pressure is applied to the 
 portion of gas at A B. 
 The volume of gas is 
 thereby reduced in ac- 
 cordance with the law. 
 
68 
 
 CERTAIN GENERAL LAWS OF MATTER. 
 
 These laws are statements of facts capable of direct 
 demonstration. When portions of gases of different 
 densities are brought into direct contact, or are sepa- 
 rated by a porous partition only, the molecules of both 
 
 FIG. 42. Jacques Alexandra Ce"sar Charles. Born in 1746; died in 1823. 
 Celebrated French physicist. 
 
 gases are discovered to be in active motion. Incident- 
 ally the gases intermingle. These properties of gases 
 are displayed by the various forms of the diffusiometer. 
 If a portion of gas is confined within the porous cell 
 of a diffusiometer, this gas projects its molecules into 
 
FIG. 43. One form of Torricellian barometer, with its cathetometer. In this case 
 the barometer is EF. The apparatus in the figure is intended to measure the pressure 
 r>r mercury upon a portion of gas in the tube AB. 
 
7<3 CERTAIN GENERAL LAWS OF MATTER. 
 
 the passages of the porous walls, and they pass through 
 and out. Simultaneously, and in the same manner, any 
 external gas projects its molecules inward. But the 
 rate of passage is found to be different if the gases have 
 different densities. This is proved by the motion of the 
 liquid in the gauge-tube connecting with the cell of the 
 diffusiometer. It is observed that the molecules of the 
 
 FIG. 44. Graham's apparatus for showing diffusion of hydrogen gas. The tube A 
 contains hydrogen gas. The top of the tube is closed by a porous wafer. The hydrogen 
 gas escapes so rapidly into the air that the atmospheric pressure upon the mercury in the 
 trough is able to force the mercury up into the tube A, 
 
 lighter gas always move with greater rapidity. A series 
 of careful experiments has afforded the basis for the law 
 already stated. 
 
 The following is an illustration of this law : A given 
 bulk of oxygen gas is found by experiment to weigh six- 
 teen times as much as the same volume of hydrogen 
 gas. The density of oxygen is then said to be sixteen 
 (it being customary to adopt hydrogen gas as the 
 
CERTAIN GENERAL LAWS OF MATTER. J\ 
 
 standard of density for gases). Now hydrogen gas is 
 found experimentally to diffuse itself into oxygen gas 
 four times as rapidly as oxygen gas diffuses into it. 
 
 The Law of Henry and of Dalton, of the Relation of 
 Pressure to the Solubility of a Gas in Water : 
 
 When a given gas is exposed to water 
 under a constant temperature, the volume 
 of the gas dissolved by the water varies 
 directly as the pressure acting at the time. 
 
 The amount of gas dissolved by water 
 varies with the nature of the gas. It 
 also varies with the temperature, being 
 in general less at higher temperatures. 
 It also varies with the pressure acting. 
 This last is the only one of the three 
 conditions that can be described in the 
 form of a law. By the law as given, it 
 appears that a gas resting on the sur- 
 face of water is dissolved by the water 
 to a certain extent. If now (other eon- 
 
 FIG. 45. Syphon 
 
 ditions being appropriate) the pressure, for containing water 
 for example, is doubled, the amount of 2^!* carb n 
 gas dissolved will also be doubled. It 
 likewise follows that if the pressure is, for example, 
 halved, the amount of gas dissolved will be halved ; in 
 other words, under lessened pressure gas dissolved in 
 water actually comes out of the water, escaping with 
 effervescence. If water, charged with gas under pres- 
 sure, is allowed to flow from a syphon, the gas imme- 
 diately leaves the water with effervescence. 
 
72 CERTAIN GENERAL LAWS OF MATTER. 
 
 The Law of Definite Proportions : 
 
 The same compound always contains the same atoms, 
 united in the same proportions by weight (and by volume 
 when they are gaseous], and with the same molecular 
 arrangement. 
 
 Example: Pure water always contains in each mole- 
 cule only one atom of oxygen and only two atoms of 
 hydrogen. It always contains these atoms in approx- 
 imately the following proportions, by weight : 
 
 Hydrogen .... 2 parts by weight. 
 
 Oxygen 16 parts by weight. 
 
 18 
 
 It always contains these atoms in the following pro- 
 portions, by volume : Two volumes of water vapor when 
 decomposed yield approximately : 
 
 Hydrogen two volumes. 
 
 Oxygen one volume. 
 
 The molecular arrangement is believed to be such 
 that the oxygen atom is somehow between the two hy- 
 drogen atoms, an arrangement which may be expressed 
 by the formula, H O H. 
 
 This law contains several statements. Some of them 
 are capable of experimental demonstrations ; some of 
 them are not. But the latter are based upon a multi- 
 tude of observed facts which strongly suggest their 
 truth. 
 
 The law as a whole is implicitly and safely relied upon 
 in all chemical experiments and in the conduct of the 
 great chemical industries of the world. 
 
CERTAIN GENERAL LAWS OF MATTER. 73 
 
 The Two Laws of Multiple Proportions : 
 
 1. When the elementary substance A chemically unites 
 with the elementary substance B in more than one propor- 
 tion by weight (and in case of gaseous elements, by volume 
 as well), they form more than one compound, and the 
 several compound substances so produced possess well- 
 marked and distinctly different properties. 
 
 2. The several amounts by weight (and in case of gas- 
 eous elementary substances, by volume also) of B, that 
 may combine with the same amount of A, bear a very 
 simple relation to each other. 
 
 As a general example illustrating this law, suppose 
 that the elementary substances A and B unite in the 
 several proportions expressible by the formulas : 
 
 A B m , A B n , A B , A B p , A B q . It is found that the 
 five compounds formed are essentially different sub- 
 stances. It is found that several amounts of B repre- 
 sented by m, n, o, p, q, bear very simple ratios to each 
 other. 
 
 As a special example the compounds of nitrogen and 
 oxygen may be taken. 
 
 These two substances unite in five different propor- 
 tions by weight and gaseous volume. 
 
 They produce five distinctly different compounds, each 
 having special characteristics of its own. They are the 
 following : 
 
 Nitrogen Monoxide (N^O). 
 
 This is composed of 28 parts nitrogen by weight 
 and 1 6 " oxygen " " 
 44 
 
74 CERTAIN GENERAL LAWS OF MATTER. 
 
 Nitrogen Dioxide (N 2 O 2 or NO). 
 This is composed of 28 parts nitrogen by weight 
 and 32 " oxygen " " 
 60 
 
 Nitrogen Trioxide 
 This is composed of 28 parts nitrogen by weight 
 and 48 " oxygen " " 
 76 
 
 Nitrogen Tetroxide (N 2 O^ or NO 2 ). 
 This is composed of 28 parts nitrogen by weight 
 and 64 " oxygen " " 
 92 
 
 Nitrogen Pentoxide (N 2 O S ). 
 
 This is composed of 28 parts nitrogen by weight 
 and 80 " oxygen " " 
 108 
 
 Evidently in these five compounds the several weights 
 of oxygen combined with the constant weight of nitro- 
 gen bear the simple ratios I : 2 : 3 : 4 : 5. (For further 
 discussion of these compounds, see pp. 232, 236.) 
 
 NOTE. It will be observed that in the five compounds just cited in illus- 
 tration of the laws under consideration, the amount of nitrogen is taken as 
 a basis of comparison, and its weight is represented by the number 28. 
 This number has been used because it has been found, after a great multi- 
 tude of most carefully devised and executed experiments, that it has some 
 special significance in this case. // is believed to represent twenty-eight 
 microcriths ; i.e. the weight of the amount of nitrogen present in one mole- 
 cule of each of the substances referred to. 
 
 In its first stages quantitative chemical analysis makes its statements in 
 percentage form. In the cases of the nitrogen compounds referred to, 
 such statements are given in columns two and three of the following 
 table: 
 
CERTAIN GENERAL LAWS OF MATTER. 
 
 75 
 
 1 
 
 2 
 
 3 
 
 4 
 
 5 
 
 
 PER CENT i 
 
 JY WEIGHT. 
 
 WEIGHT-RATIO. 
 
 VOLUME-RATIO. 
 
 
 Nitrogen 
 
 Oxygen 
 
 Nitrogen : Oxygen 
 
 Nitrogen : Oxygen 
 
 Nitrogen monoxide 
 
 63-7I 
 
 36.29 
 
 = -57 (0 
 
 i: (I) 
 
 Nitrogen dioxide 
 
 46.75 
 
 53.25 
 
 : 1.14 (2) 
 
 I : I (2) 
 
 Nitrogen trioxide 
 
 36.91 
 
 63.09 
 
 :i-7i (3) 
 
 I'll (3) 
 
 Nitrogen tetroxide 
 
 30-5 
 
 69.50 
 
 : 2.28 (4) 
 
 1:2 (4) 
 
 Nitrogen pentoxide 
 
 25.98 
 
 74.02 
 
 : 2.85 ( 5 ) 
 
 i = 4 (5) 
 
 If in these several compounds the amount by weight of nitrogen in each 
 is taken as a common unit of comparison, then the amounts by weight of 
 oxygen will be .57: 1.14: 1.71 : 2.28: 2.85, and these numbers appear at 
 once by inspection to be to each other as i : 2 : 3 : 4 : 5. 
 
 It is plain then that the several amounts of oxygen in these compounds, 
 combined with the same amount of nitrogen, bear to each other the very 
 simple ratio stated as I : 2 : 3 : 4 : 5. 
 
 In its most advanced stages, quantitative chemical analysis employs 
 for each elementary substance a number representing its atomic or molec- 
 ular weight. Thus the atomic weight of nitrogen is believed to be 14 
 microcriths, and its molecular weight 28 microcriths. The atomic weight 
 of oxygen is believed to be 16 microcriths, and its molecular weight 32 
 microcriths. 
 
 Returning now to the table given, it is discovered that the volume-ratios 
 afforded by the five compounds show yet more marked simplicity. Not 
 only do the several volumes of oxygen bear to each other the simple ratios 
 1:2:3:4:5, but they also in each case bear a very simple ratio to the 
 constant volume of the other element, the nitrogen. (See the laws of 
 Gay-Lussac.) 
 
 Gay-Lussac's Three Laws of Combination of Gases : 
 
 1. When tivo or more gases combine, the volumes of 
 these gases bear very simple ratios to each other. 
 
 2. When tiuo or more gases combine to form a product 
 wJiich can remain a gas, the volume of the gas so formed 
 
76 CERTAIN GENERAL LAWS OF MATTER. 
 
 bears a very simple ratio to the volume of each of the com- 
 ponent gases. 
 
 3. The weights of combining volumes of gaseous 
 
 FIG. 46. Gay-Lussac. Born 1778; died 1850. 
 
 elements bear very simple ratios to their atomic 
 weights. 
 
 (In all these cases it is understood that, when under 
 comparison, the gases are at constant temperatures and 
 pressures?) 
 
CERTAIN GENERAL LAWS OF MATTER. 
 
 77 
 
 Many illustrations of the signification of these state- 
 ments might be given. Two useful ones are presented 
 here. 
 
 First Illustration drawn from Hydrochloric Acid Gas. 
 
 - The principal points to be mentioned in connection 
 with the matter here under consideration may be ex- 
 
 pressed by the following equations : 
 
 H 2 
 
 + ' C1 2 
 
 One molecule of 
 
 One molecule of 
 
 Hydrogen, 
 
 Chlorine, 
 
 2 
 
 7 1 
 
 parts by weight. 
 
 parts by weight. 
 
 73 
 
 2HC1 
 
 Two molecules of 
 Hydrochloric acid, 
 
 73 
 parts by weight. 
 
 73 
 
 H 
 
 I 
 H 
 
 I 
 
 Cl- 
 
 -Cl 
 
 H 
 
 1 
 -Cl 
 
 I 
 
 H 
 
 I 
 -Cl 
 
 I 
 
 I + I 
 
 2 71 
 
 parts by weight. parts by weight. 
 
 i +35i 
 36* 
 
 parts by weight. parts by weight. 
 
 FIRST FACT. Hydrogen gas and chlorine gas chemically combine. 
 
 SECOND FACT. As a result they produce a new gas called hydro- 
 chloric acid gas, having properties different from those of the components. 
 
 THIRD FACT. When hydrogen and chlorine combine, they do so in 
 the proportions of two volumes of chlorine and two volumes of hydrogen. 
 
 FOURTH FACT. The resulting gas has the bulk of four volumes; in 
 other words, there is neither permanent contraction nor permanent expan- 
 sion as a result of the act of union. 
 
 FIFTH FACT. The combining volumes of hydrogen gas and chlorine 
 gas have the weight-ratio of 2: 71 = i : 35.5. Now the accepted atomic 
 weight of hydrogen is I, and that of chlorine is 35.4. 
 
 Evidently then the facts stated sustain the statements of the law. 
 
 Second Illustration drawn from Water Vapor. The 
 
 following equations are applicable in this case : 
 
CERTAIN GENERAL LAWS OF MATTER. 
 
 2H 2 
 
 Two molecules of 
 Hydrogen, 
 
 4 
 parts by weight. 
 
 One molecule of 
 Oxygen, 
 
 32 
 parts by weight. 
 
 2H 2 
 
 Two molecules of 
 Water vapor, 
 
 36 
 parts by weight. 
 
 H 
 
 1 
 -H 
 
 I 
 
 
 H 
 
 I 
 -H 
 
 I 
 
 I + 
 
 16 + 16 2 + 16 2 H- 16 
 
 4 32 36 
 
 parts by weight. parts by weight. parts by weight. 
 
 FIRST FACT. Hydrogen gas and oxygen gas chemically unite. 
 
 SECOND FACT. As a result they produce a new gas or vapor called 
 hydric oxide or water vapor, having properties different from those of the 
 component gases. 
 
 THIRD FACT. When hydrogen and oxygen combine, they do so in the 
 proportions of four volumes of hydrogen and two volumes of oxygen. 
 
 FOURTH FACT. The resulting gas has the bulk of four volumes; that 
 is, the same weight of matter has been packed into smaller space by the 
 influence of the chemical union which has taken place. Thus there has 
 been a permanent reduction from a total of six volumes to a total of four 
 volumes. The ratio of 6 : 4 = 3 : 2, and is a very simple one. 
 
 FIFTH FACT. The combining volumes of hydrogen gas and oxygen 
 gas have the weight-ratio of 4: 32 = 2: 16. Now the accepted atomic 
 weight of hydrogen is i, and that of oxygen is 16. 
 
 Evidently, then, the facts stated under this illustration sustain the laws 
 as given. 
 
 Avogadro's and Ampere's Hypothesis of the Size of 
 Gaseous Molecules : 
 
 Equal volumes of all substances, when in tJie gaseous 
 state, and under like conditions of temperature and pres- 
 sure, contain the same number of molecules. 
 
CERTAIN GENERAL LAWS OF MATTER. 79 
 
 Evidently the hypothesis declares that if, under cer- 
 tain conditions, one cubic foot of oxygen gas contains n 
 molecules of oxygen, then under the same conditions 
 one cubic foot of nitrogen gas, one cubic foot of hydro- 
 gen gas, one cubic foot of compound gases, as carbon 
 dioxide (CO 2 ), ammonia gas (NH 3 ), each contain n mol- 
 ecules of their respective substances. 
 
 This hypothesis seems to follow directly from the 
 laws of Boyle and of Charles. For if the material 
 groups we call molecules exist at all, and if expansion 
 and contraction of gases are in fact due to the mov- 
 ing apart or the moving together of these material 
 groups, then the observed exact correspondence of the 
 laws of such expansion and contraction, even in differ- 
 ent substances, points conclusively to the existence 
 of equal numbers of molecules in equal bulks of 
 gases. 
 
 The hypothesis of Ampere is not the expression of a 
 distinct and easily verified fact. It is rather the most 
 reasonable explanation of a series of facts which cannot 
 well be correlated without it. 
 
 As an example of its bearing, the following chemical illustration may be 
 given : 
 
 FIRST. Ammonia gas (NH 3 ) and hydrochloric acid gas (HC1) may 
 be proved, by processes independent of this hypothesis, to have the for- 
 mulas here assigned them. 
 
 SECOND. They are found by analysis to have the respective molecular 
 weights 17 and 36.4. 
 
 THIRD. It is found experimentally that they always combine in the 
 proportion of 1 7 parts by weight of ammonia gas to 36.4 parts by weight 
 of hydrochloric acid gas. Hence they unite molecule for molecule. 
 
 FOURTH. It is found that they unite in equal volumes. 
 
 Whence these equal volumes appear to contain equal numbers of 
 molecules. 
 
8O CERTAIN GENERAL LAWS OF MATTER. 
 
 The Relation of Diffusion of Gases to Ampere's Law. That 
 
 the facts relating to diffusion of gases afford a remarkable confirmation of 
 the law of Avogadro and of Ampere, may be made apparent from the fol- 
 lowing example and explanations : 
 
 Suppose a rubber balloon containing hydrogen gas and exposed to the 
 air. The hydrogen gas is subject to two pressures from without, the con- 
 tractile force of the rubber, and the weight of the atmosphere. Why then 
 is it not reduced to yet smaller bulk ? Because of a resistance to pressure 
 that it possesses in common with other forms of matter. In gases this 
 resistance is called tension, or elasticity, or expansive power. But the 
 facts of diffusion prove that all gaseous molecules are in particularly active 
 motion, though with different rates. We are thence led to believe that the 
 tension of the hydrogen in the balloon referred to is due to the impact of 
 the moving hydrogen molecules; that is, to their outward blows against the 
 enclosing walls of the balloon. 
 
 Again, suppose a balloon containing oxygen, but otherwise in every 
 way similar to that containing hydrogen, just discussed. The portion 
 of oxygen gas will have a weight sixteen times that of the hydrogen gas. 
 The oxygen gas possesses tension, and this is due to the outward impact of 
 the oxygen molecules against the walls of its containing vessel. 
 
 Now it is to be noted that in this second case the impact is equal to the 
 impact of the hydrogen molecules in the other case; for in both cases the 
 same external pressures are overcome. 
 
 The laws of mechanics show that the impact of any moving body may 
 be expressed as equal to one-half its mass multiplied by the square of its 
 velocity. Then the impact of a moving molecule of hydrogen and of oxy- 
 gen may be expressed respectively as follows : 
 
 i (impact of molecule of hydrogen) = J mv 2 ; 
 i' (impact of molecule of hydrogen) m'v 1 ' 2 . 
 
 But it has been shown already that in the case of equal volumes of the 
 two gases, 
 
 whence mv 2 =m'v' 2 . 
 
 Taking the velocity of the oxygen molecule (z/) as the unit of compari- 
 son of velocity and calling it I, and substituting for /// and m' the ratios of 
 their respective molecular weights, i and 16, we obtain, 
 
 a= 16, 
 
 v= 4. 
 
CERTAIN GENERAL LAWS OF MATTER. 8 1 
 
 This result means that when the velocity of the oxygen molecule is 
 called i, the velocity of the hydrogen molecule is 4. Now this is in fact 
 the rate of motion of the molecules as proved by Graham. And the result 
 obtained from the course of reasoning here pursued has involved but one 
 supposition ; namely, that the two equal balloons, or in fact any two equal 
 volumes of the gases, under like conditions of temperature and pressure, 
 contain equal numbers of molecules. 
 
 The correct result attained contributes materially to place the hypothe- 
 sis of Avogadro upon a mathematical foundation. 
 
CHAPTER VIII. 
 
 CERTAIN FORMS OF ENERGY CLOSELY CONNECTED 
 WITH CHEMICAL CHANGES. 
 
 HEAT AND ELECTRICITY. 
 
 IT has been remarked that " a chemical operation pre- 
 sents two aspects to the investigator ; it involves a 
 change in the form or distribution of matter and a 
 change in the form or distribution of energy." 
 
 Two forms of energy are especially involved in chem- 
 ical changes : they are heat and electricity. 
 
 These subjects belong in a certain sense to the de- 
 partment of physics, yet by their sources, uses, and 
 effects, they are so closely connected with chemistry 
 that they admit of brief discussion here. 
 
 HEAT. 
 
 The invisible agency by whose transfer sensations of 
 warmth and cold are produced, is itself called heat. 
 Two kinds of heat may be distinguished : 
 
 1. Absorbed heat is that which resides in a hot body, 
 often remaining in it for a considerable time. It is 
 transferred to another body, mainly by contact. 
 
 2. Radiant heat is heat in the act of passing with 
 great velocity (about 190,000 miles per second) through 
 
 space, whether vacuous or otherwise ; radiant heat may 
 82 
 
CERTAIN FORMS OF ENERGY. 83 
 
 be either dark heat or luminous heat (the latter form 
 being commonly known as light). 
 
 Theories of the Nature of Heat. The dynamical the- 
 ory of heat, and that now generally accepted, supposes 
 that all matter as well as all space is pervaded by an 
 extremely delicate and elastic medium called the ether. 
 This theory regards (i) absorbed heat as a vibration of 
 the molecules of matter ; (2) radiant heat as an undula- 
 tory movement in the ether. 
 
 Heat as Motion. That heat is not a form of matter 
 appears to be shown by a variety of facts. For exam- 
 ple, neither does addition of heat to a body increase its 
 weight, nor does loss of heat by a body diminish that 
 weight. 
 
 The quantity of heat in a given system is capable 
 of indefinite increase ; again, it can be destroyed as 
 material substances cannot. 
 
 That heat is a form of energy in other words, of 
 motion appears to be suggested by a multitude of 
 phenomena. Of these a few may be mentioned. 
 
 FIRST. The general quantitative relations between heat and mass- 
 motion are very simple. A given amount of mechanical motion may be 
 changed into a certain definite amount of heat and no more; and on the 
 other hand, a given amount of heat is capable of generating only a certain 
 fixed amount of mass-motion. 
 
 Some of the contrivances ordinarily used to effect such interchange are 
 mperfect and involve large losses during the transformations; these, how- 
 ever, are not losses of the total amount of energy, but only of that partic- 
 ular form of it which the appliance or machine may be intended to afford. 
 Hence the strength of the argument is not impaired. 
 
 SECOND. The sources of heat are well explained by this view. They 
 are friction, percussion, chemical action, the sun. (The internal heat of 
 the earth need not be discussed here.) 
 
8 4 
 
 CERTAIN FORMS OF ENERGY. 
 
 Friction and percussion involve a diminution of mass-motion or its 
 entire quenching. But these have not been destroyed ; they appear to 
 have been merely transformed into minute molecular motions. 
 
 Chemical action evolves heat when certain substances combine. The 
 true source of this heat appears to be that molecular percussion or atomic 
 bombardment which is sustained when a myriad of atoms of one kind 
 clash into combination with a myriad of another kind. 
 
 The sun gives out an enormous amount 
 of heat. Only a minute fractional part of 
 it is received by this earth. (But this is 
 a large amount as compared with man's 
 ordinary means of producing energy.) But 
 this tremendous and continual transfer does 
 not appear to diminish the weight of the 
 giver nor to increase that of the receiver. 
 
 Again, it appears more rational to be- 
 lieve that the incredible velocity of radiant 
 heat is associated with a progressive flow 
 of energy rather than with an actual trans- 
 portation of matter. 
 
 THIRD. The effects of heat are best 
 explained by this view. The principal of 
 these effects are the following : (a) tem- 
 perature, () expansion and contraction, 
 (c) change of state, (d) work, (<?) light, 
 (/) chemical combination and decompo- 
 sition, () electricity. 
 
 FIG. 47 . - Apparatus for gradu- O) Temperature. When a portion 
 
 ating a thermometer at the freezing- of matter gains or loses heat, the effect 
 
 point of water. easiest and oftenest noticed is rise or fall 
 
 of temperature. (But it is well known 
 
 that in some cases this effect is altogether wanting. See latent heat, p. 47.) 
 
 What, then, is signified by the temperature of a body? Evidently its 
 state of sensible thermal equilibrium or want of equilibrium as compared 
 with some other body. 
 
 Temperature, then, is relative. It expresses the condition of a body in 
 answer to the questions, does this body give heat to our persons? or, does 
 it take heat from them? does this body give or withdraw heat from some 
 other certain neighboring body with which it may be placed in contact? 
 
CERTAIN FORMS OF ENERGY. 
 
 To one of these questions our own nervous systems give a more or 
 less distinct answer; to another we get an answer by observing some 
 convenient secondary effect, such as the expansion of matter in some 
 thermoscope. 
 
 The terms warm and cold mean, then, the transfer of a certain force in 
 one direction or another toward a given body or from it. And it is sup- 
 posed that this transfer results in an increase or a decrease of molecular 
 motion. 
 
 But it is supposed that no known body exists in the condition of pos- 
 sessing absolutely no heat; that is, no molecular motion. Such a condi- 
 tion might be characterized as at a tem- 
 perature of absolute zero (this temperature 
 in assumed to be minus 273 degrees ordi- 
 nary centigrade). Finally, it can easily be 
 shown that temperature, whether judged 
 by our sensations or by thermoscopes, is 
 no certain index of the real amount of heat 
 possessed by a body. 
 
 () Expansion. With few excep- 
 tions all bodies, whether solid, liquid, or 
 gaseous, expand with addition of heat and 
 contract upon withdrawal of it. In these 
 changes the heat certainly produces a mass- 
 motion. It is observed in the alteration of 
 the bulk of the whole mass of the body 
 acted on, but apparently due to an increase 
 or diminution of the motion of the mole- 
 cules. 
 
 FIG. 48. Steam box for use in 
 graduating a thermometer at 100 C. 
 
 (V) Change of State. When the molecular motion just referred to 
 becomes so great as to carry the molecules of a solid or of a liquid beyond 
 the range of those cohesive forces which characterize its previous state, 
 it changes to the next less dense form of matter. In other words, addition 
 of heat may in most cases change a solid to a liquid and a liquid to a gas. 
 Again, withdrawal of heat may so restrict the molecular motion as to bring 
 the molecules of a gas near enough together to make them subject to such 
 cohesive forces as bind them into a liquid, or even to a solid. 
 
 There are some substances that have not yet been made to undergo 
 change of state. Carbon, for example, has not yet been changed from the 
 solid to the liquid form, much less to the gaseous. But it can hardly be 
 
86 
 
 CERTAIN FORMS OF ENERGY. 
 
 called an exception to the general statement. For in the first place the 
 number of such substances is steadily decreasing; witness the recent lique- 
 faction of the so-called permanent gases, oxygen, hydrogen, nitrogen, and 
 the like. In the second place, all experience shows that whenever refrac- 
 tory solids are liquefied and vaporized, it is in consequence of addition of 
 heat; and whenever liquids not previously frozen are solidified, it is in 
 
 consequence of withdrawal of 
 heat. (See p. 58.) 
 
 (</) Chemical Combi- 
 nation and Decomposi- 
 tion. These subjects are 
 important, but they are ex- 
 plained later. 
 
 0) Light. For the pur- 
 poses of this book this sub- 
 ject may be presented in 
 connection with spectrum an- 
 alysis. 
 
 SPECTRUM ANALYSIS. 
 
 Spectrum analysis is much 
 used in chemistry. It de- 
 pends upon the following 
 facts : 
 
 1. Highly heated solids 
 give out white light; i,e. a 
 mixture of rays of light of 
 various colors. 
 
 2. Highly heated gases or 
 
 vapors give out light of a characteristic color; i.e. either monochromatic 
 light (composed of rays of a single quality), or polychromatic light 
 (composed of rays of a character such that they cannot make up white 
 light). 
 
 3. A ray of light when it passes through a prism is bent out of its 
 course, i.e. suffers refraction; while a bundle of rays passing through a 
 prism suffers dispersion; that is, the different rays of which the bundle is 
 composed are bent out of course in different degrees. 
 
 FIG. 49. Glass prism in a convenient mounting 
 for use in experiments upon the refraction of light. 
 
CERTAIN FORMS OF ENERGY. 87 
 
 The Spectrum. The term spectmm is applied to that peculiar pic- 
 ture which appears when a narrow beam of light, after passing through a 
 prism, is allowed to fall upon a screen. 
 
 Thus white light when passed through a prism gives a spectrum that 
 contains an enormous number of rays. These rays are of all the different 
 colors and shades of the rainbow, and they appear to be the same what- 
 ever the chemical constitution of the body giving out the white light. The 
 spectrum is called a continuous one. 
 
 But colored light produces a discontinuous spectrum. Moreover, the 
 
 Fig. 50. Refraction of light by water. A beam of light coming from the rod at the 
 bottom of the water is bent out of course so that when it comes to the eye it appears to 
 arrive from a different point, and thus the rod, in fact straight, appears to be bent. 
 
 character of such a spectrum varies with the chemical substance produc- 
 ing it. 
 
 The orange light emitted by highly heated sodium appears upon ordi- 
 nary examination to be monochromatic. In a certain sense it is so. But 
 critical investigation shows that its well-known color is made up of several 
 minutely different shades. 
 
 The Spectroscope. The following are a few of the fundamental 
 principles upon which the use of the spectroscope depends : 
 
 I. A spectroscope is a contrivance for examining light. In its simplest 
 form it consists of three parts, a narrow opening to admit the beam of 
 light to be tested; a prism or series of prisms to disperse the rays of light; 
 a small telescope to bring the spectrum to the eye. 
 
88 CERTAIN FORMS OF ENERGY. 
 
 2. A beam of light when passed through a suitable spectroscope yields 
 a spectrum showing of what ray or rays the beam consists. 
 
 3. A beam of light may be tested just as it comes from its source; thus 
 any substance may by some method be so heated as to become luminous, 
 that is, to give out light, and such light may be passed through the 
 spectroscope. 
 
 One method of heating is by an ordinary Bunsen lamp. 
 
 A second method applicable to solids and liquids is to cause some form 
 
 FIG. 51. Refraction of light by a glass prism. A beam of light passing through an 
 aperture at a would, if uninterrupted, fall upon the screen at d. If, however, it is inter- 
 rupted by the prism b, it is bent out of course, i.e. refracted, and falls upon the screen at 
 a lower point. At the same time the different rays composing the beam are refracted 
 differently, thus producing a spectrum. 
 
 of electric discharge to flow from points of the solid to be tested, or over 
 the surface of the liquid. 
 
 Gases may be made luminous by passing an electric discharge through 
 glass tubes containing minute quantities of them. 
 
 4. Highly heated solids and liquids in general give out white light that 
 is not characteristic, but is the same for all of them. 
 
CERTAIN FORMS OF ENERGY. 89 
 
 5. Highly heated vapors and gases in general give out light having 
 colors that are peculiar, and characteristic of the atoms present. 
 
 FIG. 52. Adjustable slit for use upon a spectroscope. 
 
 The denser the vapor, the greater the light; hence metallic vapors give 
 out strongly luminous rays. 
 
 FIG. 53. Spectroscope (of one prism) for the examination of luminous flames. 
 
 6. When light falls upon portions of matter, it is capable of at least 
 three different dispositions : 
 
 Certain substances allow nearly all the rays of a complex beam of light 
 to pass through them. 
 
9 o 
 
 CERTAIN FORMS OF ENERGV. 
 
 Other bodies absorb certain rays and allow certain others to pass 
 through them; the fact of such absorption is shown sometimes by the 
 color of the body itself or sometimes by the spectroscope. 
 
 A third class contains bodies that are opaque; that is, they forbid any 
 rays of light to pass through them, but they sometimes give back reflected 
 light of a peculiar color. 
 
 FIG. 54. Apparatus for examining the spectrum of a liquid. Upon the passage of an 
 electric current through the apparatus, the liquid is highly heated, and affords a luminous 
 mass between the points AB. Thereupon this mass may be examined by the spectroscope. 
 
 The spectroscope is capable of showing whether light as originally 
 emitted has been modified by the body upon which it has fallen. 
 
 As a result of the study of spectrum analysis it is believed that the light 
 which characterizes each element is due to the atomic motion peculiar to 
 that element. In fact, certain elements form colored compounds such as 
 give similar spectral lines whether heated or cold, and so appear always to 
 maintain the same rate of atomic motion (didymium). 
 
CERTAIN FORMS OF ENERGY. 9 1 
 
 That light is a form of motion is further sustained by the fact that spec- 
 trum lines appear displaced, owing to the rapid advance toward or retire- 
 
 FIG. 55. Geissler tube. It is so constructed that a portion of gas contained in it 
 mny be highly heated by an electric current. The narrow portion of the tube may be 
 placed before the slit of the spectroscope for examination. 
 
 ment from the earth of certain comets and other celestial bodies. This 
 displacement in the spectrum is analogous to the change of pitch of rapidly 
 moving sonorous bodies, as the whistles on moving locomotives. 
 
 FIG. 56. Direct vision spectroscope, employed to examine a colored liquid. The 
 kind of dye-stuff in a liquid is often determined by this method of examination. 
 
 (/) Work. Heat may accomplish external or internal work. The 
 former product is evident to us in various visible forms of mass-motion. 
 
92 CERTAIN FORMS OF ENERGY. 
 
 Internal work is done when some natural molecular force is over- 
 come. Thus this is accomplished when the cohesive power of a solid is 
 so overpowered that liquid is produced. Ice when melted occupies a 
 diminished bulk, so that in this case no external work results. 
 
 FIG. 57. Spectroscope attached to a telescope for the purpose of examining the pro- 
 tuberances on the outside of the disk of the sun. (The portrait represents J. Norman 
 Lockyer, the celebrated English astronomer.) 
 
 The internal work of heat is explained by supposing that in this case 
 there is produced some internal motion, like rotation of molecules, for exam- 
 ple, rather than one like a translation of them. 
 
CERTAIN FORMS OF ENERGY. 93 
 
 ELECTRICITY. 
 
 The phenomena of electricity already known are so 
 numerous, varied, and subtile as to defy complete ex- 
 planation. Of the various theories of its nature that 
 have been suggested, none seem on the whole so satis- 
 factory as that it is some form of energy ; in other words, 
 a form of motion. The exact character of this motion 
 cannot at present be stated. It is probable that in some 
 cases it is a motion of translation of molecules ; oftener, 
 perhaps, it is a motion of mere rotation or similar polar- 
 ization of molecules without change of position. Many 
 of the phenomena seem to demand the intervention of 
 some medium like the ether, whether the same as 
 that supposed for the explanation of radiant heat or 
 only a somewhat similar one, it is impossible to declare. 
 However objectionable the theory of an ether or of 
 one or more fluids is, yet in the present state of 
 knowledge something of the sort seems indispensable 
 for purposes of explanation. 
 
 Note the tendency to use such expressions as "the 
 electric current." That there is any current of matter is 
 not probable ; but there certainly is a progressive trans- 
 fer of electric energy. This transfer is marked by a 
 velocity nearly double that of light. 
 
 Its Sources. It is a very suggestive fact that what- 
 ever known source of electricity is invoked, there is 
 always an evident consumption of force. This becomes 
 apparent from a mere enumeration of some of its direct 
 sources. 
 
 Such sources are : the energy of friction; the energy of mere mechani 
 cal separation of certain bodies in contact; mere application of heat; 
 
90 CERTAIN FORMS OF ENERGY. 
 
 mere change of temperature; mechanical separation of magnetically at- 
 tracted bodies; chemical combination and decomposition. 
 
 The modern system of producing electricity for purposes of illumina- 
 tion illustrates, in an interesting manner, several transformations of force. 
 
 FIG. 60. Electric light produced by a Bunsen battery and employed in a magic 
 lantern for the purpose of projecting the image of a microscopic object upon the screen. 
 
 This system generally includes at least four contrivances: first, the fire-box; 
 second, the boiler and engine; third, the dynamo; fourth, the lamp. In 
 the fire-box, energy of the chemical affinity of burning coal is transformed 
 into the energy of heat; in the boiler and engine the energy of heat is 
 
CERTAIN FORMS OF ENERGY. 
 
 97 
 
 transformed into the energy of mass-motion; in the dynamo the energy of 
 mass-motion is transformed into the energy of electricity; in the lamp, the 
 energy of electricity is transformed into the energy of light and heat. 
 
 Its Effects. That the effects of electricity are strik- 
 ingly suggestive of some form of motion is evinced by a 
 brief reference to a few of them. 
 
 FIG. 61. Method of precipitating silver or gold from a solution of the metal upon 
 an ornamental object. In this case the electric current from a Bunsen battery is em- 
 ployed. The current from a dynamo will also serve. 
 
 (a) One of the most early and easily observed of the effects is mechan- 
 ical motion; thus electrified bodies readily exhibit direct attraction and 
 repulsion of mass. Mechanical apparatus can also be kept in motion by a 
 proper application of electricity. 
 
 () Electricity gives rise, by direct transformation, to other admitted 
 forms of force, such as heat, light, and magnetic polarity. 
 
 (c) Electricity produces motion in chemical molecules. In the pro- 
 cesses of electro-plating and other forms of electrolysis it effects a drawing 
 
98 CERTAIN FORMS OF ENERGY. 
 
 apart of portions of matter previously firmly bound together by chemical 
 affinity. 
 
 (</) The very peculiar phenomena of conduction, insulation, and electric 
 induction, seern to favor the view here presented; for it is hardly possible 
 to conceive of such results springing from a transfer of matter, even of a 
 most tenuous kind. 
 
CHAPTER IX. 
 THE ATTRACTIONS OF MASSES. 
 
 GRAVITATION. 
 
 GRAVITATION is a force by which considerable por- 
 tions of matter are mutually attracted, whatever their 
 size, nature, distance apart, or the intervening medium. 
 
 Law. The gravitating attraction between material 
 objects is directly proportional to their masses, and in- 
 versely proportional to the squares of the distances between 
 their centres of gravity. 
 
 In accordance with this law, then, if two bodies, each 
 containing a unit of mass, gravitate toward each other 
 with a certain force, then when one of those bodies has 
 its mass doubled or trebled, the gravitating force is mul- 
 tiplied by two or three ; and if at once the mass of one 
 body is doubled, and the mass of the other is trebled, 
 then the gravitating force is doubled by reason of the 
 one increase, and trebled by reason of the other ; that 
 is, it is increased in the proportion of 2 X 3 = 6 times. 
 
 Again, if the masses remain unchanged, and the dis- 
 tance between the centres of gravity of the bodies is 
 increased to two or to three units of distance, then the 
 gravitating force is reduced respectively to 
 
 H- orto H- 
 
 99 
 
IOO THE ATTRACTIONS OF MASSES. 
 
 If the distance stated is reduced to one- half or one- 
 third, then the gravitating force is increased to four 
 times or nine times, respectively. 
 
 Gravitation is the great agent of stability in nature. 
 It regulates the motions of the heavenly bodies. By its 
 influence objects on the surface of the earth retain their 
 positions instead of being cast forth into space. 
 
 Its value is best estimated by considering the results 
 which would follow its suspension, supposing the latter 
 to be possible though in fact it is not. 
 
CHAPTER X. 
 
 THE ATTRACTIONS OF MOLECULES. 
 
 I. COHESION. 
 
 COHESION is an attractive force acting at insensible 
 distances between molecules of the same kind. Besides 
 
 FIG. 62. Wire-drawing machine. The coarser wire on the reel A passes through a 
 small opening in the steel plate f. It is wound as a finer wire upon the drum B. The 
 operation illustrates a resistance of solids to change of form. (The metal remains solid 
 during the operation.) 
 
 this cohesive force, molecules of the same kind are influ- 
 enced by an expansive tendency (undoubtedly due to 
 heat). 
 
 In solids the cohesive force manifests itself in the 
 resistance they offer to any derangement of the shape 
 of the solid ; that is, of the arrangement of molecules 
 
'162'' ' THE' ATTRACTIONS OF MOLECULES. 
 
 within the mass. Thus solids offer resistance to in- 
 crease or decrease of volume, also to flexure and to 
 rupture, and, indeed, to any alteration of shape, even 
 though not involving increase or decrease of bulk. 
 
 In gases the opposite extreme is found. They seem 
 to possess little or no cohesive force ; the expansive 
 tendency predominates. A few cubic inches of gas 
 placed in a vacuous receiver, of any shape, and of any 
 size (not exceeding forty or fifty miles in height), would 
 doubtless soon expand so as to fill the receiver. Appar- 
 ently the remoteness of the molecules of a gas from one 
 , another is an important feature ; they are thereby re- 
 moved more from the influence of their specific cohesive 
 forces, and thus are made capable of greater freedom of 
 motion. 
 
 Liquids exist under conditions that are in a certain 
 sense intermediate between those of solids and of gases. 
 
 Liquids manifest cohesion in their tendency to as- 
 sume the globular form (that form affording the most 
 compact arrangement for a given number of centrally 
 attracted objects). 
 
 Liquids display the existence of the expansive ten- 
 dency in their easy evaporation. 
 
 Polarity. There is a striking difference between 
 solids and liquids as to the adjustments of the molec- 
 ular forces. In liquids the cohesive forces seem to be 
 balanced about the centres of the molecules, so that 
 these molecules are free to move about each other, and 
 to occupy equally well many positions with respect to 
 each other. In solids, on the other hand, there exists 
 polarity ; i.e. the cohesive forces seem to reside out of 
 
THE ATTRACTIONS OF MOLECULES. IO3 
 
 the centres of the molecules, and in certain centres of 
 force which may be called poles. 
 
 This polarity not only compels solids to offer that 
 resistance to derangement of shape already referred to. 
 It also produces the phenomena of crystallization. 
 
 Crystallization. In general, a portion of matter in 
 the act of changing from the gaseous or the liquid to 
 the solid form manifests a tendency toward a definite 
 arrangement of molecules. With the exception of a few 
 animal and vegetable products every solid affects a defi- 
 nite polyhedral form, although it may manifest this ten- 
 dency only under favorable circumstances. A body 
 possessing such a form is called a crystal. 
 
 It seems proper to assume that the crystalline condi- 
 tion is the normal one for all solids. 
 
 It is true that certain distinctively animal and vege- 
 table matters the so-called organized matters as- 
 sume the cellular form rather than the crystalline. They 
 are not as thoroughly exceptional, however, as might at 
 first appear. (See p. 105.) 
 
 Cleavage. Certain well-crystallized substances, when 
 broken, are found to possess the property of cleavage in 
 a marked degree : if pulverized, or otherwise subdivided, 
 they undergo fracture with far greater ease in certain 
 directions than in others. A given portion of Iceland 
 spar, for example, having a well-defined rhombohedral 
 form, may be broken up into smaller rhombohedrons 
 rather than into masses of indefinite shape. So a mass 
 of galena, if broken, forms rectangular or cubical masses. 
 Even when these substances are pulverized, examination 
 
IO4 THE ATTRACTIONS OF MOLECULES. 
 
 by the microscope shows, in each case, the strongly 
 marked crystalline tendencies ; each particle of powder 
 shows itself to be a little crystal, or mass of crystals. 
 The same principle is observed in other crystalline sub- 
 stances, though perhaps in less marked degree. 
 
 In general, it may be assumed that any crystalline substances when 
 broken up into small particles will show crystalline fracture, and in the 
 powder will show a multitude of little crystals bearing a strong resemblance 
 to the large crystal whence it came. The cutting of diamonds and other 
 precious stones depends upon this general principle; i.e. that layers may 
 be cleaved off in certain directions better than in others. It is very plain, 
 then, that a crystal possesses not merely external symmetry the laws of 
 its structure govern most thoroughly its internal parts. It may be imag- 
 ined that, if it were practicable to reduce the size of a given large crystal 
 by removal of its outer layers, one by one, the time would come when, the 
 limit of the single molecule being reached, this molecule itself if capable 
 of remaining solid would be found to possess something equivalent to a 
 crystalline form. It would be either a miniature of the large crystal or 
 else one geometrically related to it. 
 
 Theoretically, at least, it may be considered that 
 a single molecule first solidifies, and then other mole- 
 cules build themselves upon it as upon a nucleus. It 
 may be assumed that they pile themselves up upon these 
 outer faces in accordance with some definite geometrical 
 law. These statements strengthen the conviction that 
 crystalline form is not a mere external and superficial 
 peculiarity of substances, but is the product of a funda- 
 mental and essential law of them. Probably it is closely 
 connected with their atomic and molecular constitution. 
 Mitscherlich's law (see p. 234) represents an attempt to 
 deal with this relationship. It may be expected that in 
 the future yet other and more definite connection will 
 be shown between the crystalline form and chemical 
 constitution. 
 
THE ATTRACTIONS OF MOLECULES. IO5 
 
 Apparent Exception. It is commonly accepted as a principle that 
 in a certain sense organized bodies (see p. 154) do not conform to these 
 statements. Perhaps in another sense they will ultimately be found in 
 harmony with it. Thus certain organized bodies, like muscle, are made 
 up, not of one substance, but of several different substances, arranged in 
 the cellular form. If these substances could be separated one from an- 
 other, and then each reduced to the solid form, perhaps they would then 
 appear as a set of crystalline compounds. This latter statement is made 
 with the general admission, already presented in another place, that differ- 
 ent substances well recognized as crystallizable assume the crystalline form 
 with different degrees of ease. In other words, in order to crystallize, sub- 
 stances demand conformity to many conditions. 
 
 Crystalline Systems. The various crystalline forms 
 recognized have been classified in six so-called systems ; 
 and, moreover, a substance crystallizing in a given sys- 
 tem is capable of certain variations within that system. 
 In this way there exist within a given system not only 
 the simple forms, but also what are called compound 
 forms and hemihedral forms. 
 
 Certain substances crystallize in shapes that are 
 readily recognized and referred to their proper sys- 
 tems. Others assume such complicated forms, or else 
 such imperfectly developed shapes, or else crystallize in 
 such minute portions, that their recognition is difficult. 
 Sometimes a special apparatus called a goniometer is 
 employed to determine the particular crystalline form. 
 
 The several systems are presented in brief but com- 
 pact form in the following tables and diagrams : 
 
io6 
 
 THE ATTRACTIONS OF MOLECULES. 
 
 
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THE ATTRACTIONS OF MOLECULES. 
 
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IO8 THE ATTRACTIONS OF MOLECULES. 
 
 The Process of Crystallization. - - The process of 
 crystallization is a variety of the process of solidifica- 
 tion ; but it is one that is dependent upon a highly 
 specialized arrangement of the particles of the solid. 
 Hence it must be supposed that solidifying molecules 
 take up certain determinate motions before they have 
 placed themselves in those symmetrical and orderly 
 
 FIG. 69. Crystals formed in a mass of metallic bismuth by slow cooling 
 of the melted metal. 
 
 ranks required by the crystalline condition. This 
 special kind of motion is not consistent, however, with 
 the two fluid states of matter, nor yet with the rigid 
 one. It seems to best take place in that transition 
 period which extends between them. It might be 
 expected, then, that crystallization would be facilitated 
 by increasing as far as possible the extent of this 
 transition state. This is found to be, in fact, the case. 
 Whenever the progress of solidification is prolonged, 
 the tendencies toward crystallization are favored. 
 
THE ATTRACTIONS OF MOLECULES. 
 
 Crystals are usually produced by 
 the following means : 
 
 (a) The Slow Cooling of Vapors. 
 
 The formation of crystals of snow 
 from water-vapor in the atmosphere 
 is an example of this method. 
 
 The formation of crystals of iodine 
 is another example. 
 
 FIG. 70. Section of a 
 crucible in which melted 
 sulphur has been allowed 
 to crystallize by slow 
 cooling. 
 
 (b) The Slow Cooling 
 of Liquids produced by 
 Fusion. The crystalli- 
 zation of melted sulphur, 
 
 FIG. 71. Diagram showing crystalline form 
 assumed by sulphur during slow cooling from 
 the melted form. 
 
 of melted bismuth, and of 
 melted zinc are examples of 
 this method. 
 
 (c) The Slow Cooling of Liq- 
 uids produced by Solution. 
 
 This is the method by which 
 most crystals produced in the 
 arts are formed. A very fa- 
 miliar example is rock candy, 
 
 which is crystallized by cooling the liquid produced by 
 dissolving cane sugar in hot water. 
 
 FIG. 72. A diagram showing crys- 
 talline form in which sulphur is found 
 in nature. 
 
IIO THE ATTRACTIONS OF MOLECULES. 
 
 Multitudes of crystalline salts are manufactured by 
 chemists by this method. 
 
 FIG. 73. Method of producing crystals of rock candy by slow cooling of the solution 
 of cane sugar in water. The crystals collect upon threads stretched through the liquid. 
 
 (d) Slow Evaporation of Liquid Solutions. The ma- 
 jority of solid chemical salts known may be dissolved in 
 
 FIG. 74. Crystals of potassic nitrate (saltpetre) formed by the slow cooling 
 of a solution of the salt in boiling water. 
 
 water and thus changed temporarily to the liquid form. 
 If such a liquid is evaporated, the water may be expelled 
 
11 
 
 II 
 
 
 in 
 
112 THE ATTRACTIONS OF MOLECULES. 
 
 and the solid caused to reappear. If the expulsion of 
 the water is conducted very slowly, the solid reappears 
 so slowly that its molecules have time to arrange them- 
 selves in the form of crystals. 
 
 An example of this method is found in the manufac- 
 ture of common salt, which is generally crystallized from 
 its solution in water, by means of slow evaporation. 
 
 FIG. 76. Rock crystals found in nature. The substance is silicic oxide (SiO 2 ). 
 forms in hexagonal prisms surmounted by hexagonal pyramids. 
 
 Theoretically, a crystal once formed in a solution and 
 continuing to increase in size (by every face of every 
 set of faces receiving a deposit of the same thickness) 
 would produce an ideally perfect crystal. But as a fact, 
 distortion almost always results. By the change in 
 specific gravity in the liquid, from loss of the solid 
 substance, currents are generated, and different parts of 
 
THE ATTRACTIONS OF MOLECULES. 113 
 
 the crystal are subjected to different conditions. Again, 
 by the crystal becoming attached to other crystals, or to 
 the side or bottom of the vessel, the several faces 
 receive unequal deposits ; yet every face always remains 
 parallel to its original position and the interfacial angles 
 are constant. 
 
CHAPTER XL 
 
 THE ATTRACTIONS OF MOLECULES (continued}. 
 II. ADHESION. 
 
 ADHESION is a form of attractive force, exerted at in- 
 sensible distances, between molecules of different kinds. 
 
 (A) ADHESION BETWEEN SOLIDS AND SOLIDS. 
 
 Of this kind of adhesion there are many examples. 
 The adhesion of a piece of wood to another of different 
 kind by means of a layer of glue involves two illustrations ; 
 for the solidified glue adheres to each kind of wood. 
 
 The rock known as granite is composed of three 
 different and separate materials, quartz, felspar, and 
 mica, easily recognized by inspection ; they are held 
 together by a form of adhesion. 
 
 There are certain cases in which two solids when brought in contact 
 liquefy, or set up some very evident chemical change. Thus when solid 
 ice and solid salt are placed together, each exerts on the other a very 
 remarkable attractive force by reason of which the ice melts and the salt 
 dissolves in the water formed. 
 
 The phenomena of this operation, and others similar to it, are discussed 
 more properly under Dissolving of Solids in Liquids (p. 118) and Chemi- 
 cal Action (pp. 169 and 171). 
 
 (B) ADHESION BETWEEN SOLIDS AND LIQUIDS. 
 
 Of this kind of adhesion there are many forms worthy 
 of consideration here. 
 114 
 
THE ATTRACTIONS OF MOLECULES. 
 
 FIRST FORM. Moistening. 
 
 This form is exemplified by many solids and liquids. 
 Thus, a glass rod dipped in water and then withdrawn is 
 found to retain some water upon its surface. 
 
 FIG. 77. Disposition of apparatus for showing the adhesion of a solid to the surface 
 of a liquid. The upper part of the figure represents one pan of the balance. In the other 
 pan of the balance (not shown in the cut) weights may be added until the plate (shown at 
 the bottom of the cut) is pulled away from contact with the liquid. 
 
 SECOND FORM. Capillary Attraction. 
 
 When a tube, open at both ends, is dipped into liquid 
 contained in a considerably larger vessel, tnere may be 
 three cases (under this title). 
 
n6 
 
 THE ATTRACTIONS OF MOLECULES. 
 
 The case oftenest observed is that in which the liquid 
 in the tube rises to some distance above the general 
 level of the liquid in the vessel. A tube of glass dipped 
 in water displays this phenomenon. 
 
 In some cases the liquid in the tube is depressed 
 below the level of that in the vessel. A glass tube 
 dipped in mercury affords an illustration of this case. 
 
 It might be expected that the case of a liquid main- 
 taining the same level within and without the tube 
 
 FIG. 78. Capillary attraction, showing rise 
 of liquid in narrow tubes. 
 
 FIG. 79. Capillary depression, shown by 
 fall of mercury in a narrow tube of glass. 
 
 would be a rare one ; for this can only exist when there 
 prevails a certain exact balance between the amount of 
 cohesion of the liquid itself, and double the amount of 
 the adhesive force of the liquid to the material of the 
 tube. Of course exact equality is everywhere an excep- 
 tional condition of things. 
 
 THIRD FORM. Spheroidal State. 
 
 When a small portion of liquid is placed upon the 
 surface of a supporting material that is relatively highly 
 heated, the' liquid draws itself up into a globule and 
 
THE ATTRACTIONS OF MOLECULES. 
 
 117 
 
 moves about in what is called the spheroidal state. 
 The heated surface causes the liquid to evaporate 
 chiefly on its under side ; the abundant vapors thus 
 
 FIG. 80. Experiment to illustrate the spheroidal state of water. The lamp heats the 
 plate, whereupon, if drops of water are placed upon it, they remain as little spheres and 
 do not adhere to the plate. 
 
 produced afford a cushion upon which the liquid is 
 supported. Of course the liquid does not rest in actual 
 contact with the heated material. 
 
Il8 THE ATTRACTIONS OF MOLECULES. 
 
 A noticeable anomaly exists in the case described. Even upon a solid 
 surface of a very high temperature the liquid does not evaporate as rapidly 
 as in a vessel sustained at a much lower temperature. But at these lower 
 temperatures the liquid rests in contact with the solid, and the entire mass 
 of liquid receives heat by the processes of conduction and convection. 
 At very high temperatures, however, the liquid is not in contact; the 
 globule then receives heat just as other objects do at a great distance from 
 a source of heat; that is, by the process called radiation. By this means, 
 the lower surface of the globule is the portion chiefly influenced; here 
 vapors are given off in abundance sufficient to afford the supporting 
 cushion, but not sufficient to rapidly diminish the mass of the globule. 
 
 FOURTH FORM. Sohition of a Solid in a Liquid. 
 
 The dissolving of one or more liquids may involve the 
 interaction of a great many forces, so that solution in 
 its more complex varieties is worthy of careful study 
 and extended discussion. 
 
 The general opinion now prevails that substances in dilute solutions 
 exist in a condition somewhat analogous to substances in the gaseous state 
 under moderate pressure and moderately high temperature. 'This view is 
 based upon the studies of J. H. van 't Hoff and F. Raoult. 
 
 Even in the simplest forms the process of solution 
 depends on a variety of conditions. 
 
 The amount of a given solid capable of dissolving in 
 a liquid, in a given experiment, depends upon at least 
 three factors : 
 
 The nature of the solid and the liquid ; 
 
 The quantity of the solid and the liquid ; 
 
 The temperature under which the experiment is 
 conducted. 
 
 The rapidity with which in a given experiment any 
 certain (possible) amount of a solid is dissolved in a 
 given amount of liquid, depends upon the rapidity with 
 which the favorable conditions are provided. Thus 
 
THE ATTRACTIONS OF MOLECULES. I IQ 
 
 extreme fineness of comminution, agitation of the mix- 
 ture of solid and liquid, rapid addition of heat, all 
 favor rapid dissolving. 
 
 I . Nature of the Liquid and the Solid. (a) Water 
 is specifically gifted with solvent power of a remarkably 
 wide range. Moreover, it is a very abundant and widely 
 diffused substance. In many ways it seems entitled to 
 be considered the chief liquid. Now water dissolves in 
 large quantities a very large number of chemical salts. 
 As examples may be mentioned, potassic sulphate, 
 K 2 SO 4 ; sodic sulphate, Na 2 SO 4 10 H 2 O ; potassic ni- 
 trate, KNO 3 ; zinc sulphate, ZnSO 4 - 7 H 2 O ; sodic chlor- 
 ide, NaCl. In fact, it dissolves in greater or smaller 
 quantity the majority of salts known. Water also dis- 
 solves a great number of neutral bodies, of which cane 
 sugar (CtfHffiOu) may be used as an example. 
 
 (b) Carbon disulphide dissolves sulphur and some 
 other substances not soluble in water. 
 
 (c) Liquid oils dissolve many solid fats ; thus the 
 more liquid paraffins dissolve the more solid ones like 
 white paraffin wax. 
 
 (d} Liquid mercury dissolves most of the metals, as 
 potassium, sodium, gold, silver, zinc (but not iron). 
 Mercurial solutions and their more solid forms are 
 called amalgams. 
 
 (e) Melted zinc dissolves many metals, as solid cop- 
 per, gold, platinum, and others. Products of this 
 general character both before and after solidification 
 are called alloys. 
 
 (/) Diluted sulphuric acid dissolves metallic zinc and 
 other metals. 
 
I2O 
 
 THE ATTRACTIONS OF MOLECULES. 
 
 Remarks on this List. This list is merely one of examples. Other 
 examples might have been given under each head, and perhaps with equal 
 propriety. 
 
 In examples l>, c, d, e, it plainly appears that solvent power is generally 
 the greater, the greater the similarity of the solid and the solvent liqtiid. 
 
 It will be noted that, as a matter of course, solvent action is oftenest 
 observed in cases of liquids which (like water, for example) remain in the 
 liquid form at the temperatures ordinarily prevailing; it must not be for- 
 gotten that if the globe had a slightly lower climatic temperature, many of 
 these would be best known as solids; they would be thus reduced to the 
 category of those (like metallic zinc, for example) which now have to be 
 artificially raised a little in temperature before they display their solvent 
 powers. Of course, at greatly reduced temperatures, all known liquids 
 solidify, and would be thrown out of the account, just as at very greatly 
 elevated temperatures all known solids would probably liquefy, and so 
 would come into the list of liquid solvents. 
 
 Example / needs special attention. In a very just sense it belongs to 
 case a. The solvent action is properly that of water upon a chemical salt 
 zinc sulphate (ZnSO 4 7 H 2 O). For it happens that the dilute sulphuric 
 acid exerts such a chemical action upon the metallic zinc as changes it 
 into the chemical salt zinc sulphate. When dilute sulphuric acid acts 
 upon metallic zinc, the operation may be represented as follows : 
 
 Zn + H 2 SO 4 
 
 f 7H 2 
 
 + Aq 
 
 One atom of One molecule of 
 
 Seven molecules of 
 
 Indefinite amount of 
 
 Zinc, Sulphuric acid, 
 
 Water, 
 
 Water. 
 
 65 98 
 
 126 
 
 
 parts by weight. parts by weight. 
 
 parts by weight. 
 
 
 28 9 
 
 
 
 ZnSO 4 7 H 2 O + 
 
 H 2 
 
 + Aq 
 
 One molecule of 
 
 One molecule of 
 
 Indefinite amount of 
 
 Crystallized zinc sulphate, 
 
 Hydrogen, 
 
 Water. 
 
 287 
 
 2 
 
 
 parts by weight. 
 
 parts by weight. 
 
 
 289 
 
 Then the zinc sulphate (ZnSO 4 7 H 2 O) dissolves in the water present 
 in the original dilute sulphuric acid. 
 
 It ought to be noted that it seems highly probable that in all cases of 
 solution even those where a single solid dissolves in a single liquid 
 
THE ATTRACTIONS OF MOLECULES. 121 
 
 there is some chemical action. In cases at one extreme of the series the 
 action may be very marked. This is so, for example, when sulphuric 
 oxide (SO 3 ), a solid, dissolves in water; great heat is here evolved, and 
 there is produced a new substance, sulphuric acid (H 2 SO 4 ), which also 
 dissolves in water. An example of the opposite extreme is that in which 
 sugar dissolves in water. It would be difficult in such a case to demon- 
 strate that any chemical action takes place unless specially devised means 
 were employed for its detection. 
 
 2. Influence of Change of Temperature. The com- 
 prehension of this, as well as of other branches of the 
 subject, may be facilitated by a description of the opera- 
 tion of solution as advancing by stages. 
 
 When a solid is placed in a liquid, the liquid acts first 
 upon the outer layers of the solid. 
 
 Adhesion and chemical action are exerted at once. 
 The surface molecules of the solid leave the others and 
 assume the liquid form. 
 
 The liquefied portions from the original solid at once 
 diffuse themselves into some of the intermolecular spaces 
 of the bathing liquid. 
 
 True solution has now been accomplished, though it 
 may as yet be somewhat limited quantitatively. 
 
 By the very act of dissolving, as thus far described, 
 there have been created certain conditions which directly 
 oppose its further progress. 
 
 The first of these opposing conditions is a reduced temperature. It is a 
 recognized law that in all changes of a solid to a liquid, absorption of heat 
 takes place. This effect is recognized in the cooling of whatever happens 
 to be the surrounding medium. It is often stated that in liquefaction sen- 
 sible heat becomes latent heat. (While the expression latent heat of lique- 
 faction is a well-established one, it is somewhat inappropriate. It carries 
 the suggestion that a certain amount of heat has become merely concealed, 
 whereas heat in fact ceases to be as heat when it does the work of lique- 
 fying a solid.) To the foregoing should be added another important state- 
 
122 THE ATTRACTIONS OF MOLECULES. 
 
 ment. It is the following : The amount of a solid that a liquid can hold 
 in solution varies with the temperature, being in most cases the greater the 
 higher the temperature. It follows that at any given point of temperature 
 the liquid may hold dissolved a certain amount of the solid and no more. 
 When this full amount is in fact dissolved, the liquid is said to be saturated. 
 In case of many solids and liquids, tables have been constructed showing 
 by the graphical method the quantities producing saturation at each of a 
 series of temperatures. 
 
 From what has been said it is plain that if it is desired that the dissolv- 
 ing operation shall go on, heat must be added. 
 
 The second of the opposing conditions is the local saturation of the sol- 
 vent liquid. By reason of this saturation the portions of liquid lying imme- 
 diately about the solid may become incapable of further dissolving action, 
 while more remote portions may not as yet have begun to act. This ces- 
 sation is sometimes associated with the fact that the solid rests at the 
 bottom of the liquid, and the solution, being heavier than the original 
 liquid, naturally rests upon and covers the solid. There are three ways of 
 overcoming this difficulty. One way is to agitate the liquid mechanically. 
 A better way is to suspend the solid in a perforated basket hung in the 
 upper layers of the liquid; then the solution, as it becomes saturated, sinks 
 by its own weight, and is promptly replaced by portions of fresh liquid. 
 A third method, employed to advantage in combination with the second, 
 is heating; this produces convective currents in the liquid. 
 
 3. The Quantity of the Solid and Liquid. This point 
 needs no discussion in addition to the statements relat- 
 ting to saturation already introduced. 
 
 Deliquescence. This is a form of solution. It is ex- 
 emplified by certain substances that have such a strong 
 attraction for water that they even absorb that moisture 
 existing as vapor in the atmosphere. They draw this 
 water to themselves in such quantity that they soon 
 cease to be solid ; for they liquefy by dissolving in the 
 water absorbed. 
 
 This topic is naturally associated with the adhesion of 
 solids and gases. (See p. 130.) 
 
THE ATTRACTIONS OF MOLECULES. 123 
 
 Freezing Mixtures. In some cases the loss of heat 
 associated with and due to liquefaction is very great. 
 Thus, when ice and salt are mixed, the ice melts and 
 the salt dissolves in the water so formed. Thus both 
 liquefy. The amount of heat absorbed from surround- 
 ing objects is very great, and the cold so produced is 
 utilized in many operations in the arts. 
 
 The mixture remains liquid at temperatures much 
 below that at which both constituents when separate 
 would be solid. This clearly shows that the liquefac- 
 tion is not due to heat alone, but involves also some 
 specific influence of adhesion or chemical union or both 
 together. It is of similar nature to the phenomena 
 already mentioned under the title eutexia. (See p. 49.) 
 
 This topic has certain relations to the adhesion of 
 solids to solids, but is more closely affiliated with the 
 solution of solids in liquids. 
 
CHAPTER XII. 
 THE ATTRACTIONS OF MOLECULES (continued). 
 
 II. ADHESION (continued-}. 
 
 THE study of the conditions under which solids dis- 
 solve in liquids naturally leads to a consideration of 
 those under which solids may be separated again from 
 liquids holding them in solution. But it is not intended 
 here to extend the discussion to the formation of pre- 
 cipitates by sudden chemical reactions. 
 
 THE SEPARATION OF A SOLID FROM A LIQUID. 
 
 The comprehension of this subject may be facilitated 
 by a few typical examples. These will develop the fol- 
 lowing simple but important principle : As dissolving is 
 favored by increase of quantity of solvent and by addition 
 of heat, so separation of a solid from its solvent is favored 
 by decrease of quantity of liquid and by decrease of heat. 
 The withdrawal of heat is almost always practicable ; the 
 decrease of quantity of solvent is practicable in cases of 
 certain liquids, like water, carbon disulphide, alcohol, 
 ether, and others that easily evaporate. 
 
 FIRST EXAMPLE. Cane Sugar. 
 
 If a saturated aqueous solution of cane sugar has some of its water 
 removed by evaporation, a portion of sugar corresponding to the amount of 
 124 
 
THE ATTRACTIONS OF MOLECULES. 
 
 125 
 
 water so removed immediately settles out in the solid form. Incidentally 
 this sugar forms crystals. 
 
 FIG. 81. The vacuum-pan for producing rapid evaporation of water from sugar solu- 
 tions. The vapor as fast as it is formed, and also the air in the apparatus, is rapidly 
 withdrawn by a powerful pump. Whereupon further evaporation takes place, and the 
 sugar syrup is brought to a condition such that it will readily crystallize. 
 
 Again, if a saturated aqueous solution of cane sugar is reduced in tem- 
 perature, a portion of sugar corresponding to the amount of heat with- 
 
126 
 
 THE ATTRACTIONS OF MOLECULES. 
 
 FIG. 82. Crystals produced in a 
 vessel by the slow evaporation of a 
 liquid produced by solution. 
 
 drawn immediately settles out in the solid form. In this case, also, the 
 solidifying sugar incidentally crystallizes. 
 
 Both these means are, in fact, employed in the arts for the manufacture 
 of sugar on the large scale. 
 
 SECOND EXAMPLE. Alum. 
 
 Potassic sulphate (K 2 SO 4 ) and aluminic sulphate (A1 2 [SO 4 ] 3 ) may be 
 dissolved together in water. When the clear solution so formed is either 
 
 evaporated or cooled, crystals of a new 
 double salt separate. This salt is called 
 alum. It is potassio-aluminic sulphate, 
 and the crystals are found to have the 
 composition represented by the formula 
 
 K 2 S0 4 , A1 2 (S0 4 ) 3 .2 4 H 2 0. 
 
 Under these circumstances, then, the 
 salts have the power of drawing to 
 
 themselves, by reason of chemical affinity for it, a definite amount of water, 
 called in this case, and similar ones, water of crystallization. Many other 
 salts have the power of combining chemically 
 with water in this way. In fact, it is believed 
 to be probable that all substances that dissolve 
 in water combine with it chemically, though the 
 demonstration of the fact of combination some- 
 times involves difficulties. 
 
 THIRD EXAMPLE. Sodic Sulphate. 
 
 Sodic sulphate presents a peculiar and inter- 
 esting form of the same tendency manifested by 
 alum; i.e. to combine with water under proper 
 conditions. 
 
 Thus there exist three salts the same in 
 composition except as respects water differing 
 merely according to the circumstances under 
 which they solidify. 
 
 These salts are : 
 
 FIG. 83. Burnt alum. 
 When alum is heated in a 
 crucible, it puffs up on ac- 
 count of the escape of water 
 of crystallization in the form 
 of steam. 
 
 Anhydrous sodic sulphate 
 Heptahydrated sodic sulphate 
 Dekahydrated sodic sulphate 
 
 7 H 2 O. 
 Na 2 SO 4 . ioH 2 O. 
 
THE ATTRACTIONS OF MOLECULES. I2/ 
 
 It is not necessary to undertake a detailed account of the conditions 
 under which these several compounds are produced. It is sufficient to 
 state, in general, that the formation of one rather than another is a matter 
 of temperature mainly. The general rule is that in solutions of lower 
 temperatures more water combines with the salt; in solutions of higher 
 
 FIG. 84. Dr. Frederick Guthrie, lately Professor of Physics at the Royal School of 
 Mines, London. Born in 1833; died in 1886. 
 
 temperatures a kind of dissociation takes place, and crystals are formed 
 containing less water. 
 
 FOURTH EXAMPLE. So die Chloride. 
 
 Common salt affords an illustration of the general principle just illus- 
 trated by sodic sulphate, only in the case of common salt the principle is 
 extended to very low temperatures indeed. Crystals of common salt 
 
128 THE ATTRACTIONS OF MOLECULES. 
 
 formed at ordinary temperatures are anhydrous; they have the formula 
 NaCl. When a suitable solution is cooled considerably below the freezing- 
 point of water, two kinds of crystals may be formed one variety having 
 the formula NaCl 2 H 2 O; the other variety, formed at still lower tempera- 
 tures, having the formula NaCl io|H 2 O. Crystals formed in this way 
 below the freezing-point of water are called, by Guthrie, cryohydrates. 
 
 The properties of the cryohydrates of common salt help to explain the 
 well-known fact that salt water does not freeze except at temperatures 
 much below 32 F. Salt water may be considered as a special chemical 
 compound, the cryohydrate of common salt. This cryohydrate is charac- 
 terized by a melting-point (and, what is the same thing, a solidifying- 
 point) which happens to be below o C. 
 
 Efflorescence. Most crystals containing water of crys- 
 tallization may give it off by mere influence of heating. 
 The amount of heat required varies with the substances : 
 in some cases the ordinary heat of the atmosphere is 
 sufficient. The result of such expulsion is a breaking 
 of the crystals into a non-crystalline powder. The crys- 
 tals are x said to effloresce 
 
 FIFTH EXAMPLE. Metallic Lead. 
 
 Melted lead is of course a liquid; when it is slowly cooled, it permits 
 the formation of solid crystals. Many other melted metals and alloys do 
 the same, but melted lead affords a good example, because in many large 
 lead works this crystallization is continually carried on on an enormous 
 scale. Lead, as produced from the ore, contains a minute amount of sil- 
 ver diffused through it. By Pattinson's process for extraction of this 
 silver the melted mass is cooled and thus partly crystallized; solid crystals 
 of nearly pure lead thus separate, and upon their removal they are found 
 to have left most of their silver in the melted portion of lead remaining. 
 From this the silver is finally extracted. 
 
 Alloys. The term alloy was originally applied to a 
 mixture of gold and silver melted together with or with- 
 out other metals. The term is now applicable to all 
 
THE ATTRACTIONS OF MOLECULES. 
 
 129 
 
 mixtures or compounds of metals with each other, ex- 
 cept those containing mercury, which latter are called 
 "amalgams." 
 
 On melting two metals together, complete assimila- 
 
 FIG. 85. Pattinson furnace for separating crystals of pure lead (from its solution in 
 melted argentiferous lead) by slow cooling. 
 
 tion takes place in some cases ; in others it does not. 
 Thus, silver does not readily alloy with iron. 
 
 FIG. 86. Top view of Pattinson furnace, showing the kettles in which argentiferous 
 lead is melted. 
 
 The physical properties of an alloy are, in certain 
 cases, the mean of the properties of the metals of 
 which it is composed ; in other cases they are widely 
 different. 
 
I3O THE ATTRACTIONS OF MOLECULES. 
 
 Matthiessen has divided the metals that form alloys into two classes: 
 
 FIRST. Those which impart to their alloys their own properties : lead, 
 tin, zinc, and cadmium. 
 
 SECOND. Those which do not : the other metals. 
 
 He regards the alloys of class first as solidified solutions of one metal 
 in the other. The metals of class second he considers enter into alloys in 
 allotropic form. 
 
 (C) ADHESION BETWEEN SOLIDS AND GASES. 1 
 
 A solid, when immersed in a gas and then withdrawn, 
 retains upon its surface a thin film of gas, somewhat as 
 a solid is wetted by dipping in water. 
 
 Further, some solids absorb into their intermolecular 
 spaces a great bulk of gas so much indeed that in some 
 cases the absorbed gas occupies a volume even smaller 
 than it would if condensed to the liquid state by itself. 
 
 The metal palladium is remarkable for absorbing or 
 occluding, at ordinary temperature, eight hundred times 
 its bulk of hydrogen gas. The late Professor Graham, 
 of London, who observed this property of palladium, con- 
 sidered the solid thus formed to be an alloy, and to con- 
 tain hydrogen in the solid form. The quantities of the 
 two elements are in this case approximately in the pro- 
 portion of the weight of one atom of hydrogen to one 
 atom of palladium, so that it has been suggested that 
 the substances may be in chemical combination. 
 
 The metal platinum has the same power as palladium, though to a less 
 degree. 
 
 If a warm piece of platinum foil is placed in a current of mixed illu- 
 minating gas and air, the foil absorbs portions of all the gases. In so doing 
 it condenses them to such a degree as to bring the molecules very near to 
 each other, even within that minute distance through which chemical 
 
 1 Section (Z?) is at p. 114. 
 
THE ATTRACTIONS OF MOLECULES. 131 
 
 attraction can be exerted. Chemical union does in fact take place, as is 
 evidenced by the production of light and heat and other phenomena of 
 true combustion. 
 
 Hannay and Hogarth have shown that in some cases 
 a gas, brought in contact with a solid, dissolves the latter 
 quickly into itself. 
 
 FIG. 87. Dbbereiner's lamp, showing the adhesion of hydrogen gas to platinum. 
 The bottom of the lamp is a hydrogen generator. Dilute sulphuric acid acts upon the 
 mass of zinc B. Hydrogen rises in the little bell-glass, and streaming from the tip at f, 
 and falling upon the mass of spongy platinum at G, takes fire. The burning hydrogen 
 lights the oil lamp M. 
 
 (D) ADHESION BETWEEN LIQUIDS AND LIQUIDS. 
 
 In general, the adhesion of liquids to liquids so far 
 exceeds their respective cohesive forces that the liquids 
 may be mixed in all proportions. 
 
 In general, heat favors this sort of diffusion. 
 
 Thus water and ordinary alcohol, when mixed in any 
 proportion whatever, mingle throughout by virtue of 
 their own attractive forces. On the other hand, when 
 water and ordinary ether are mixed, only a certain small 
 amount of the ether dissolves in the water ; the ether in 
 
132 
 
 THE ATTRACTIONS OF MOLECULES. 
 
 excess of this amount forms a separate layer upon the 
 top of the water. 
 
 FIG. 88. Apparatus for demonstrating the fact and the amount of liquid diffusion. 
 A given liquid is placed in the vessel B. A solution to be tested is placed in the vessel 
 A , provided with a glass cover. At a certain point of time the cover of A is removed. 
 The material in A at once commences to diffuse into the liquid B. After a proper period 
 of time has elapsed, the cover is replaced upon A . A portion of the liquid B is then 
 tested, and the amount of material that has diffused from A into B in the given number 
 of minutes or hours is determined. 
 
 There are many well-known examples of two liquids 
 which scarcely mix at all ; water and oil, water and 
 mercury, are such. 
 
 FIG. 89. Graham's apparatus for dialysis. 
 
 Osmose of Liquids. In case of two liquids separated by a porous 
 septum (it being granted that there exists adhesion between the liquids, 
 and a difference in the amounts of adhesion of the two liquids for the sep- 
 
THE ATTRACTIONS OF MOLECULES. 
 
 133 
 
 turn), the liquid which wets the septum the better passes through the more 
 rapidly. 
 
 FIG. 90. Dialyzing apparatus separated. The upper vessel is called the dialyzer. 
 It consists of a ring open at top and bottom, the bottom opening being covered with a 
 membranous material, held in place by a stout rubber ring. 
 
 Dialysis. The process of dialysis can be displayed by means of a 
 suitable vessel divided by a kind of membranous partition into two com- 
 
 FIG. 91. Graham's apparatus for illustrating dialysis. A crystallizable substance placed 
 in the vessel a, called the dialyzer, passes by liquid diffusion into the liquid b. 
 
 partments. If pure water is placed in one compartment and the aque- 
 ous solution of some crystallizable substance in the other, dialysis takes 
 
134 
 
 THE ATTRACTIONS OF MOLECULES. 
 
 place; that is, the crystallizable substance makes its way through the 
 diaphragm into the other compartment. Non-crystallizable substances (for 
 this purpose called colloids) are not capable of this kind of transfer. Evi- 
 dently the crystallizable substances pass through the diaphragm by a kind 
 of osmose. 
 
 (E) ADHESION BETWEEN LIQUIDS AND GASES. 
 
 Water and many other liquids have the power of dis- 
 solving gases, though in very different proportions. 
 
 FIG. 92. Ammonia fountain. The vessel A contains at first ammonia gas. As 
 water from B passes up through the little tube, the ammonia gas dissolves so rapidly in 
 the water as to produce diminished pressure. Whereupon the atmospheric pressure 
 upon the surface of water in B forces the water into the vessel A as in a fountain. 
 
 The amount of gas absorbed by a liquid upon which it 
 exerts no chemical action depends upon 
 
 I. The nature of the gas and liquid ; 
 II. The pressure to which they are exposed (the 
 amount of gas absorbed varies directly as the pressure) ; 
 
THE ATTRACTIONS OF MOLECULES. 135 
 
 III. The temperature (with few exceptions, tne solu- 
 bility of a gas in a liquid is greater, the lower the tem- 
 perature). 
 
 Evidences of the difference in the amounts of gas 
 dissolved by a stated amount of water may be found in 
 the following table : 
 
 FIG. 93. Disposition of apparatus for showing the adhesion of atmospheric air to 
 water. Water containing air is placed in flask A. Upon boiling this water air is expelled 
 and some steam is formed. The steam and air pass into the bell-glass C. The air col- 
 lects at the top of the bell-glass. The water-vapor condenses on the surface of the 
 mercury. 
 
 TABLE 
 
 SHOWING AMOUNTS, BY VOLUME, OF SEVERAL GASES SPECIFIED, DISSOLVED 
 BY IOOO VOLUMES OF WATER AT 32 F. 
 
 Amount of water used, 1,000 volumes. 
 
 " " hydrogen gas dissolved, 19 " 
 
 " " nitrogen gas " 20 " 
 
 " oxygen gas " 41 
 
 " " carbon dioxide gas (CO. 2 ) " !>796 " 
 
 " " hydrosulphuric acid gas (H 2 S) " 437 " 
 
 " sulphur dioxide gas (SO 2 ) " 68,861 " 
 
 " " ammonia gas (NH S ) " 1,049,600 " 
 
136 THE ATTRACTIONS OF MOLECULES. 
 
 The very large amounts in several of these cases are 
 believed to be due to the definite chemical union of the 
 gases with the water to form new compounds. 
 
 It is a fact worthy of mention that molten silver has the power of draw- 
 ing oxygen from the air and dissolving it in a quantity equal to twenty 
 
 FIG. 94. Apparatus for illustrating diffusion of gases. If a heavier gas is placed in 
 the lower flask and a lighter gas is placed in the upper flask, and the stop-cocks are 
 opened, it is found experimentally that the lighter gas diffuses rapidly downward into the 
 other, and that the heavier gas diffuses upward (although more slowly) into the lighter. 
 
 times its own volume. When the silver solidifies, this gas is violently 
 expelled. (The same principle is manifested by water; upon freezing, it 
 expels the oxygen and nitrogen it previously dissolved from the air.) 
 
 (F) ADHESION BETWEEN GASES AND GASES. 
 
 The extraordinary tendency of gases to intermingle 
 and interdiffuse has already been discussed under the 
 
THE ATTRACTIONS OF MOLECULES. 137 
 
 title of diffusion of gases. This tendency is so strong that 
 it overcomes the greatest differences of specific gravity. 
 
 These phenomena are not mainly due to adhesion, how- 
 ever, though there are grounds for believing that there 
 is such a thing as gaseous adhesion. Thus Regnault 
 has shown that when a liquid evaporates in the air, 
 more vapor rises than when it evaporates into the same 
 volume of vacuous space. 
 
 The tendency of gases to intermingle seems to be 
 mainly a development of their tension or expansive 
 power. This phenomenon is due to that motion within 
 the mass which the molecules of all kinds of matter 
 even the most rigid possess. 
 
 But the molecules of the gaseous form of matter are 
 almost uninfluenced by cohesion. Hence they manifest 
 this intermolecular motion to the most striking degree. 
 And so when gases themselves are compared, it can be 
 proved that the molecules of the lightest ones move 
 with the greatest rapidity. Of course the ample spaces 
 between the molecules of a gas offer great opportunities 
 for the entrance of the molecules of another gas. 
 
 The Terrestrial Atmosphere. The atmosphere of our 
 globe affords a splendid example of gaseous diffusion 
 constantly at work on a large scale. 
 
 I. The atmospheric air consists mainly of a mixture 
 of oxygen gas and nitrogen gas, in the following propor- 
 tions : 
 
 COMPOSITION OF ATMOSPHERIC AIR. 
 
 By volume. By weighf. 
 
 Oxygen 20.9 per cent. 23.1 per cent. 
 
 Nitrogen 79.1 " 76.9 " 
 
FIG. 95. Balance for showing that certain gases are heavier than the atmosphere. 
 The one jar contains atmospheric air. When a heavier gas is poured into the other jar, 
 the needle of the balance is boldly deflected. 
 
THE ATTRACTIONS OF MOLECULES. 
 
 139 
 
 Now any given measure of oxygen gas is sixteen times 
 as heavy as the same measure of the standard gas, hydro- 
 gen ; but nitrogen gas is only fourteen times as heavy 
 as hydrogen. Yet in our atmosphere the heavier oxy- 
 gen does not settle out, but remains thor- 
 oughly intermingled with the nitrogen. 
 
 2. The respiration of living animals 
 and the burning of all our chief fuels 
 are constantly casting into the atmos- 
 phere immense quantities of a heavy 
 gas, carbon dioxide (CO 2 ). This gas is 
 twenty-two times as heavy as the stand- 
 ard gas, hydrogen. Of course, therefore, 
 it is much heavier than the oxygen or 
 the nitrogen of the atmospheric air ; it 
 does not settle out from the air, how- 
 ever, but promptly intermingles with it 
 and remains intermingled. 
 
 NOTE I. On the density of atmospheric air. 
 
 The air contains minute amounts of a multitude of 
 gases, but oxygen and nitrogen so largely predominate 
 that only these need be taken .into the account here. 
 
 The density of the air is somewhere between the 
 densities, 1 6 and 14, of its two chief constituents : it is 
 about 14.4. 
 
 FIG. 96. Reg- 
 nault's method of 
 suspending, from the 
 balance-pan, a globe 
 containing a gas to 
 be weighed. A globe 
 of similar volume is 
 also suspended from 
 the other pan. 
 
 I volume of oxygen gas, weighing 16 units . . . . 16. units. 
 
 4 volumes of nitrogen gas, each weighing 14 units. . 56. " 
 
 5 volumes of mixture (air) will weigh 72. " 
 
 I volume of air will weigh 14.4 " 
 
 NOTE II. On the density of carbon dioxide gas (CO 2 ). 
 
 By actual weighing, in comparison with an equal volume of the standard 
 gas, hydrogen, this gas has been found to have the density 22; i.e. to 
 weigh, bulk for bulk, 22 times as much as hydrogen. 
 
 The density may be computed from the molecular weight as follows : 
 
I4O THE ATTRACTIONS OF MOLECULES, 
 
 Formula of a Molecule of Carbon Dioxide Gas (CO%). 
 
 Weight of one atom of carbon 12 microcriths. 
 
 " " two atoms of oxygen ( 1 6 X 2) ... 32 " 
 
 " " one molecule of carbon dioxide . . 44 " 
 " " one molecule of hydrogen, H 2 (i X 2) 2 " 
 
 Hence a molecule of carbon dioxide weighs twenty-two times as much 
 as a molecule of hydrogen. 
 
 But all gaseous molecules have the same size; hence, any volume of 
 carbon dioxide weighs twenty-two times as much as the same volume 
 of hydrogen. 
 
 NOTE III. Of course, in weighing atmospheric air and other gases, 
 pressure and temperature must be considered. The pressure must be 
 measured by some form of barometer. The temperature must be measured 
 by some form of thermometer. 
 
CHAPTER XIII. 
 THE ATTRACTION OF ATOMS. 
 
 CHEMICAL AFFINITY. 
 
 CHEMICAL affinity is an agency which acts at in- 
 sensibly small distances, and tends to produce combina- 
 tions of certain atoms and molecules of matter into 
 groups of a precisely determinate kind. 
 
 The characteristics of this agency cannot be described 
 in a few words. To it are referred a multitude of 
 phenomena, displaying under different circumstances 
 the greatest variety of action. Such differences are for 
 example : 
 
 As to the original quantity and intensity of the activity 
 itself. 
 
 As to the conditions under which its active powers are 
 displayed. 
 
 As to the methods by which it works. 
 
 As to the sphere of activity extremely narrow in a 
 certain sense and extremely wide in another. 
 
 As to the results accomplished by it. 
 
 The Conditions favoring Chemical Change. I . This 
 force manifests its chief activity between atoms or mole- 
 cules of different kinds. 
 
 Thus, an atom of hydrogen has affinity for another 
 atom of hydrogen, and the two may unite to form a 
 
 141 
 
142 THE ATTRACTION OF ATOMS. 
 
 molecule of hydrogen, expressible by H H, also written 
 Hz. Again, an atom of chlorine has affinity for another 
 atom of chlorine, and these two may unite to form a 
 molecule of chlorine expressible by the formula Cl Cl, 
 or C1 2 . But when a molecule of hydrogen is brought in 
 contact with a molecule of chlorine, the two generally 
 surfer decomposition, so that a rearrangement may take 
 place and two new molecules of hydrochloric acid (HC1) 
 may be produced. This chemical change may be ex- 
 pressed by the following equation : 
 
 H 2 + C1 2 2HC1 
 
 One molecule of One molecule of Two molecules of 
 
 Hydrogen, Chlorine, Hydrochloric acid, 
 
 2 71 73 
 
 parts by weight. parts by weight. parts by weight. 
 
 Evidently the atom of chlorine has more affinity for 
 an atom of hydrogen than for another atom of chlorine. 
 And an atom of hydrogen has more affinity for an atom 
 of chlorine than for another atom of hydrogen. 
 
 2. It is manifested between different substances with 
 very different, though definite, degrees of force. Thus 
 the metal gold and the metal iron oxidize (that is, com- 
 bine with oxygen) with different degrees of ease ; but 
 it is always the iron that oxidizes the easier. 
 
 3. Certain physical conditions are of great importance 
 in connection with chemical action. When physical 
 conditions are favorable, chemical action proceeds with 
 great vigor ; when they are unfavorable, the same pro- 
 cesses sometimes fail to advance at all, or they may be 
 even reversed. Under unfavorable conditions chemical 
 affinity appears either not to exist or to be dormant. 
 
THE ATTRACTION OF ATOMS. 143 
 
 The following are some of the physical conditions which determine 
 chemical changes changes that may have as their prominent features 
 either the building up or the breaking down of molecules : 
 
 (rt) The Liquid Condition. Some substances that chemically 
 unite when mixed as solutions, manifest no affinity when they are mingled 
 in the solid form. Thus, solid tartaric acid and solid hydro-sodic carbonate 
 when mingled manifest no change. When water is added, however, each 
 solid dissolves, and a chemical change at once ensues, hydro-sodic tartrate, 
 carbon dioxide, and water being formed. 
 
 The chemical change may be expressed as follows : 
 
 H 4 4 (C 4 H 2 2 ) 
 
 One molecule of 
 Tartaric acid, 
 
 1 5 
 parts by weight. 
 
 + 2 HNaCO 3 
 
 Two molecules of 
 Hydro-sodic carbonate, 
 1 68 
 parts by weight. 
 
 2C0 2 + 
 
 (H 2 Na 2 )0 4 (C 4 H 2 2 ) 
 
 + 2H 2 
 
 Two molecules of 
 
 One molecule of 
 
 Two molecules of 
 
 Carbon dioxide, 
 
 Hydro-sodic tartrate, 
 
 Water, 
 
 88 
 
 194 
 
 36 
 
 parts by weight. 
 
 parts by weight. 
 
 parts by weight. 
 
 318 
 
 The equation indicates that water is actually formed by the operation; 
 it appears evident, therefore, that the water which acted as the solvent was 
 not demanded in the building up of the molecules produced, but did, in 
 fact, act as a favoring physical agent. 
 
 It appears to be proved, however, that certain solid bodies, finely pul- 
 verized, thoroughly mixed, and then subjected to great pressure, produce 
 new compounds as the result of the pressure (and not of the heat attend- 
 ant). The quantities of the compounds changed appear to increase with 
 the duration of the pressure and its amount, as well as the fineness and 
 thoroughness of intermingling of the powders. 
 
 Thus, in a certain experiment, mixtures of dry, pure precipitated baric 
 sulphate and sodic carbonate were subjected to a pressure of six thousand 
 atmospheres under varying conditions of temperature and duration of the 
 pressure. Afterward the product was tested. After a single compression 
 the amount of baric carbonate produced was about one per cent; the solid 
 
144 THE ATTRACTION OF ATOMS. 
 
 block produced was pulverized and compressed again, when five per cent 
 of barium carbonate was produced; further treatment brought it up to 
 eleven per cent. 
 
 It has been concluded that 
 
 1. A sort of diffusion takes place in solid bodies. 
 
 2. Matter assumes under pressure a condition relative to the volume it 
 is obliged to occupy. 
 
 3. For the solid state, as for the gaseous, there is a critical tempera- 
 ture above or below which changes by simple pressure are no longer 
 possible. 
 
 () Heat. Many substances, when practically in contact with each 
 other, do not combine chemically unless the whole or a portion of the 
 mass is raised to some definite point of temperature. When this point is 
 reached, union at once commences. 
 
 The process of combustion of ordinary fuels affords an appropriate illus- 
 tration. If a portion of a mass of coal is heated in the air to the point at 
 which union with oxygen takes place, the phenomena of combustion (a 
 form of chemical union) are witnessed. The chemical change initiated 
 may be expressed in part as follows : 
 
 c + o, co 2 
 
 One atom of One molecule of One molecule of 
 
 Carbon, Oxygen, Carbon dioxide, 
 
 12 32 44 
 
 parts by weight. parts by weight. parts by weight. 
 
 44 44 
 
 It is an interesting fact that generally the combustion of the first por- 
 tions of the coal evolve, by the act of chemical union, sufficient heat to 
 raise yet other portions to the igniting point. This process, repeated, ena- 
 bles the operation to proceed from portion to portion so long as the supply 
 of carbon and oxygen are kept up unless, indeed, some unfavorable 
 physical condition is allowed to supervene. 
 
 There are numerous other examples known, in which chemical action is 
 stimulated by an amount of heat insufficient to produce light. 
 
 In fact, addition of heat is the method oftenest used for developing or 
 arousing chemical affinity. 
 
 Thermolysis and Dissociation. Another, and at first seemingly 
 inconsistent chemical effect of heat, ought to be mentioned here. It has 
 
THE ATTRACTION OF ATOMS. 
 
 145 
 
 already been pointed out that addition of heat expands material bodies, 
 and even changes solids and liquids to the gaseous form. (See p. 43.) 
 These effects are believed to be essentially associated with a motion of the 
 particles of the body, such that the molecules are moved farther and far- 
 ther apart, and even beyond the range of influence of those cohesive forces 
 
 FIG. 97. Henri St. Clair Deville, distinguished French chemist, noted for his discoveries 
 in the chemistry of high temperatures, dissociation, for example. 
 
 that bind them into solid and liquid masses. It would be quite consistent 
 with this view if still greater addition of heat were found to be sufficient to 
 drive even atoms apart from each other, and so to place them beyond the 
 minute distances within which the force of chemical affinity is exerted. 
 This would result in a decomposition of compound molecules and a lessen- 
 
146 THE ATTRACTION OF ATOMS. 
 
 ing of the number of atoms capable of existing together in elementary 
 molecules. 
 
 Now the experiments of Deville and others fully confirm these sugges- 
 tions. It is, in fact, proved that certain substances, as water, for example, 
 may be decomposed into their elements by influence of high temperature 
 alone. In this, and some similar cases, the elements may reunite when the 
 temperature of the mixture falls slightly. This kind of temporary decom- 
 position is called dissociation. It may be added that light and electricity, 
 as well as heat, are in some cases capable of accomplishing it. 
 
 In another class of cases, of which ammonia gas (NH 3 ) may serve as 
 an example, the molecule '^permanently broken up; that is, its elementary 
 substances do not, by fall of temperature, rejoin to produce the original 
 compound. In such cases the operation is called thermolysis. 
 
 It is also observed that certain elementary substances, as sulphur, for 
 example, manifest a gradual lessening of their relative vapor densities as they 
 are raised to higher and higher temperatures. This lessening of vapor 
 density is accepted as an indication that the molecules contain fewer and 
 fewer atoms; that is, undergo dissociation. The methods of Victor Meyer 
 and others have directed attention to this subject. 
 
 Heat often produces a modification of the relative chemical attractions 
 of bodies. Thus, at ordinary temperatures, sulphuric acid is capable of 
 displacing boric acid from its salts in solutions. At high temperatures 
 the red heat, for example the chemical affinities are reversed : boric acid 
 displaces sulphuric acid. 
 
 In a few cases chemical decomposition is producible by mechanical 
 means, as, for example, in certain explosive compounds; but it is proba- 
 ble that the mechanical is not always the immediate cause. In the familiar 
 cases where mechanical percussion produces decomposition of certain 
 explosives, evidently the heat generated by the percussion is the true 
 
 (<:) Light. This agent, as usually produced by luminous bodies, is 
 by no means a homogeneous one; the prism shows it to be divisible into 
 thousands of kinds of energy, characterized by greater or less differences. 
 The white light, as emitted by most of its sources, has at least three classes 
 of rays, luminous rays of various colors, non-luminous chemical rays, non- 
 luminous heat rays. The non-luminous chemical rays, called also actinic 
 rays, have a specific power of determining the chemical union of certain 
 elements and the chemical decomposition of certain compounds. 
 
 Thus chlorine gas and hydrogen gas, when mixed in a dark room, do 
 
THE ATTRACTION OF ATOMS. 
 
 147 
 
 not readily unite; when such a mixture is exposed to sunlight, almost 
 instantaneous combination ensues. 
 
 The decomposing influence of certain rays of light is displayed in the 
 photographic print, the substance decomposed being argentic chloride. 
 
 (</) Electricity. The influence of electricity in connection with 
 chemical action is manifested in at least four different forms. 
 
 FIRST. Operations coming under the head of electrolysis. 
 
 Electricity of low tension, such as that produced by the galvanic bat- 
 tery, is capable of most important influences on chemical compounds. In 
 
 FIG. 98. Apparatus showing how an electric current may precipitate a metal 
 from its solution. 
 
 processes of electro-plating with copper, nickel, silver, gold, and other 
 metals, it sets in motion an invisible current by which atoms of metal are 
 driven away from the metallic plate called the anode, then into the mole- 
 cules of acid or metallic salt dissolved in the plating-bath, and thence upon 
 the surface of the object to be plated, called the cathode. The deposition 
 of the metal is merely incidental; the molecular transfer is the important 
 feature. Metals may be dissolved by this method if desired. Again, non- 
 metals may be made to combine, or, if combined, may be separated in like 
 fashion by the current. 
 
 SECOND. When a current, in the form of an electric arc, passes through 
 compounds or through mixtures of elementary substances, chemical changes 
 often occur. One of the most marked illustrations of this kind of influ- 
 
148 
 
 THE ATTRACTION OF ATOMS. 
 
 ence is in the direct union of carbon and hydrogen whereby the substance 
 known as acetylene (C 2 H 2 ) is formed. One of the most important features 
 of interest in connection with this operation is the fact that acetylene rep- 
 resents a starting-point for the synthesis, or building up, of organic com- 
 pounds directly from their elements. 
 
 THIRD. Another form of action is by the influence of the electric spark 
 from a Ruhmkorff coil, a Holtz machine, or similar appliance. In this way 
 distinct chemical action is stimulated over a limited field. The field, how- 
 ever, may be widened by continuance of the electric discharge. 
 
 FIG. 99. Apparatus for decomposing water into its components, hydrogen and 
 oxygen, by means of a galvanic current generated by a Bunsen battery. 
 
 Such an electric discharge, in the form of sparks flowing from platinum 
 terminals through dry atmospheric air, gives rise to a direct union of the 
 oxygen and nitrogen. Brown fumes of N. 2 O 4 or NO 2 are thus formed. 
 These with water may form nitric acid (HNO 3 ). 
 
 Probably lightning discharges form nitric acid, in this way, and thus 
 contribute to the available nitrogen of the soil. 
 
 FOURTH. The silent electric discharge produces certain marked effects, 
 of which the most noteworthy is the change of ordinary oxygen into ozone. 
 
 In all these cases the action may be twofold. There is the true electric 
 influence, and, especially in cases of the second and third methods already 
 referred to, there is the additional influence of the heat connected with the 
 arc or with the luminous discharge. 
 
g S 
 
 l! 
 * 
 
 '35 
 
150 
 
 THE ATTRACTION OF ATOMS. 
 
 (<?) Vital Processes of the Higher and Lower Living Beings. 
 
 The vital powers of the higher orders of animal and vegetable beings have 
 most marked influence upon chemical action. Thus vast numbers of com- 
 pounds are recognized as existing in living animals and plants that have 
 not yet been produced without the intervention of vital force. A few crys- 
 
 -^ -J^ / 
 
 FlG. ioi. Ruhmkorff, the celebrated manufacturer of electric instruments, and 
 inventor or the Ruhmkorff coil. 
 
 tallizable substances, ordinarily the products of living organisms, have lately 
 been produced by circuitous chemical operations without intervention of 
 life. Doubtless others will be in future. 
 
 Certain processes of acetic, butyric, and other fermentations, purely 
 chemical in their nature, have been shown to be due to the presence and 
 action of microbes, and not to go on in their absence. 
 
THE ATTRACTION OF ATOMS. 15! 
 
 Organic and Inorganic Compounds. Chemists long ago recog- 
 nized certain differences between the substances found in distinctly animal 
 and vegetable matters, on the one hand, and the substances found in min- 
 eral matters, on the other between those things which constitute organ- 
 isms like animals and plants, as opposed to non-living substances like clay, 
 iron-rust, alum, saltpetre, etc. 
 
 Animal matters and vegetable matters are the products of bodies pos- 
 sessing organs. Organs are parts having specific functions. Thus the 
 stomach is an organ possessing the function of digestion, and the lungs are 
 
 FIG. 102. The Bunsen battery, F, energizes the Ruhmkorff coil, E, affording a 
 series of electric sparks in the flask A. The flask contains a mixture of nitrogen and 
 oxygen in a very dry condition. The sparks lead the two gases to combine. 
 
 organs possessing the function of respiration. Again, the leaves, the 
 flowers, the seeds, the roots, of plants, are separate organs, and they 
 possess special and very different functions of the living vegetable to which 
 they belong. Accordingly, substances derived from vegetables and animals 
 are called organic. Non-living objects, as rocks and other mineral and 
 earthy substances, do not possess organs, and they have long been called 
 inorganic. 
 
 This division of matters into organic and inorganic was formerly thought 
 an essential one ; it is not now considered so. It is now known that the 
 chemical changes of living animals and plants are governed by the same laws 
 as those prevailing in the changes of rocks and other lifeless forms of matter. 
 
152 
 
 THE ATTRACTION OF ATOMS. 
 
 Grounds for this Division. Chemistry is still, however, commonly 
 divided into the two great departments, inorganic chemistry and organic 
 chemistry; but this division is recognized as a matter of convenience 
 mainly. 
 
 FIG. 103. Yeast plant, illustrating the formation of additional cells by fission. 
 
 Three reasons, which may be mentioned, why the distinction is still 
 maintained, are : 
 
 FIRST. The number of organic compounds is very great. 
 
 FIG. 104. Globules of wheat starch, as FIG. 105. Globules of potato starch, 
 
 seen under the microscope, showing cellular as seen under the microscope, showing 
 structure. cellular structure. 
 
 SECOND. These compounds perform varied and important offices in 
 connection with human beings in their growth and nourishment in health, 
 and in their treatment in illness. 
 
 THIRD. The processes of analysis and the methods of investigation in 
 
THE ATTRACTION OF ATOMS. 
 
 153 
 
 Organic compounds are slightly different, as a whole, from those that 
 serve for the study of inorganic. 
 
 Definition of Organic Chemistry. The inorganic and the or- 
 ganic worlds are, however, so closely allied in some respects, and certain 
 of the substances of the one have such close and natural affiliations 
 with those of the other, that it is often found difficult to determine 
 where shall be placed the line of demarcation between these two great 
 natural groups. In fact, chemists have not found the definition incidentally 
 introduced in a preceding paragraph sufficiently distinct. To make it 
 more so, organic chemistry has been sometimes called the chemistry of the 
 carbon compounds. It has sometimes been called the chemistry of the 
 
 FIG. 106. Globules of corn starch as seen under the microscope, showing 
 cellular structure. 
 
 hydrocarbons. Again, the following still more rigid and scientific state- 
 ment is often employed : organic chemistry includes those compounds in 
 which the atoms of carbon are directly united either with other atoms of 
 carbon, or with atoms of hydrogen, or with atoms of nitrogen 1 
 
 Two Classes of Organic Compounds. There is one distinction 
 between the classes of organic compounds themselves that ought not to be 
 omitted here. The members of the organic family differ very much in 
 their properties, according as they are crystalline or cellular. Crystalline 
 organic compounds, of which cane sugar may be taken as a familiar and 
 suitable example, are numerous. These compounds are closely allied in 
 some respects to inorganic compounds. They do not seem to have so 
 
 1 From the Author's work, " Beginner's Handbook of Chemistry." 
 
154 THE ATTRACTION OF ATOMS. 
 
 close a relation to the vital processes as might at first be supposed. But 
 those organic compounds that are cellular, such, for example, as the 
 different varieties of starch, the fibre of wood, and the fibre of lean meat, 
 are much removed from inorganic bodies, and seem to bear a peculiar and 
 close relation to the vital forces. In general, cellular organic compounds 
 are called organized ; while the non-cellular organic compounds are called 
 non-orga nized. 
 
 Compound bodies then are divided from a certain point of view into 
 two great classes, inorganic and organic. The organic are again divided 
 into two classes, organized and non-organized. 
 
CHAPTER XIV. 
 
 THE ATTRACTION OF ATOMS (continued). 
 THE CHEMICAL WORK OF MICRO-ORGANISMS. 
 
 IT is difficult to form a proper conception of the vast 
 amount of chemical work accomplished by those ex- 
 
 I/M 
 
 FIG. 107. Autograph letter of Louis Pasteur, being an order for board for hydrophobia 
 patients undergoing treatment at the Pasteur Institute. 
 
 ceedingly minute parasitic plants called micro-organisms, 
 microbes, bacteria, etc. Of late years, following the 
 work of Pasteur and Koch, many observers have indus- 
 
5 6 
 
 THE ATTRACTION OF ATOMS. 
 
 triously studied this subject. As a result, microbes or 
 bacteria (the word bacterium is used in a general as well 
 as in a special sense) are now recognized as of high im- 
 portance in chemistry. 
 
 FIG. 108. Dr. Robert Koch, celebrated investigator In the field of bacteriology. 
 
 These organisms are exceedingly numerous as varieties 
 and yet more as individuals. They are most widely dif- 
 fused. They exist in air (though not largely in sea air 
 nor even in the air of large sewers), in water (though in 
 
THE ATTRACTION OF ATOMS. 157 
 
 varying quantities and kinds), in the soil (though in 
 most cases, not at great depths). 
 
 They effect many kinds of decomposition of mole- 
 cules a work essentially chemical. Some of it is of 
 industrial interest in manufactures and in agriculture ; 
 some of it leads to the wonderful fermentative and 
 putrefactive processes of the world ; some of it goes on 
 as the chief factor in diseases of the higher animals, and 
 indeed in the normal digestive operations of them. 
 
 FIG. 109. Microscopic infusoria such as are found in stagnant natural water. 
 
 (Thus it is observed that not all microbes are pathog- 
 enic : some are distinctly beneficial to living animals.) 
 
 General Description of Microbes. The organisms in question 
 consist of very minute cells often not greater in length than one ten- 
 thousandth of an inch. 
 
 Yet, when floating in the air, they cannot pass through a small plug of 
 loose cotton fibres. 
 
 In form they vary very much. The common forms are the globular 
 (micrococcus), the keyhole-shaped (like two spheres in contact, or partly 
 run together) , the rod-shaped (bacillus form), the comma-shaped, the spiral 
 shaped. 
 
158 
 
 THE ATTRACTION OF ATOMS. 
 
 The cells are often grouped in a tolerably definite way in filaments 01 
 chains; sometimes they are gathered in great irregular masses. 
 
 A given cell usually consists of a sac of mycoprotein enclosing homo- 
 geneous protoplasm. 
 
 FIG. no. aa, mycoderma vini; bb, mycoderma aceti (earlier stage of development); 
 cc, mycoderma aceti (advanced stage of development) . 
 
 Classification and Nomenclature of Bacteria. Micro-organ- 
 isms have been divided into two sections, (i) the Endosporea and (2) 
 the Arthrosporea, 
 
 The former consists of but one genus, sporobacterium, which has four 
 recognized species. 
 
 The latter (Arthrosporea) has two genera, bacterium and micrococcus, 
 
THE ATTRACTION OF ATOMS. 159 
 
 The genus bacterium is the more numerous, having at least twenty-five 
 distinct species, among which are the bacteria found in the human body 
 in the diseases of consumption, pneumonia, cholera, diarrhoea, typhoid 
 fever, and glanders; also the forms which are found in foul ponds and 
 sewage : it also includes the vinegar ferment, which converts ethylic 
 alcohol into acetic acid. 
 
 FIG. in. Mycoderma aceti, or mother of vinegar, as seen (enlarged 500 diameters) 
 under the microscope. 
 
 The genus micrococcus has eight species now enumerated, among which 
 are the bacteria of small-pox, erysipelas, scarlet fever, and others. 
 
 Growth of Microbes. The cells of micro-organisms 
 are capable of extremely rapid multiplication generally 
 
i6o 
 
 THE ATTRACTION OF ATOMS. 
 
 by fission or some modification of it. Sometimes fission 
 takes a course whereby forms like spores are produced. 
 By such processes a single bacterium cell may multiply 
 in twenty-four hours to more than a billion individuals 
 like itself. 
 
 ' 1 ' 1 ""'" ' ' : " ' .' ' "' ' 
 
 FIG. 112. Apparatus used in Pasteur's laboratory in Paris; boiler (sterilizer) heated 
 by gas, for destroying microbes by steam under high pressure; oven for culture of certain 
 microbes at a definite temperature; oven for sterilizing tubes and flasks by hot air. 
 
 The number of microbes in some kinds of food is very 
 great. Thus, it has been computed that in the case of 
 certain kinds of Swiss cheese one pound of the article 
 possesses a microbian population greater than the human 
 population of the terrestrial globe, 
 
FIG. 113. Large oven used in Pasteur's laboratory for the culture of certain selected 
 microbes. (It is provided with an automatic gas regulator.) 
 
162 
 
 THE ATTRACTION OF ATOMS. 
 
 Micro-organisms are influenced in their growth by prevailing conditions; 
 some conditions are highly favorable, some unfavorable. 
 
 I. They need a certain temperatiire, varying in particular cases (100 
 F. is generally favorable). They are best killed by very high temperatures 
 
 FIG. 114. Sterilizing apparatus. It is for the purpose of destroying bacteria in such 
 substances as may be placed in the flasks. The apparatus consists essentially of a strong 
 vessel, made tight, and provided with a safety valve and steam gauge. It has jackets to 
 prevent loss of heat and condensation of steam. Upon applying a strong gas flame 
 underneath the vessel, the water in the vessel is raised to the boiling-point. High pres- 
 sure steam is produced. The flasks are therefore subjected to a temperature sufficient to 
 destroy any microbes in them. The flasks are then withdrawn all at once by means of 
 the caster. The plugs of sterilized cotton in the necks of the flasks prevent subsequent 
 access of microbes from the air. 
 
 (212 F. and upward). Some, however, succumb at even moderately low 
 temperatures (50 F. and downward). 
 
THE ATTRACTION OF ATOMS. 
 
 163 
 
 2. They flourish best in presence of moisture. A small amount will 
 serve. Moderately dry dust containing them, when put under proper con- 
 ditions with moisture, shows life by the multiplication of the varieties 
 present. Bui- thorough desiccation is fatal. 
 
 FIG. 115. Louis Pasteur, celebrated for his studies in the diseases produced by 
 micro-organisms. 
 
 3. Some of them need atmospheric air; some flourish best in absence 
 of it. 
 
 As bacteria are destitute of chlorophyll, they do not obtain nutrition by 
 decomposing carbon dioxide of the air under influence of sunlight. 
 
 4. They must have suitable pabulum. Farinaceous matters are good; 
 albuminoid or other nitrogeneous substances are very favorable; meat 
 extract is excellent. 
 
THE ATTRACTION OF ATOMS. 
 
 Some bacteria attack only dead organized tissues; others attack and 
 disorganize tissues of living beings. 
 
 Some obtain nitrogen from as simple compounds as ammonia gas 
 
 
 a l.irge 
 soi iltfer 
 
 i] n 
 
 (' i: i c rc 
 
 tHi 
 
 1:1 1 
 
 Ci tm- 
 
 
1 66 THE ATTRACTION OF ATOMS. 
 
 enable them to produce, in the aggregate, great quanti- 
 ties of such compounds as they are able to form. 
 
 The general tendencies of bacterial growth involve a 
 breaking-down of complex molecules into somewhat sim- 
 pler ones, although this is not the invariable result. 
 
 Microbes produce certain special compounds as has 
 already been suggested. 
 
 1. They accomplish the following important transformations : 
 Cane sugar to ethyl alcohol ; 
 
 Glycerin to ethyl alcohol, and thence to butyl alcohol ; 
 
 Cane sugar to gum or mannite ; 
 
 Grape sugar or milk sugar or glycerin to lactic acid and butyric acid; 
 
 Urea to ammonic carbonate ; 
 
 Hippuric acid to benzoic acid ; 
 
 Albumens to ptomaines ; 
 
 Nitrogenous matters to nitrates. 
 
 2. They produce certain groups of substances possessed of general 
 properties, of which the following may be noted : 
 
 Substances having marked agreeable or disagreeable odors; 
 
 Substances having brilliant colors; 
 
 Substances called, in general, ptomaines having eminently poison- 
 ous properties, as tyrotoxicon in milk and cheese. 
 
 The ptomaines just referred to are alkaloids of a highly poisonous char- 
 acter, generally resulting from a morbid decomposition of albuminoids under 
 the influence of microbes. 
 
 The leucomaines are analogous poisonous alkaloids, but they are pro- 
 duced by the ordinary physiological processes of the higher animals, and 
 thus are capable of being decomposed and excreted under the normal 
 action of the appropriate organs, of which, apparently, the liver is the 
 most effective. 
 
 It was formerly held that the morbid conditions recognized in animals 
 affected by certain contagious and infectious diseases were due directly to 
 the specific microbes present. At present the abnormal action of the organ- 
 ism is referred rather to the poisonous ptomaines produced by the microbes. 
 
 Usefulness of Bacteria in the Organic World. One of 
 the most marked features in the life-processes of the 
 
THE ATTRACTION OF ATOMS. l6/ 
 
 higher animals and plants is the circulation of certain 
 atoms. That is, there seems to be a definite and rather 
 small stock of certain useful elements, like nitrogen, 
 phosphorus (and to these may be added, with less force, 
 potassium and even carbon), which are in a continual 
 state of transfer. This " stock " is absorbed from the 
 soil by living plants ; it is then absorbed by living 
 animals. The bacteria assist the process of animal 
 digestion, whereby the vegetable molecules are altered. 
 Upon the death of animals the current stock returns to 
 the soil, thence to be employed by a new set of growing 
 plants, and later by a new population of living animals. 
 Without microbes the "stock" would be withdrawn 
 from circulation in living animals or vegetables, and 
 locked up inactive in dead bodies. Upon the death of 
 the animal, the microbes set up those processes of 
 putrefaction and decay, whereby the stable molecules in 
 the dead bodies become available for the food of grow- 
 ing plants. 
 
CHAPTER XV. 
 
 THE ATTRACTION OF ATOMS (continued}. 
 MODES OF CHEMICAL ACTION. 
 
 As a result of the operation of chemical affinity, 
 molecules are changed in a variety of ways. 
 
 The following are some of the principal ones : 
 
 I. Elementary or compound molecules may directly 
 combine : 
 
 Zn 
 
 r- Ci 2 
 
 ZnCl 2 
 
 One atom of 
 
 One molecule of 
 
 One molecule of 
 
 Zinc, 
 
 Chlorine, 
 
 Zinc chloride, 
 
 65 
 
 7 1 
 
 136 
 
 parts by weight. 
 
 parts by weight. 
 
 parts by weight. 
 
 J 36 136 
 
 2. An element or group of elements may displace 
 another element or group : 
 
 2HC1 + Zn ZnCl, + H 2 
 
 Two molecules of One atom of One molecule of One molecule of 
 
 Hydrochloric acid, Zinc, Zinc chloride, Hydrogen, 
 
 73 65 136 2 
 
 parts by weight. parts by weight. parts by weight. parts by weight. 
 
 138 138 
 
 But in some cases the displacement may be by gradual 
 stages. Thus marsh gas (CH 4 ) may have its hydrogen 
 replaced by chlorine, atom by atom, until all is removed. 
 
 168 
 
THE ATTRACTION OF ATOMS. 169 
 
 Thus the following compounds may be progressively 
 formed : 
 
 CH 3 C1, 
 
 CH 2 C1 2 , 
 
 CHC1 3 , 
 
 CC1 4 . 
 
 3. An element or group of elements in one molecule 
 may exchange places with an element or group of ele- 
 ments in another molecule : 
 
 CuSO 4 + 
 
 Ba(NO,) a 
 
 BaSO 4 
 
 f Cu(N0 3 ) 2 
 
 One molecule of 
 
 One molecule of 
 
 One molecule of 
 
 One molecule of 
 
 Cupric sulphate, 
 
 Baric nitrate, 
 
 Baric sulphate, 
 
 Cupric nitrate, 
 
 159 
 
 261 
 
 233 
 
 187 
 
 parts by weight. 
 
 parts by weight. 
 
 parts by weight. 
 
 parts by weight. 
 
 420 420 
 
 4. There may be a rearrangement of elements or 
 groups of elements within single molecules of a sub- 
 stance : 
 
 (NH 4 )O(CN) changes spontaneously into N 2 H 4 (CO) 
 
 One molecule of One molecule of 
 Ammonic cyanate, Urea, 
 
 60 60 
 
 parts by weight. parts by weight. 
 
 ~6o 60 
 
 5. There may be a direct decomposition of a certain 
 molecule into others of a different kind : 
 
 2 H.,O may be decomposed into 2 H 2 + O 2 
 
 Two molecules of Two molecules of One molecule of 
 
 Water, Hydrogen, Oxygen, 
 
 36 4 32 
 
 parts by weight. parts by weight. parts by weight. 
 
I/O THE ATTRACTION OF ATOMS. 
 
 The Sphere of Chemical Action. The sphere of 
 chemical action is evidently that of the individual mole- 
 cule ; as a result of chemical change, molecules change 
 their components. This sphere is a very limited one 
 when looked at with reference to the minuteness of a 
 single molecule. It is one of very wide range when it 
 is remembered that all material substances are made up 
 of molecules, and that the character of the molecules 
 determines the character of the mass. Chemical change, 
 therefore, is most fundamental, altering substances in 
 their ultimate recesses. Changing the molecules in 
 which the identity of substances reside, it changes the 
 identity of masses themselves. Thus all the kingdoms 
 of nature owe to chemical action the variety of sub- 
 stances produced in their normal or abnormal growth, 
 while geologic and cosmic changes involve chemical 
 action and reaction on the largest scale. 
 
 The Results of Chemical Action. The effects pro- 
 duced by chemical change are recognized as of the most 
 striking kind ; and this is true both in natural and in 
 artificial processes. t 
 
 Some of the principal effects noticed are changes of 
 physical condition, as a substance originally solid or 
 liquid or gaseous, at a given temperature, may change 
 to another of these conditions ; changes of color, odor, 
 taste, or other physiologic or toxic effect ; change of 
 volume: thus sometimes chemical action draws atoms 
 closer together. As already stated (p. 78), two volumes 
 of hydrogen gas and one volume of oxygen gas, when 
 chemically actuated, unite to form a new substance 
 (water- vapor), occupying only two volumes altogether. 
 
THE ATTRACTION OF ATOMS. I/I 
 
 Sometimes there is no reduction, but rather expansion : 
 thus gunpowder, a solid, experiences chemical change 
 when slightly heated, and produces an immense volume 
 of gas. Sometimes neither expansion nor contraction 
 takes place. 
 
 Sometimes chemical affinity produces such violent or 
 bizarre effects that there can be no question that it is in 
 active exercise. In other cases, where two or more sub- 
 stances might be supposed to undergo chemical change, 
 the evidences are so slight as to make the very exist- 
 ence of the chemical action difficult to substantiate. 
 
 Among all these various, and in many cases inexplica- 
 ble results, two principles are constantly recognized, 
 the indestructibility of matter and the indestructibility 
 of force. 
 
 General Laws of Chemical Action. The following are 
 a few general laws relating to the results of chemical 
 action : 
 
 The Law of Insolubility. When there are brought 
 together solutions that contain several elements such as 
 would, if united, form a compound that is ordinarily 
 insoluble in the liquid present, this insoluble compound 
 will usually be formed and will appear as a precipitate. 
 
 This law is subject to certain limitations, yet it is of 
 sufficiently wide application to sometimes enable the 
 chemist to predict the formation of a given substance 
 that may never have been produced before in that par- 
 ticular way. 
 
 This law finds illustration in the following equations : 
 
 HC1 +Ag(N0 3 )=AgCl-rH(N0 3 ); 
 NaCl + Ag(N0 3 ) = AgCl + Na(NO 3 ) ; 
 XC1 +AffY =AffCl + XY. 
 
172 THE ATTRACTION OF ATOMS. 
 
 The Law of Volatility. When there are brought 
 together substances whose reaction can produce a gas 
 or a substance that is volatile at the temperature of the 
 experiment, such volatile or gaseous substance generally 
 will be formed, and will be liberated with effervescence. 
 
 The Indestructibility of Matter. The amounts of 
 weighable matter taking part in a chemical change are 
 definite ; and the sum of the weights of the products is 
 always equal to the sum of the weights of the factors. 
 
 The Indestructibility of Energy. The amounts of en- 
 ergy involved in chemical changes are definite. When 
 the elements of a chemical compound are drawn apart, 
 a certain amount of energy is usually absorbed. When 
 the same elements come together to form a compound, 
 a certain amount of energy is evolved. Now the amounts 
 of energy in these two cases are equal. 
 
 It is true that the energy absorbed or evolved in such 
 cases may vary in kind. It may be the energy of heat 
 or that of light or that of electricity in some of its modi- 
 fications, or it may be some combination of these. But, 
 in any event, the facts sustain the doctrine called the 
 conservation of energy, which involves the view that it 
 is impossible for us to create or to destroy energy, just 
 as it is impossible for us to create or to destroy matter. 
 All that we can do is to change the particular form 
 which the energy shall, for the time being, assume. 
 
 Criteria of Chemical Action. It has already been 
 stated as a fundamental principle that natural phe- 
 nomena arrange themselves in series in which the 
 individual members differ from their immediate neigh- 
 bors by minute and sometimes almost indistinguishable 
 
THE ATTRACTION OF ATOMS. 1/3 
 
 details. Thus it may be expected that the drawing of 
 distinct lines of division will often be impracticable. 
 This statement is applicable to those different kinds 
 of action called chemical action and physical action. 
 While a multitude of operations are readily recognized 
 as manifesting distinct evidences of chemical change, 
 and at a distance from these may be produced changes 
 that are referable distinctly to cohesive and physical 
 forces, there are between these extremes phenomena in 
 which the definite signs of the one or the other kind of 
 action become less and less marked, or entirely fade 
 away. 
 
 Thus, on a given occasion an observer may be reason- 
 ably in doubt whether certain intermingled or adjacent 
 substances undergo or do not undergo what is properly 
 described as chemical change. 
 
 It is therefore desirable to consider systematically the 
 evidences upon which a decision must be reached. The 
 following may be accepted as a guiding principle : Gain 
 a thorough acquaintance with all the characteristics that 
 generally attend undoubted chemical changes ; then, in 
 a doubtful case, observe whether one or several single 
 characteristics are distinctly evident, and whether one 
 or several characteristics can be recognized, if only in a 
 feeble and rudimentary degree. 
 
 The following are the chief evidences of well-marked chemical action : 
 
 (<z) The generation of certain physical forces, as heat, light, electricity. 
 These are important indications, since appliances have been produced 
 capable of detecting very minute amounts of heat and of electrical dis- 
 turbance. 
 
 (/;) The production of new molecules. These may be recognized as 
 follows : 
 
 I. They possess new chemical composition; that is, they contain either 
 
174 THE ATTRACTION OF ATOMS. 
 
 new elements or else they contain the original elements in new propor- 
 tions by weight (and in case of gaseous elementary substances, by volume 
 as well). This evidence involves the important law of definite proportions. 
 (See p. 72.) 
 
 2. They possess properties differing more or less distinctly from those 
 of the original elementary or compound molecules which, in the case in 
 question, are supposed to have been subject to chemical change. The 
 changes to be looked for are in the following features : 
 
 The color; degree of opacity; refracting power for light; 
 
 The taste; physiologic and toxic properties; 
 
 The conducting power for heat, light, and electricity; 
 
 The density of the substance in the solid, liquid, or gaseous condition; 
 
 The melting and boiling points; 
 
 The degree of solubility in solvents. 
 
CHAPTER XVI. 
 
 THE ATTRACTION OF ATOMS (continued}. 
 THERMO-CHEMISTRY. 
 
 Introduction. It has long been recognized that many 
 chemical changes give rise to an evolution of heat. 
 Sometimes the amount evolved is very large. Indeed, 
 practically all man's artificial heat is the product of 
 chemical combination. 
 
 It is also well known that the amount of heat evolved 
 by the combustion or other chemical reaction of a 
 certain weight of one substance is very different from 
 that given out by a corresponding chemical reaction of 
 an equal weight of another substance. 
 
 At a given moment of time any material substance or 
 thing possesses certain internal and external relations 
 of parts, and contents of heat and other forces, the sum 
 total of which may be called its condition. Now any 
 change whatever of its condition implies either some 
 alteration of the arrangement of its parts externally or 
 internally, or some alteration by increase or decrease of 
 the amount of its forces. In either case the change 
 requires for its initiation the application of some ex- 
 ternal force, which may, perhaps, be small in amount. 
 But the change, when once it has been instituted, either 
 absorbs or liberates a large amount of energy of some 
 kind, generally that of heat. 
 
 175 
 
176 THE ATTRACTION OF ATOMS. 
 
 Reactions of the sort referred to are sometimes classed as direct, or 
 exothermic (those in which heat is evolved), and indirect, or endothermic 
 (those in which heat is absorbed). 
 
 It is worthy of note that the amount of heat involved by the union of 
 substances in an exothermic reaction is exactly equal to the amount of heat 
 that would be required to subsequently decompose the substance produced 
 by such operation. 
 
 The facts here stated seem to be merely forms of the general law of 
 nature, that changes in the arrangement of material substances cannot be 
 accomplished without the aid of force. In other words, to bring about a 
 certain change, heat or other force must be supplied, and the energy 
 appears to be somehow taken in by the compound. In the reversal of the 
 same operation the energy taken in is given out : then force is evolved, 
 as heat or in some other form, equivalent in amount to that originally 
 absorbed. 
 
 Within a few years efforts have been made to learn and state with 
 exactness the amounts of heat afforded by all the more prominent chemical 
 tions. 
 
 <The exact studies of Alexander Naumann, Julius Thomsen, Marcellin 
 Berthelot, and others rank among the classical scientific researches of this 
 era. The whole subject has also been carefully reviewed in recent treatises 
 by Pattison Muir and others. 
 
 Laws of Thermo-Dynamics. First Law. There is 
 a definite quantitative relation between the amount of 
 work done and the quantity of heat produced or 
 destroyed. 
 
 Second Law. If all the heat in any body or system 
 of bodies is at the same temperature, no mechanical 
 work can be obtained from that body or system except 
 by bringing it into contact with another body at a lower 
 temperature. 
 
 The important principle stated by Clerk Maxwell may 
 with propriety be presented here : 
 
 " The total energy of any material system is a quan- 
 tity which can neither be increased nor diminished by 
 any action between parts of the system, though it may 
 
THE ATTRACTION OF ATOMS. I// 
 
 be transformed into any of the forms of which energy 
 is susceptible." 
 
 The Law of Maximum Work. Berthelot States this 
 law as follows : 
 
 " Every chemical change accomplished without the 
 addition of energy from without tends to the formation 
 of that body or system of bodies, the production of which 
 is accompanied by the evolution of the maximum quan- 
 tity of heat." 
 
 Muir makes a critical examination of this statement, and thinks that 
 Berthelot has fallen into error. Thus consider at the outset the suggestion 
 that any chemical change can be accomplished without the addition of 
 energy from without. Initial outward influence seems necessary to change 
 the condition of any portions of matter not in actual process of change. 
 
 Thermal Units. The amount of heat absorbed or 
 evolved in chemical operations is usually represented in 
 thermal units called calories. 
 
 Thomsen uses the water calory, and his unit of heat 
 is the amount of heat necessary to raise one gramme or 
 one kilogramme of water through one degree measured 
 in the neighborhood of the eighteenth to the twentieth 
 degree of the centigrade thermometer. 
 
 Berthelot prefers to use water at zero degrees centigrade. 
 
 In some cases an ice calory is used, the heat being 
 measured by the amount of ice that may be changed 
 from the solid to the liquid form without rise of tempera- 
 ture (one ice calory is equal to 80.025 water calories). 
 
 It should be noted, also, that sometimes the so-called 
 large calories (C) are used, and sometimes small calo- 
 ries (c). The large calory relates to the kilogramme of 
 water, the small calory to the gramme of water. 
 
1/8 
 
 THE ATTRACTION OF ATOMS. 
 
 The heat of combustion of an element is sometimes 
 defined as the amount of heat evolved by the perfect 
 combustion of one gramme or one kilogramme of the 
 substance. 
 
 Sometimes the heat of combustion means the quantity of heat produced 
 by the chemical change of a number of grammes represented in a certain 
 reaction of the substance. 
 
 In accordance with the first definition, 
 the heat of combustion of hydrogen is the 
 amount of heat produced by burning one 
 gramme of it. In accordance with the 
 second definition, the heat of combustion 
 of hydrogen is the amount of heat pro- 
 duced by burning two grammes of it 
 (representing one molecule of hydrogen) 
 in accordance with the equation 
 
 H 2 + O = H 2 O. 
 
 Calorimeters. In the ex- 
 periments of thermo-chemistry 
 several different kinds of ap- 
 pliances have to be used. 
 
 The forms of chemical change 
 are so varied as to the sub- 
 stances taking part, as to the 
 substances ultimately produced, 
 and as to the conditions attend- 
 ing the progress from the one 
 set of substances to the other, 
 that a thermal study of these 
 of apparatus specially adapted 
 
 calorimeters are : First, an inte- 
 e.g. of glass or of platinum, in 
 
 FIG. 117. Simple form of calor- 
 imeter. The vessel E represents a 
 box placed on a felt or wooden sup- 
 port, and provided in its interior 
 with a non-conducting wreath for 
 thermal insolation. The thermom- 
 eter registers the heat generated in 
 the process of a chemical reaction. 
 
 changes demands forms 
 to the different cases. 
 
 The essential parts of 
 rior vessel of some sort, 
 
THE ATTRACTION OF ATOMS. 
 
 179 
 
 which the chemical operation proceeds. Second, one or 
 more delicate and accurate thermometers, to be used in 
 
 FIG. 118. One of Berthelot's calorimeters, constructed for observing the heat gen- 
 erated by mixing chemical substances in solution. The outer jackets are for the purpose 
 of preventing heat from passing from the room into the apparatus or from the apparatus 
 outward. The beaker in which the experiment is performed is placed upon pointed sup- 
 ports for thermal insolation. A stirrer, a, is provided to diffuse throughout the solution 
 the heat generated. A delicate thermometer suspended in the solution registers the 
 temperature. 
 
 measuring the rise of temperature of the 
 TJiird, several protecting coatings or chan 
 
 operation, 
 chambers, such 
 
i8o 
 
 THE ATTRACTION OF ATOMb. 
 
 as vessels of water or of air, covered with felt. FourtJi, 
 mechanical stirrers to agitate the water of the outer 
 chamber so that tHe heat absorbed may be evenly dis- 
 
 FIG. 119. Calorimeter devised by Berthelot for experiments upon the union of cer- 
 tain gases. The products of chemical union are collected in the spiral S and the bulb R. 
 The heat generated is absorbed in the liquid surrounding the spiral. The thermometer 
 registers the heat produced. The vessel containing the spiral is thermally isolated by 
 the several jackets placed about it. 
 
THE ATTRACTION OF ATOMS. l8l 
 
 tributed. FiftJi, in some cases the room in which the 
 experiments are performed is most carefully maintained 
 at a uniform temperature. Sixth, sometimes the ex- 
 periments have a duration of several days, so that the 
 water employed as a protective coating (and the air 
 of the room also) may acquire a uniform tempera- 
 ture. 
 
 I. One general form of apparatus is suited to experiments on the 
 relations of solids with water or other liquids. Under this head come such 
 cases as the solution of many salts in water, the reaction between water 
 solutions of acids and water solutions of alkalies, and the like. 
 
 II. Another kind of apparatus is necessary in the study of combustion 
 processes. Such are those in which solids, liquids, or gases are burned in 
 oxygen gas, or are made to unite by a process analogous to oxygen com- 
 bustion with sulphur, chlorine, bromine, or other substance. 
 
 III. Another form may be necessary in the study of the violent changes 
 involved in the action of chemical agents on certain organic matters, as, for 
 example, of nitric acid on sugar. 
 
 IV. Some chemical operations may demand apparatus for their indi- 
 vidual treatment, as, for instance, when two gases act on each other at 
 ordinary temperature : the union of nitrogen dioxide and oxygen affords an 
 illustration. Again, the action of certain substances on others proceeds 
 slowly, and special apparatus may be needed to deal with such changes : 
 the action of oxygen on a solution of sodic thiosulphate (sodic hypo- 
 sulphite) affords an illustration. 
 
 Difficulties Experienced. Exact determinations of the 
 amounts of heat evolved or absorbed by chemical opera- 
 tions are attended with certain difficulties : 
 
 FIRST. There are the mechanical difficulties associ- 
 ated with the construction and use of the apparatus 
 required. 
 
 SECOND. In certain distinctly connected series of 
 chemical changes heat is both absorbed and evolved. 
 The computation is thereby complicated. 
 
1 82 THE ATTRACTION OF ATOMS. 
 
 The chemical union of hydrogen and chlorine would be a simple one if 
 it were properly expressed by the equation H + Cl = HC1. In this expres- 
 sion a simple and direct union of two atoms is described. 
 
 But the true change taking place when hydrogen and chlorine unite is 
 believed to be expressed by the equation H ? -f C1 2 = 2 HC1. This expres- 
 sion describes something more complicated than the foregoing. It ex 
 presses at least three operations; viz. a decomposition of a molecule of 
 hydrogen, a decomposition of a molecule of chlorine, the union of an atom 
 of hydrogen with an atom of chlorine (this particular operation being 
 twice repeated). Now it may be safely assumed that the decomposition of 
 the hydrogen molecule and the decomposition of the chlorine molecule 
 both absorb heat, while the union of the atoms of hydrogen with the 
 atoms of chlorine evolves heat. Evidently, then, the total amount of heat 
 observed in such an operation represents a remainder equal to the excess 
 of the evolved over the absorbed heat. 
 
 H'=H- (h'+h). 
 
 H' represents net observed evolution of heat. 
 
 .//represents amount of heat evolved by actual chemical union of the 
 substances. 
 
 h represents amount of heat absorbed by decomposition of one of the 
 molecules involved. 
 
 h f represents amount of heat absorbed by decomposition of the other 
 molecule involved. 
 
 THIRD. In many cases of chemical change the heat 
 actually generated may be partly expended in raising 
 the temperature of the solids, liquids, or gases produced. 
 Of course any such rise of temperature as is observed in 
 the experiment must therefore be subjected to correction 
 because of the different specific heats of the substances 
 present. Thus the experimenter in thermo-chemistry 
 must make a careful study of specific heat, and the 
 results of thermo-chemistry cannot be relied upon unless 
 correct results in specific heat are employed. 
 
 FOURTH. In many cases the final result is complicated 
 because the substances first produced dissolve in the 
 water present, absorbing or evolving heat by this opera- 
 tion. 
 
THE ATTRACTION OF ATOMS. 183 
 
 Range of the Subject. - - Evidently the field of thermo- 
 chemistry is a wide one. 
 
 As a method of observation it has been brought to 
 bear upon many diverse forms of action. Thus it has 
 been applied to the study of the following classes of 
 subjects : 
 
 FIRST. The amounts of heat of formation of certain 
 binary compounds, as the compounds of hydrogen, 
 chlorine, bromine, iodine, sulphur, oxygen, and other 
 elements by various methods. These include energetic 
 changes in which various substances, elementary or 
 compound, unite with oxygen or other elements by a 
 process analogous to combustion. 
 
 SECOND. The heats afforded by neutralization of 
 alkalies, like soda, potash, and others by acids. 
 
 THIRD. The heats of solution of solids, the dilution of 
 liquids, and hydration generally. 
 
 FOURTH. The phenomena of dissociation, and the so- 
 called abnormal vapor densities. 
 
 FIFTH. Certain allotropic and isomeric substances. 
 Thus there has been made an examination of the differ- 
 ences in the thermal value of the combustion of different 
 kinds of sulphur, different kinds of phosphorus, different 
 kinds of carbon, different kinds of silicon, as well as of 
 different isomeric compounds of the organic series, with 
 a view of detecting, if possible, the differences of molec- 
 ular structure of these elements and compounds. 
 
 Results. While it is not possible as yet to state 
 many distinct laws as to the heat of chemical union, it 
 has been noticed that certain operations of a similar 
 chemical type involve approximately equal amounts of 
 
184 THE ATTRACTION OF ATOMS. 
 
 heat, even when the particular substances taking part 
 are all different. 
 
 The amount of heat produced when certain substances are dissolved in 
 water affords important information as to the condition of such substances 
 in solution. Thus it is well known that upon mingling sulphuric anhydride 
 (SO 3 ) with water, heat is produced. A critical study of this operation 
 shows plainly that a large proportion of the heat afforded is produced by 
 the addition of the first molecule of water, and a large proportion is also 
 produced by the addition of the second molecule of water. Muir says that 
 there can be little or no doubt that the various results point to the formation 
 in aqueous solutions of sulphuric anhydride (SO 3 ), of one definite hydrate 
 having the formula H 2 SO 4 , and not of other hydrates. 
 
 The same general results are obtained by the study of the solution of 
 other acids. 
 
 " The difference commonly expressed in the terms water of constitution 
 and water of crystallization is evidently, so far as may be judged from 
 thermo-chemical data, strictly a difference of degree and not of kind." 
 
 A few examples of the numerical results may be pre- 
 sented here. 1 
 
 (a) Union of Elements. 
 
 H +C1=HC1+ 22,000 cal. 
 H + Br = HBr + 8,440 " 
 H +1 -HI - 6,040 " 
 H 2 +O = H 2 O+68,36o " 
 H 2 + S =H 2 S + 4,740 " 
 S + O 2 = SO 2 +71,080 " 
 
 (d) Union of Compounds with Water. 
 
 H 2 SO 4 + H 2 O = (H 2 O -H 2 SO 4 ) +6,379 cal. 
 
 KOH - 3 H 2 O + 197 H 2 O = (KOH - 200 H 2 O) + 2,75 1 " 
 
 1 Muir, Elements of Thermal Chemistry. 
 
THE ATTRACTION OF ATOMS. 185 
 
 (c) Union of Acids and Bases. 
 
 2 HC1 + Na 2 O = 2 NaCl + H 2 O + 27,480 cal. 
 
 2 HI + Na 2 O = 2NaI + H 2 O + 27,360 " 
 
 In this last set of equations the similarity of the number of calories 
 evolved at once suggests similarity in the operations in question. This 
 similarity is noticeable in many other cases among elements and com- 
 pounds of the same family group. 
 
 Indeed, Thomsen classifies the acids in a general way into sets some- 
 what as follows : 
 
 FIRST. Acids whose heats of neutralization are about 20,000 cal. 
 Examples are 
 
 Nitrous acid ............. HNO 2 , 
 
 Hypochlorous acid .......... HC1O. 
 
 Carbonic acid ............ H 2 CO 3 , 
 
 Metaboric acid ............ H 2 B 2 O V 
 
 SECOND. Acids whose heats of neutralization are about 25,000 cal. 
 Examples are 
 
 Chromic acid ............ H 2 CrO 4 , 
 
 Succinic acid ............ C 4 H 6 O 5 . 
 
 THIRD. Acids whose heats of neutralization are about 27,000 cal. 
 Examples are 
 
 Hydrochloric acid ........... HC1, 
 
 Hydrobromic acid ........... HBr, 
 
 Hydriodic acid ............ HI, 
 
 Nitric acid ............. HNO 3 , 
 
 Chloric acid ............. HC1O 3 , 
 
 Bromic acid ............. HBrO 3 , 
 
 lodic acid ............. HIO 3 , 
 
 Formic acid ..... ........ HOCHO, 
 
 Acetic acid ............. HOC 2 H 3 O. 
 
 Most of the acids examined by Thomsen belong to this group. 
 
1 86 THE ATTRACTION OF ATOMS. 
 
 FOURTH. Acids whose heats of neutralization are greater than 27,000 
 cal. (generally from 28,000 to 32,500 cal.). 
 Examples are 
 
 Hydrofluoric acid HF, 
 
 Sulphurous acid H 2 SO 3 , 
 
 Sulphuric acid H 2 SO 4 , 
 
 Selenic acid H 2 SeO 4 , 
 
 Metaphosphoric acid HPO 3 , 
 
 Phosphorous acid H 3 PO 3 , 
 
 Oxalic acid H 2 O 2 (C 2 O 2 ). 
 
CHAPTER XVII. 
 
 THE ATTRACTION OF ATOMS (continued}. 
 THEORIES OF THE NATURE OF CHEMICAL ATTRACTION. 
 
 THE force whatever may be its nature that leads 
 substances to undergo chemical changes is often called 
 chemical affinity. This force is capable of overcoming 
 a certain amount of resistance ; again, a certain amount 
 of force is necessary to undo its work. It also bears 
 definite quantitative relations to other forces, such as 
 heat, light, and electricity : definite amounts of chemical 
 energy are necessary to the production of unit amounts 
 any one of them, and definite amounts of one of them are 
 necessary to the production of a unit of chemical energy. 
 
 A very large amount of information has been secured 
 as to its ways of working, etc., but no entirely satisfac- 
 tory explanation of its nature has yet been offered. 
 Views as to the nature of chemical attraction have 
 changed from time to time as one phase of thought or 
 another has been dominant. They have in a marked 
 manner reflected the spirit of the time as aroused by 
 some great discovery. 
 
 i. Early Views. The general opinion of the alche 
 mists was that somehow or other substances of like kind 
 or origin tend to combine. 
 
 This view is evidently inadequate the more, in that 
 
 187 
 
1 88 THE ATTRACTION OF ATOMS. 
 
 it is now known that the most active chemical unions are 
 between substances that are in many ways most unlike. 
 
 2. Newton's Theory. Newton's discovery of the uni- 
 versal attraction of masses was naturally and easily ex- 
 tended to the minute particles of matter, and chemical 
 attraction was then held to be one form of general 
 attraction. This view was advanced by Newton, and 
 later was supported by Berthollet. These philosophers 
 considered, however, that the universal tendency of 
 bodies towards each other was somewhat modified by 
 the minuteness of the particles between which chemical 
 changes are capable of taking place. 
 
 3. The Theory of a Special Force. Chemical attrac- 
 tion has been viewed as a unique kind of force. This 
 theory, widely accepted during the past hundred years, 
 is that atoms and molecules are gifted by the Creator 
 with certain specific tendencies to unite, and that with 
 certain fixed degrees of force. This supposes the pos- 
 session by atoms of an inherent property called chemical 
 affinity. The name applies in a general way to an un- 
 explained energy somehow residing in the atom. 
 
 4. The Electrical Theory. Chemical attraction has 
 been thought to be a phase of electrical energy. Davy, 
 Dumas, Becquerel, Ampere, Berzelius, Gmelin, and 
 others have held some form of electrical theory of 
 chemical action. The general notion has been that 
 atoms and molecules are naturally or may be artificially 
 charged with varying amounts and kinds of electricity. 
 By reason of their condition in this respect they are 
 mutually attracted or repelled, and with varying degrees 
 
THE ATTRACTION OF ATOMS. 
 
 189 
 
 of force, somewhat as electrified masses of matter are. 
 This theory derives support from certain important and 
 well-defined facts. For bodies artificially electrified 
 often manifest thereby stronger chemical attractions ; 
 
 FIG. 120. Sir Humphry Davy, Bart. Born in Penzance, England, December 17 
 1778; died in Geneva, Switzerland, May 29, 1829. 
 
 " Davy, when not yet thirty-two years old, occupied, in the opinion of all those who 
 could judge of such labors, the first rank among the chemists of this or any other age." 
 
 again, chemical action yields as a product a definite 
 quantity of electricity. 
 
 5. The Theory of Motion. Williamson's theory is 
 that chemical attraction is a form of motion. This view 
 
FIG. I2i. Andre"-Marie Ampere. Born at Lyons, January 20, 1775; died at Marseilles 
 June 10, 1836. (The portrait is from a statue erected at Lyons.) 
 
THE ATTRACTION OF ATOMS. IQI 
 
 accepts the modern idea of constant atomic and molec- 
 ular movement. It suggests that in atoms of all or- 
 dinary molecules a rapid but regulated interchange is 
 going on, so that in certain cases a given atom may be 
 continually moving from one molecule into another. A 
 transfer of this kind could not be easily detected among 
 molecules of the same kind, but among molecules of 
 different kinds it would effect just such changes as are 
 recognized in many chemical operations. Thus two or 
 more molecules of hydrochloric acid (HC1,HC1) might 
 make an interchange of atoms without any easily appre- 
 ciable alteration of properties of the substance. But 
 when a molecule of argentic nitrate and a molecule of 
 hydrochloric acid are brought into contact, the inter- 
 change might produce two new molecules possessing 
 properties easily recognized as different from those of 
 the original two, 
 
 Ag(NOs) + HC1 AgCl + HNO 3 
 
 Argentic nitrate + Hydrochloric acid = Argentic chloride + Nitric acid. 
 
 This theory, moreover, not only explains such simple 
 operations as that just referred to ; it is capable of 
 affording an adequate reason for certain of the more 
 obscure phenomena of chemical change. 
 
 Comment on these Theories. Chemical attraction 
 must be looked upon as a force having in it something 
 of general powers and something of highly specialized 
 ones. Thus any theory of it ought to include the notion 
 that all substances tend to come toward each other, for 
 apparently all chemical substances will combine there 
 is merely a difference in the strength of this tendency 
 in different cases. And so if masses of matter gravitate 
 
THE ATTRACTION OF ATOMS. 
 
 toward each other, why not molecules and even atoms ? 
 Where shall be drawn the line at which gravitative 
 force ceases ? 
 
 Further, it must be admitted that substances are found to possess 
 certain natural qualities inexplicable ones. What is this but a declara- 
 tion that they are gifted, at their original creation, with specific powers? 
 
 Again, the close connection of electricity with chemical force is 
 indubitable. 
 
 The modern notion of constant movement of all forms of matter applies 
 with peculiar appropriateness to atoms and molecules, and seems to be 
 inseparable from any idea of so intimate a change as the chemical. 
 
 There is no impropriety in considering chemical attraction as a complex 
 rather than a simple form of force. Certainly the rich variety of its modes 
 of action and of its results must sustain such a view. 
 
 Thus each of the theories stated contains truths. The acute observers 
 and thinkers who have held them could not have been entirely misled. 
 Each theory singly is merely inadequate. Probably in one that is adequate 
 there must be combined the truth included in each of those stated and 
 more, too, as chemistry advances. 
 
 What is wanted, then, is a compact statement, sufficiently comprehen- 
 sive to embrace in harmonious union the various principles known to be 
 involved in chemical change. 
 
CHAPTER XVIII. 
 ATOMIC WEIGHT. 
 
 METHOD OF WORK AND METHOD OF DESCRIPTION. 
 
 Introduction. Whatever theory an individual may 
 hold with respect to the existence of atoms, in the 
 most distinct import of that word, it cannot be reason- 
 ably questioned that substances combine chemically in 
 accordance with certain approximately fixed proportions, 
 and that these proportions may be expressed in tolerably 
 exact numerical form. 
 
 As a matter of fact, chemists assign to each elemen- 
 tary (and compound) substance a certain representative 
 number. Such numbers are certainly combining num- 
 bers. They are probably much more. For elements, 
 they represent at least an approach to atomic weights, 
 and for compounds, at least an approach to molecular 
 weights. They are, in fact, compact, single expres- 
 sions, of the best form now known, embracing at once 
 in harmonious union, weight ratios, volume ratios, vapor 
 densities of elements, vapor densities of compounds, 
 specific heats of elements, specific heats of compounds, 
 substitution powers of elements and compounds, and 
 even other relations besides. 
 
 " It is true it may be questioned whether there is an absolute uniformity 
 in the mass of every ultimate atom of one and the same chemical element. 
 Probably atomic weights merely represent a mean value around which the 
 
 193 
 
194 ATOMIC WEIGHT. 
 
 actual atomic weights of the atoms vary within certain narrow limits. 
 When, therefore, it is said, e.g. that the atomic weight of calcium is 40, 
 the actual fact may well be that whilst the majority of the calcium atoms 
 really have the atomic weight of 40, some are represented by 39.9 or 40.1, 
 a smaller number by 39.8 or 40.2, and so on. The properties which we 
 perceive in any element are thus the mean of a number of atoms differing 
 among themselves very slightly, but still not identical. " * 
 
 In case of numbers accepted as atomic weights, the rule in this discus- 
 sion is as follows : When the number involves a fractional part, this part is 
 so modified that quantities less than .05 are rejected, while quantities equal 
 to or greater than .05 are counted as I of the next higher denomination. 
 
 Practical Importance of Atomic Weights. The num- 
 bers adopted as atomic weights have unquestioned 
 practical value. They serve as a basis for the calcula- 
 tions of the chemist in his analytical processes, and also 
 as a foundation for the work of all the great chemical 
 manufacturing industries of the world. Their value 
 depends in part upon the invariability of the numerical 
 laws of combination through all chemical mutations. 
 
 Labor expended in securing Atomic Weights. The 
 
 numbers adopted have so great an importance that 
 chemists have devoted their highest skill and their most 
 assiduous labor to the exact ascertainment of them. 
 (Examples may be found in the work of Berzelius on 
 many elements, that of Stas on many elements, es- 
 pecially silver, of Crookes on thallium, Mallet on alu- 
 minium and gold, Cooke on antimony, and Rayleigh, 
 Cooke, and others on oxygen.) 
 
 The different elements afford different kinds of information : Thus the 
 atomic weight adopted to-day for one element may be worthy of far greater 
 confidence than that adopted for another. 
 
 1 W. Crookes. 
 
ATOMIC WEIGHT. 
 
 195 
 
 And again, while the atomic weight ultimately accepted in a given case 
 ought to be one which harmonizes with the entire body of chemical and 
 physical knowledge, it must be expected that there should remain a few 
 
 FIG. 122. Jons Jakob Berzelius. Born in East Gothland (in Sweden) , August 20, 1799; 
 died August 7, 1848. 
 
 exceptional cases incapable of immediate explanation. It is acknowledged 
 to be a matter of no slight difficulty to fix upon the atomic weight as dis- 
 tinguished from some simple multiple or fraction of it : in some cases, 
 owing to insufficient data, it is at present impossible. 
 
196 ATOMIC WEIGHT. 
 
 In any event the work demands two distinct kinds 
 of operations : first, the experimental part, involving 
 numerous tests and analyses ; second, a work that is 
 even higher and more difficult ; i.e. the reasoning part 
 the drawing of the proper inferences from the body of 
 experimental facts accumulated. 
 
 Since Dalton's first attempt to determine atomic 
 weights these constants have assumed, little by little, 
 an increasing interest. At present the effort to secure 
 the most exact numerical expressions for them is con- 
 sidered by chemists a work of the highest importance. 
 
 Methods of Determination. In determining the atomic 
 weight of a given element, a connected series of steps 
 must be taken. The following is a brief outline of 
 them : 
 
 First Step. Adopt a suitable unit for the system. 
 
 Second Step. Fix a basis upon which shall be 
 selected the compounds (of the element sought} to be 
 studied. 
 
 Third Step. Proceed to make gravimetric analyses 
 of the selected compounds. From these discover 
 directly a combining number for the element. 
 
 Fourth Step. Make choice of an atomic weight for 
 the element from the various multiples or submultiples 
 of the combining number discovered. In doing this, be 
 guided by certain facts combined and applied in accord- 
 ance with definite principles. 
 
 Thus, learn 
 
 The vapor density of the element ; 
 
 The vapor density of its compounds ; 
 
 The volume composition of the compounds ; 
 
ATOMIC WEIGHT. 1 97 
 
 The specific heat of the element ; 
 
 Any other suitable data. 
 
 Fifth Step. Confirm tJie foregoing choice as fully as 
 possible. In doing this, employ as many chemical and 
 physical facts as possible. With this in view, study the 
 clement and its compounds in connection with molec- 
 ular formulas. 
 
 These involve a consideration of 
 
 Molecular grouping ; 
 
 Specific heats of compounds ; 
 
 The boiling-points of compounds ; 
 
 The crystalline forms ; 
 
 Such other relationships as may be useful. 
 
 Sixth Step. Bring all the atomic weights obtained 
 into one table and arrange them in an appropriate order. 
 This has been attempted by Newlands, Mendeleeff, 1 and 
 many others. The two investigators mentioned have 
 been among the most successful. 
 
 NOTE. It may be noted here that it is the gravimetric analyses that 
 give the results that are numerically capable of the highest degree of accu- 
 racy. Indeed, they afford the only decisive foundation. The numbers 
 they afford are certainly combining numbers. 
 
 On the other hand, the various vapor densities and the various specific 
 heats are mainly valuable as guiding in the choice between the several 
 multiples of a number already learned. 
 
 The Method of Discussion. It is plain that the subject in hand 
 is an extended one. There is some difficulty even in the selection of a 
 method of presenting it. Several ways are open. 
 
 It is not well to follow here the exact historical course of the subject, 
 for this has been marked by tentative and even erroneous views. However 
 instructive these may be to the experienced chemist, they can only embar- 
 rass the beginner. 
 
 1 Transliteration of Russian words : Nature, xli. 396; xlii. 6, 77, 316. 
 
198 ATOMIC WEIGHT. 
 
 It seems preferable to carry the student over a course directly leading 
 to what is now accepted as truth ; but the course to be selected should 
 accord with the natural progress and be shaped by a sound pedagogic 
 method. 
 
 In the following discussion certain numbers are very quickly adopted. 
 
 This is not improper. But it should not be forgotten that, as already 
 intimated, the historical progress of chemistry has involved a much more 
 circuitous route to reach these numbers than the discussion suggests. 
 
CHAPTER XIX. 
 ATOMIC WEIGHT (continued). 
 FIRST STEP: A UNIT ADOPTED. 
 
 THE subject involves the search for certain numbers. 
 Now it must be remembered that all numerical expres- 
 sions of mixed mathematics involve the use either ex- 
 pressed or implied of some unit; and often the unit 
 chosen depends more upon convenience than upon any 
 other consideration. These statements apply to atomic 
 weights. Some unit has to be selected. At the present 
 day the one almost universally accepted is the weight of 
 a single atom of hydrogen, a weight extremely small, 
 but one that it is possible (though not necessary) to 
 express in terms of every-day weights. 
 
 It is believed that the weight of a single atom of hydrogen is equal 
 to 35 grammes divided by io 23 , an amount about equal to one and one 
 quarter ounces divided by one million million million million. This minute 
 weight has received the special name microcrith. When, therefore, it is 
 said that the atomic weight of oxygen is 16, the meaning is that a single 
 atom of oxygen weighs 16 microcriths. 
 
 A Different Unit might be used. It must be remem- 
 bered that this selection of the atom of hydrogen as the 
 standard is a matter of convenience partly, being based 
 on the fact that no other atom has so low a weight. 
 The weight of any other element might be used if it 
 were found to be more convenient. 
 
 199 
 
2OO ATOMIC WEIGHT. 
 
 Dalton used hydrogen in his first table of atomic 
 weights. Subsequently Berzelius and others recom- 
 mended using oxygen as the standard, calling its weight 
 loo. 1 This latter system, however, was found to have 
 the objection of affording in many cases numbers too 
 large for convenience. Thus, when the atomic weight 
 of oxygen equals 100, the atomic weight of uranium 
 becomes about 1494. 
 
 Certain practical objections have been urged recently against the em- 
 ployment of the atom of hydrogen as the unit with the atomic weight, I. 
 
 Thus it is very difficult to decide upon the exact ratio of the weight of 
 the hydrogen atom to that of the oxygen atom. A number of ratios have 
 been obtained as a result of extremely careful investigation. Thus it has 
 been placed as low as H : O : : 1 : 15.869, and as high as H : O : : I : 1 6.010. 
 But since oxygen is the starting-point for the determination of the atomic 
 weights of a great many other elements, any error in the adopted ratio 
 of H : O is transferred to nearly all the other atomic weights. Thus, if 
 O = 15.869, then the atomic weight of uranium = 237.14. If O = 16.010, 
 the atomic weight of uranium = 239.25. 
 
 To avoid this difficulty it has been proposed, while using hydrogen as 
 the nominal basis of the system, to use oxygen as the practical basis; to 
 call the atomic weight of oxygen 16, and then to hereafter determine what 
 the exact atomic weight of hydrogen is. At present it would be about 
 1.0025; but it might be expected to be slightly modified from time to time 
 as the ratio of the combining portions of oxygen and hydrogen is deter- 
 mined with greater and greater exactness. Meanwhile, changes in this 
 ratio would not necessarily alter the entire scheme of atomic weights. 
 
 This proposition, to adopt oxygen with atomic weight 16 as the unit, has 
 received the approval of many eminent chemists. 
 
 SECOND STEP: SELECTION OF THE COMPOUNDS AND 
 THE PROCESSES TO BE EMPLOYED. 
 
 In a certain sense every chemical compound (and 
 every chemical process) is capable of contributing some- 
 
 1 Meyer, L., and Seubert, K., American Chemical Journal, vii. 96. 
 
ATOMIC WEIGHT. 2OI 
 
 thing to the knowledge of atomic weights ; and, in fact, 
 the study of a large number of them is necessary. But 
 certain ones are far more serviceable than others. 
 
 The compounds must be selected, then, with definite 
 principles in mind, and they should conform to as many 
 as possible of the following requisites : 
 
 1. They should be such as can be prepared in a form 
 possessing a high degree of purity. 
 
 Moreover, in purifying the materials used for analysis 
 the so-called "fractional" methods should be employed. 
 
 2. As many different compounds as practicable should 
 be tested. This helps the analyst to avoid "constant" 
 sources of error. 
 
 3. They should be such as are capable of having their 
 composition determined with a high degree of exactness. 
 
 With this in view the analyses should require as simple 
 processes as possible. 
 
 " Improvements made of late in manipulative methods and apparatus 
 have tended to reduce very much the magnitude of what are commonly 
 called ' fortuitous ' errors in quantitative determinations of matter, and to 
 increase greatly the accuracy. No one nowadays would undertake the 
 determination of an atomic weight of one of the better-known elements 
 without taking such elaborate precautions as must practically insure pretty 
 close concordance of results when obtained by the same method applied 
 in the same hands. But such mere close agreement is not alone sufficient." 
 
 4. They should be such as possess a molecular con- 
 dition that is simple and can be distinctly ascertained. 
 
 Thus it is well to employ compounds of only two 
 elements. Again, if a given pair of elements forms a 
 series of compounds, that one containing the smallest 
 amount of the element under consideration is most 
 likely to contain one atom of it. 
 
 Again, compounds that are gaseous at ordinary tern- 
 
2O2 ATOMIC WEIGHT. 
 
 peratures or that can be easily vaporized, are employed. 
 (See p. 37.) 
 
 5. In reactions depended upon, only such other 
 elements should be concerned as may be counted 
 among those of which the atomic weights are already 
 known with the nearest approach to exactness. 
 
 Thus, (#) Compounds with hydrogen are preferred when they are 
 practicable ; for hydrogen has generally been adopted as the basis of the 
 system, and so involves little or no error. 
 
 Indeed, there are four compounds of hydrogen so well suited to this 
 purpose that they have been called the type compounds of modern chemis- 
 try; they are 
 
 Hydrochloric acid (hydrogen and chlorine) ; 
 
 Water (hydrogen and oxygen) ; 
 
 Ammonia gas (hydrogen and nitrogen) ; 
 
 Marsh gas (hydrogen and carbon). 
 
 (^) In case hydrogen compounds are impracticable, then the com- 
 pounds selected should, if possible, be such as contain certain atoms that 
 have been compared directly and quantitatively with hydrogen. 
 
 The majority of the elements do not form hydrogen compounds, so 
 that, in fact, recourse is oftener had to oxygen compounds and chlorine 
 compounds. All the elements (except fluorine, and that combines 
 with hydrogen) form oxides, and oxygen itself has been compared with 
 hydrogen with a high degree of accuracy. Again, chlorine combines 
 with a great many metals, and the atomic weight of chlorine has been 
 accurately determined. 
 
 If oxygen is adopted (with the number 1 6) as the basis of the system, 
 then compounds of oxygen will be selected, even in preference to com- 
 pounds of hydrogen. 
 
 6. Further, it is desirable that as few other elements 
 as possible the assumed atomic weights of which will 
 have to be taken into account shall be involved in 
 each single reaction depended upon. 
 
 7. In selecting different processes to be applied to 
 the determination of the atomic weight of a given 
 element, it is desirable that not the same, but as many 
 
ATOMIC WEIGHT. 2O3 
 
 different other elements as possible, shall be concerned 
 in the several reactions. 
 
 Study of the Reactions. Careful preliminary study is required 
 as to the general effect of each reaction involved, and as to how it 
 may be influenced by the conditions of the experiment. For it has been 
 learned more and more of late that many reactions perhaps it should 
 rather be said all reactions which have been generally supposed to be of 
 the simplest nature, are, in reality, complex. 
 
 As many different and independent processes as can be devised 
 (reasonably free from apparent sources of error) should be employed. 
 
 Each process employed should be as simple as possible, both in the 
 kind of chemical changes involved as well as in liability to manipulative 
 
 Comparison of Results. In comparison of results, careful con- 
 sideration should be given as to the probable influence of each kind of 
 experiment; i.e. whether it tends on the whole to yield higher results or 
 lower results than the truth. 1 
 
 1 Mallet, J. W., American Chemical Journal, xii. 82. 
 
CHAPTER XX. 
 ATOMIC WEIGHT (continued). 
 
 THIRD STEP: THE EXPERIMENTAL WORK FOR SECURING 
 A FEW ATOMIC WEIGHTS. 
 
 THIS stage is a very extended one. It involves, in 
 fact, all the experimental work done and recorded up 
 to the time of forming conclusions. Now chemical 
 work has been done varying in extent and accuracy 
 upon nearly if not all substances known to civilized 
 nations. But of course, for the purpose of this discus- 
 sion, only a few of the results can be referred to. 
 
 I. 
 
 A STUDY OF CHLORINE AND ITS AFFILIATED ELEMENTS, 
 BROMINE AND IODINE (ALSO SODIUM, POTASSIUM, AND 
 SILVER). 
 
 (a) Gravimetric Composition of Certain Compounds. 
 
 Experimental Facts. A study of certain compounds of 
 chlorine, bromine, and iodine has afforded a series of 
 facts which are stated in the following table : 
 
 204 
 
ATOMIC WEIGHT. 
 
 205 
 
 FIRST TABLE. 
 
 APPROXIMATE PERCENTAGE COMPOSITION OF TWELVE COMPOUNDS. 
 EXPERIMENTAL RESULTS. 
 
 Chloride of Hydrogen. Bromide of Hydrogen. Iodide of Hydrogen. 
 Hydrogen, 2.75 p. ct. Hydrogen, 1.24 p. ct. Hydrogen, .79 p. ct. 
 Chlorine, 97.25 " Bromine, 98.76 " Iodine, 99.21 " 
 
 100.00 
 
 100.00 
 
 Chloride of Sodium. Bromide of Sodium. Iodide of Sodium. 
 
 Sodium, 39.38 p. ct. Sodium, 22.37 P- ct - Sodium, 15.37 p. ct 
 Chlorine, 60.62 " Bromine, 77.63 " Iodine, 84.63 " 
 
 100.00 
 
 100.00 
 
 Chloride of Potassium. Bromide of Potassium. Iodide of Potassium. 
 Potassium, 52.42 p. ct. Potassium, 32.83 p. ct. Potassium, 23.55 P- ct * 
 Chlorine, 47.58 " Bromine, 67.17 " Iodine, 76.45 " 
 
 100.00 
 
 100.00 
 
 Chloride of Silver. Bromide of Silver. Iodide of Silver. 
 
 Silver, 75-26 p. ct. Silver, 57-44 P- ct. Silver, 45-97 P- ct. 
 
 Chlorine, 24.74 " Bromine, 42.56 " Iodine, 54-03 " 
 
 100.00 
 
 100.00 
 
 100.00 
 
 Experimental Results of the First Table stated differently. 
 
 A consideration of the direct results given in Table I leads to the detec- 
 tion of the following facts : 
 
 1. The numbers fall into series. 
 
 2. In each of the series of chlorides, bromides, and iodides, the 
 chlorine is smaller in amount than the bromine, and the bromine is 
 smaller in amount than the iodine. 
 
 3. When the amounts of hydrogen, sodium, potassium, and silver are 
 compared, it is seen that their quantities are in the order stated; hydrogen 
 being in smallest amount, and silver in largest. 
 
 4. If the three hydrogen compounds are compared on the basis of one 
 part of hydrogen, the hydrogen series of compounds shows the following 
 composition : 
 
2O6 ATOMIC WEIGHT. 
 
 SECOND TABLE. 
 HYDROGEN COMPOUNDS. EXPERIMENTAL RESULTS. 
 
 Hydrochloric Acid. 
 Hydrogen, I. 
 Chlorine, 35.4 
 
 364 
 
 Hydrobromic Acid. 
 Hydrogen, I. 
 Bromine, 79.8 
 
 80^8 
 
 Hydriodic Acid. 
 Hydrogen, I . 
 Iodine, 126.6 
 
 127.6 
 
 5. If trial is made with the numbers obtained in Table 2 (viz. 35.4 for 
 chlorine, 79.8 for bromine, and 126.6 for iodine) in the other compounds 
 under consideration, the following very remarkable results are obtained : 
 
 THIRD TABLE. 
 
 SODIUM COMPOUNDS. EXPERIMENTAL RESULTS. 
 
 Sodic Chloride. Sodic Bromide- Sodic Iodide. 
 
 Sodium, 23. Sodium, 23. Sodium, 23. 
 
 Chlorine, 35.4 Bromine, 79.8 Iodine, 126.6 
 
 58.4 102.8 149.6 
 
 FOURTH TABLE. 
 POTASSIUM COMPOUNDS. EXPERIMENTAL RESULTS. 
 
 Potassic Chloride. Potassic Bromide. Potassic Iodide. 
 
 Potassium, 39. Potassium, 39. Potassium, 39. 
 
 Chlorine, 35.4 Bromine, 79.8 Iodine, 126.6 
 
 74.4 118.8 165.6 
 
 FIFTH TABLE. 
 
 SILVER COMPOUNDS. EXPERIMENTAL RESULTS. 
 
 Argentic Chloride. Argentic Bromide. Argentic Iodide. 
 
 Silver, 107.7 Silver, 107.7 Silver, ' 107.7 
 
 Chlorine, 35.4 Bromine, 79.8 Iodine, 126.6 
 
 I43- 1 J 87-5 2 34-3 
 
ATOMIC WEIGHT. 
 
 207 
 
 The results are brought together in the following 
 table : - 
 
 SIXTH TABLE. 
 
 EXPERIMENTAL RESULTS. 
 
 CHLORIDES. 
 
 BROMIDES. 
 
 IODIDES. 
 
 Hydric Chloride. 
 
 Hydric Bromide. 
 
 Hydric Iodide. 
 
 Per cent. 
 Hydrogen, 2.75 
 Chlorine, 97-25 
 
 Ratio, 
 i. 
 
 35-4 
 
 Per cent. 
 Hydrogen, 1.24 
 Bromine, 98.76 
 
 Ratio, 
 i. 
 
 79-8 
 
 Per cent. 
 Hydrogen, .79 
 Iodine, 99.21 
 
 100.00 
 
 Ratio, 
 i. 
 
 126.6 
 
 127.6 
 
 100.00 
 
 36-4 
 
 100.00 
 
 80.8 
 
 Sod ic Chloride. 
 
 Sodic Bromide. 
 
 Sodic Iodide. 
 
 Per cent. 
 
 Sodium, 39-38 
 Chlorine, 60.62 
 
 100.00 
 
 Ratio. 
 
 23- 
 35-4 
 
 Per cent. 
 
 Sodium, 22.37 
 Bromine, 77-63 
 
 100.00 
 
 Ratio. 
 
 23- 
 79-8 
 
 Per cent. 
 Sodium, I 5-37 
 Iodine, 84.63 
 
 Ratio. 
 
 23- 
 126.6 
 
 58-4 
 
 102.8 
 
 100.00 
 
 149.6 
 
 Potassic Chloride. 
 
 Potassic Bromide. 
 
 Potassic Iodide. 
 
 Per cent. 
 Potassium, 52.42 
 Chlorine, 47-58 
 
 Ratio. 
 
 39- 
 35-4 
 
 Per cent. 
 Potassium, 32.83 
 Bromine, 67.17 
 
 100.00 
 
 Ratio. 
 
 39- 
 79-8 
 
 118.8 
 
 Per cent. 
 Potassium, 23.55 
 Iodine, 76.45 
 
 100.00 
 
 Ratio. 
 
 39- 
 126.6 
 
 165.6 
 
 100.00 
 
 74-4 
 
 A rgentic Chloride. 
 
 Argentic Bromide. 
 
 A rgentic Iodide. 
 
 Per cent. 
 Silver, 75-26 
 Chlorine, 24.74 
 
 Ratio. 
 
 107.7 
 35-4 
 
 Per cent. 
 Silver, 57.44 
 Bromine, 42.56 
 
 Ratio. 
 
 107.7 
 79-8 
 
 Per cent. 
 Silver, 45.97 
 Iodine, 54-3 
 
 100.00 
 
 Ratio.' 
 
 107.7 
 126.6 
 
 234-3 
 
 100.00 
 
 I43-I 
 
 100.00 
 
 187.5 
 
2O8 ATOMIC WEIGHT. 
 
 Inference \, Evidently, then, the following numbers have some im- 
 portant fundamental meaning : 
 
 NUMBERS WORTHY OF CONSIDERATION. 
 Hydrogen, H, adopted as i. 
 
 ['Chlorine, Cl, found to be, 35.4 
 
 \ Bromine, Br, " " " 79.8 
 
 I Iodine, I, " " " 126.6 
 
 ( Sodium, Na, " " " 23. * 
 
 <j Potassium, K, " " " 39. 
 
 v. Silver, Ag, " " " IO 7-7 
 
 (It may be noted here, as a fact, that subsequent study and comparison 
 of all results accessible confirm the opinion that these numbers are impor- 
 tant, and are probably atomic weights.) 
 
 Inference 2. These results give the following as molecular weights : 
 
 Hydrochloric acid 36.4 
 
 Sodic chloride 58.4 
 
 Potassic chloride 74.4 
 
 Argentic chloride I43- 1 
 
 Hydrobromic acid 80.8 
 
 Sodic bromide 102.8 
 
 Potassic bromide 118.8 
 
 Argentic bromide l %7-5 
 
 Hydriodic acid 127.6 
 
 Sodic iodide 149.6 
 
 Potassic iodide I&5-6 
 
 Argentic iodide 2 34-3 
 
 Inference 3. These results suggest the following formulas : 
 
 H Cl H Br HI 
 
 Na Cl Na Br Na I 
 
 K Cl K Br K I 
 
 Ag Cl Ag Br Ag I 
 
CHAPTER XXI. 
 ATOMIC WEIGHT (continued}. 
 
 FOURTH STEP: THE CHOICE OF A PARTICULAR ATOMIC 
 WEIGHT FROM SEVERAL COMBINING NUMBERS. 
 
 (I)} The Density of Certain Elementary Gases and 
 Vapors. Experimental Fact I. When chlorine gas is 
 weighed it is found to weigh, volume for volume, about 
 35.4 times as much as hydrogen. Hydrogen is usually 
 taken as a standard of comparison for weight of gases. 
 
 The number 35.4 is called the density of chlorine gas. 
 
 Experimental Fact 2. A similar experiment is tried 
 with bromine vapor. Its vapor density is found to be 
 about 79.9. 
 
 Experimental Fact 3. A similar experiment is tried 
 with iodine vapor. Its vapor density is found to be 
 about 127. 
 
 Inference I. Certain numbers are obtained that are worthy of atten- 
 tion. Evidently their similarity to the numbers already obtained are not 
 mere coincidences. 
 
 Inference 2. The density of an elementary gaseous substance at once 
 gives its atomic weight. 
 
 Inference 3. The gaseous state is a very favorable one for study in 
 this connection : in this state bodies appear to be in a sort of equality of 
 condition that favors their comparison with each other from the point, per- 
 haps, of even other relations than atomic weight merely. 
 
 (c) The Volume Composition. Experimental Fact. 
 When hydrochloric acid gas is tested, it is found that 
 
 209 
 
210 
 
 ATOMIC WEIGHT. 
 
 two volumes of the gas yield by decomposition one vol- 
 ume of hydrogen gas and one volume of chlorine gas. 
 
 Inference I. This sustains the view assumed in the preceding study, 
 that the compound called hydrochloric acid consists of one atom of hydro- 
 gen and one atom of chlorine. (Of course it is possible that the volume 
 
 FIG. 123. Regnault's apparatus for filling a globe with gas at the temperature of o C. 
 previous to weighing, for the purpose of determining a vapor density. 
 
 composition merely teaches that the number of atoms of hydrogen and of 
 chlorine are equal; that the formula is HC1 or H 2 C1 2 or H 3 C1 3 or H M Cl n . 
 Until further information is secured, chemists assume the truth of the sim- 
 plest expression. Further information is not in fact wanting, for subse- 
 quent study strongly sustains the view that the formula is indeed HC1, and 
 no other. See p. 228.) 
 
 Experimental Facts. Similar results for hydrobromic acid and hydrio- 
 
ATOMIC WEIGHT. 
 
 211 
 
 die acid serve to confirm the figures already accepted for bromine and 
 iodine. 
 
 Inference 2. These facts sustain all the inferences previously reached. 
 
 FIG. 124. Henri Victor Regnault. Distinguished French physicist. Born at 
 Aix-la-Chapelle in 1810; died in 1878. 
 
 (d) The Vapor Density of Certain Compound Substances. 
 
 Experimental Facts. The vapor density of hydro- 
 chloric acid gas is found experimentally to be about 18. 
 This means that a given volume of hydrochloric acid 
 gas weighs 18 times as much as the same volume of 
 hydrogen gas. 
 
212 
 
 ATOMIC WEIGHT. 
 
 Evidently there exists a very simple relation between 
 the number representing the density of hydrochloric 
 acid gas and the number already adopted as its molec- 
 ular weight ; i.e. : 
 
 18 : 36 : : i : 2 
 
 Density of Molecular weight of 
 
 hydrochloric acid gas. hydrochloric acid gas. 
 
 Inference \ . Perhaps in case of other 
 compound substances the vapor density and 
 molecular weight are connected by the same 
 simple relation. Perhaps, as a rule, the 
 molecular weight (which is, from its nature, 
 difficult to determine) may be obtained by 
 multiplying by 2 the vapor density (which 
 in many cases it is easy to determine). (By 
 subsequent experiment this principle appears 
 to be sustained, and the inference is accepted 
 as a just one.) 
 
 Inference 2. Perhaps, in case of elemen- 
 tary substances, the vapor density and molec- 
 ular weight are in the ratio above suggested ; 
 that is I : 2. (By subsequent experiment this 
 principle appears to be well grounded, and 
 the inference is accepted as a just one.) A 
 remarkable result follows. 
 
 The vapor density of chlorine gas is found 
 experimentally to be 35.4; then the molec- 
 ular weight is 35.4 X 2 = 70.8. If, then, 
 the molecular weight is 70.8 (and the atomic 
 weight is 35.4), then the number of atoms 
 in the molecules is 2, and the molecular formula of chlorine is CL 2 . 
 
 (This generalization is a very important one. By subsequent experi- 
 ment it appears to be sustained with respect to most of the elements capa- 
 ble of existing in the form of gas or vapor. See p. 249.) 
 
 (e) The Specific Heats of Elements. Definition. The 
 specific heat of a substance is the amount of heat neces- 
 sary to raise one unit of weight of the substance one 
 
 FIG. 125. Method of intro- 
 ducing into the bulbyl a portion 
 of liquid, C, whose vapor density 
 is to be subsequently determined 
 by Dumas' method. 
 
ATOMIC WEIGHT. 213 
 
 degree of temperature. Water has a high specific heat, 
 and as it is usually taken as the standard, the specific 
 heats of most other substances are expressed by decimal 
 fractions. (See p. 46.) 
 
 Experimental Facts. Dulong and Petit made a great 
 many experimental determinations of the specific heats 
 of solid substances. (It is more difficult with liquids 
 and gases.) In case of elementary solids, whose atomic 
 
 FIG. 126. Hot bath, provided with thermometers for determination of vapor densi- 
 ties by Dumas' method. A small but weighed portion of liquid is placed in the globe. It 
 is then vaporized by the heat of the bath. Subsequently the tip of the flask is sealed, 
 whereupon the weight of vapor present at a certain observed temperature may be deter- 
 mined by the balance. 
 
 weights had been previously determined by other methods, 
 they found that apparently the higher the atomic weight, 
 the lower the specific heat. 
 
 Inference. They then enunciated the following law, now called the law 
 of Dulong and Petit : 
 
 The specific heats of solid elements are inversely proportional to their 
 atomic weights. 
 
2I 4 
 
 ATOMIC WEIGHT. 
 
 The law is likewise expressed in the following proportion : 
 
 Specific heat of A : Specific heat of B : : Atomic weight of B : Atomic 
 weight of A. 
 
 This proportion also discloses the following fact : The product of the 
 specific heat of any solid element by its atomic weight is a constant 
 number. 
 
 This constant is found to be about 6.3. 
 
 Again, the constant 6.3 divided by the 
 specific heat of a solid element yields as a 
 quotient the atomic weight of the element. 
 
 t 
 
 FIG. 127. Bunsen's ice calo- 
 rimeter for determining specific 
 heats of substances. 
 
 FIG. 128. Cooling apparatus for Bunsen's calo- 
 rimeter, already referred to, Fig. 127. 
 
 Bunsen's method of determining specific heats of substances may be 
 explained by reference to Figs. 127 and 128. 5 is a glass tube carefully 
 graduated or calibrated. D is an iron reservoir containing mercury; the 
 latter extends from the line ft through the tube C up into the tube 6". The 
 space B is filled with water. A is at first empty. In using the apparatus, 
 intensely cold alcohol is passed in a current through the tube A by use of 
 the apparatus, Fig. 128. In due time the water in B is frozen completely. 
 The cold alcohol is then withdrawn. Next the entire apparatus is placed in 
 melting snow or ice to bring the whole to zero centigrade. Next the posi- 
 tion of the summit of the mercury column in the tube Sis carefully observed. 
 Now one gramme of water at 100 C. is placed in the tube A. The water 
 
ATOMIC WEIGHT. 21$ 
 
 melts a certain portion of the ice in B. Thereupon contraction takes 
 place, as is indicated by the rise of mercury in B and the fall of mercury 
 in S. The point to which the summit of the mercury column in S now 
 retracts is observed. The distance this summit has traversed corresponds 
 to the specific heat of water. Subsequently the substance to be tested is 
 raised to the temperature of 100 C. Then one gramme of it is placed in 
 the tube A. Thereupon it melts another portion of ice. Further retrac- 
 tion of mercury takes place. The amount of such retraction being meas- 
 ured and compared with the retraction due to one gramme of water, gives 
 directly the specific heat of the substance in question. 
 
 Specific Heats of Compounds. The specific heats of many solid 
 compounds have been determined. In a few cases they are in general 
 harmony with the law of Dulong and Petit; in many cases they are not. 
 In some cases the specific heat of a solid compound containing two atoms, 
 and having a given molecular weight, appears to be twice as great as that 
 of a single elementary substance of the same atomic weight; and the 
 specific heat of a solid compound of three atoms appears to be three 
 times as great as that of a mere element whose atomic weight equals 
 the molecular weight of the compound. This seems to show that when 
 a compound body is heated, the rise of temperature is associated with a 
 motion of each different atom in the molecule, and not merely with that of 
 the molecule as a whole. 
 
 While, then, it requires a certain amount of heat to impart to an 
 elementary substance an amount of motion sufficient to produce a certain 
 change of temperature, in case of a compound body with the same molec- 
 ular weight more heat is required to impart to it the amount of motion 
 that affords the same temperature as that already supposed, twice as 
 much heat is required for compounds of two atoms, and three times as 
 much heat for compounds of three atoms. 
 
 Atomic Heats of Elements. The constant 6.3 is often 
 called the atomic heat of an element. The significance 
 of this expression may be explained as follows : Upon 
 taking one unit of weight of each of several elementary 
 substances, and then applying equal amounts of heat to 
 each of them, it is observed that the temperature rises 
 to different degrees in the different cases. 
 
2l6 ATOMIC WEIGHT. 
 
 Suppose, however, different weights of the substances are experimented 
 upon, say 
 
 7 parts of lithium, 
 56 parts of iron, 
 194 parts of platinum, 
 204 parts of lead; 
 
 it is then found that equal amounts of heat added to these several amounts 
 by weight produce in all the same advances of temperature. Evidently, 
 then, equal amounts of heat applied to single atoms of these substances will 
 produce the same advances of temperature. This explains the statement 
 that the atomft heats of the elements are the same. 
 
 Special Cases. Indirect Determination of Specific 
 Heats. In cases of certain elements the specific heat 
 cannot readily be determined directly. This is especially 
 true of the gaseous elements, as hydrogen, fluorine, 
 chlorine, oxygen, nitrogen. But indirect methods have 
 been devised. (In case of certain solid elements, as car- 
 bon, boron, and silicon, the specific heats are abnormal. 
 This is supposed to be due to the tendency of these 
 elements to assume allotropic modifications.) 
 
 Indirect Method for Chlorine. The specific heat of argentic chloride 
 has been learned experimentally. It is .089. Now by independent 
 methods the molecular weight of the compound is found to be 143.1, and 
 it is found to consist of two atoms, one of silver and one of chlorine. 
 Multiplying the molecular weight by the specific heat (143.1 X .089), the 
 molecular heat 12.7 is obtained. Subtracting from this number the atomic 
 heat of silver, 6.1 (as experimentally obtained), there remains 6.6 as the 
 atomic heat of chlorine indirectly determined. 
 
 Many other similar indirect determinations for chlorine have been 
 made; generally speaking, they yield the number 6.4. 
 
 Indirect Method for Carbon. The specific heat of carbon hexachloride 
 (C 2 C1 6 ) has been learned experimentally. It is .177. By independent 
 methods, its molecular weight is found to be 236.4, and it is found to con- 
 sist of eight atoms as stated. Multiplying the molecular weight by the 
 specific heat (236.4 X .177), the molecular heat, 41.8, is obtained. Sub- 
 tracting from this number six times the atomic heat of chlorine (6 X 6.4 = 
 
ATOMIC WEIGHT. 
 
 38.4) (41.8 38.4 = 3.4), there remains 3.4, which is twice the atomic 
 heat of carbon. By this means the atomic heat of carbon is 1.7 indirectly 
 determined. 
 
 Other similar indirect determinations have given the atomic heat of 
 carbon, in combination, as about 2. 
 
 Specific Heat of Bromine. Experimental Fact. The 
 specific heat of solid bromine has been found by direct 
 experiment ; it is .08432. Applying the law of Dulong 
 and Petit, i.e. dividing .08432 into the constant 6.3, and 
 there results the quotient 75. 
 
 Inference. The atomic weight of bromine is nearly 75. 
 
 But the studies of bromine already referred to (pp. 204 and 209) show 
 that the combining number is about 79.8. Its atomic weight is probably 
 either 79.8, or 2 X 79.8, or 3 X 79-8, or n X 79-8. Now, as has been said 
 before, while the specific heat does not give the exact atomic weight, it 
 enables us to decide that 79.8, and not some multiple (or fraction) of it, 
 should be accepted. 
 
 Specific Heat of Iodine. Experimental Fact. The 
 specific heat of solid iodine has been found by direct 
 experiment to be .0541. Applying the law of Dulong 
 
 63 
 
 and Petit, - = about 116. 
 .0541 
 
 Inference. The atomic weight of iodine is about 116. 
 But the results previously and otherwise obtained point to 126.6. Evi- 
 dently the specific heat confirms this selection. 
 
 Specific Heat of Sodium. Experimental Fact. The 
 specific heat of sodium is found directly to be .293. 
 
 Now - = about 22. 
 
 Inference. The atomic weight of sodium is about 22. 
 But results previously and otherwise obtained have given the number 
 23. Evidently the specific heat confirms this selection. 
 
2l8 ATOMIC WEIGHT. 
 
 Specific Heat of Potassium. Experimental Fact. The 
 specific heat of potassium is found directly to be .166. 
 
 Now ^- = about 38. 
 .100 
 
 Inference. The atomic weight of potassium is about 38. 
 But results previously and otherwise obtained have given the number 
 39. Evidently the specific heat confirms this selection. 
 
 Specific Heat of Silver. Experimental Fact. The 
 specific heat of silver is found directly to be .057. 
 
 6.3 
 
 Now = about in. 
 U 57 
 
 Inference. The atomic weight of silver is about in. 
 But results previously and otherwise obtained have given the number 
 107.7. Evidently the specific heat confirms this selection. 
 
 General Inference. If now reference is made to the 
 provisional table presented on p. 208, it is seen that the 
 numbers there given secure marked confirmation from 
 the experiments subsequently detailed. 
 
 They are repeated here as 
 
 SEVENTH TABLE. 
 
 WELL-ESTABLISHED ATOMIC WEIGHTS. 
 
 Atomic weight of hydrogen adopted as .... i. 
 
 " " " chlorine found to be . . . . 35.4 
 
 " " bromine " .... 79.8 
 
 " " iodine " .... 126.6 
 
 " " " sodium " .... 23.0 
 
 " " " potassium " .... 39.0 
 
 " " " silver " .... 107.7 
 
ATOMIC WEIGHT. 2IQ 
 
 II. 
 
 A STUDY OF OXYGEN AND SOME OF ITS COMPOUNDS. 
 
 The study of chlorine and its affiliated elements has 
 afforded a considerable number of suggestions. These 
 may well be applied to other elements. Oxygen may 
 well be studied first, because of its great importance in 
 this as well as other relations. 
 
 (a) Experimental Fact. The density of oxygen gas 
 is about 16. 
 
 Inference. The atomic weight of oxygen is about 16. 
 
 (b) Experimental Pact. The volume composition of 
 water vapor is as follows : two volumes of hydrogen and 
 one volume of oxygen. 
 
 Inference I. The formula of water is H 2 O (see pp. 226 and-229). 
 Inference 2. The molecular weight of water is about 2 + 16 = iS. 
 
 (c) Experimental Fact. The vapor density of water 
 vapor is about 9. 
 
 Inference. The molecular weight of water is about 9x2 = about 18. 
 This inference is based on results obtained under chlorine. It sustains 
 the views already adopted in this section. 
 
 (d} Experimental Fact. Gravimetric analysis shows 
 that water is made up as follows : 
 
 Hydrogen .... ii.i i parts by weight. 
 Oxygen .... 88.88 
 99-99 
 The ratios are as i : 8 or 2 : 16. 
 
220 ATOMIC WEIGHT. 
 
 Inference. These facts add support to the views previously accepted, 
 and contribute greatly to create confidence in the general principles as 
 well as the numerical results adopted. 
 
 (e) Experimental Facts. Oxygen forms compounds 
 with sodium, potassium, and silver, having the composi- 
 tion given in the following table. (As a matter of fact 
 it forms many others, but this set is selected as afford- 
 ing a strict continuity to the argument.) 
 
 EIGHTH TABLE. 
 PERCENTAGE BASIS. 
 
 Sodium and Oxygen. Potassium and Oxygen. Silver and Oxygen. 
 
 Per cents. Per cents. Per cents. 
 
 Sodium, 74.19 Potassium, 82.98 Silver, 93-O9 
 
 Oxygen, 25.81 Oxygen, 17.02 Oxygen, 6.91 
 
 100.00 loo.oo 100.00 
 
 If now the results of the eighth table are computed 
 on another basis, i.e., using the atomic weights accepted 
 for sodium 23, for potassium 39, and for silver 107.7, 
 there is afforded a new table. Its results are surprising, 
 but they present a strict statement of facts in a 
 special form merely. 
 
 NINTH TABLE. 
 ATOMIC WEIGHT BASIS. 
 
 Sodium and Oxygen. Potassium and Oxygen. Silver and Oxygen. 
 Sodium, 23 Potassium, 39 Silver, IO 7-7 
 
 Oxygen, 8 Oxygen, 8 Oxygen, 8. 
 
 3 1 47 "S-7 
 
 Inference. Either the atomic weight of oxygen is 8 instead of 16; or, 
 if it is indeed 16, then the atomic weights accepted for sodium, potassium, 
 
ATOMIC WEIGHT. 221 
 
 and silver are only one-half what they should be; or else the compounds 
 have the formulas Na. 2 O, K 2 O, Ag 2 O, respectively. This latter supposition 
 satisfies all the foregoing facts (and many others) so well, that it has been 
 universally adopted, and with it the atomic weight 16 (or thereabouts) for 
 oxygen. 
 
 On this view the following table may be arranged. 
 It is, as to numerical relations, a strict statement of 
 experimental facts. 
 
 TENTH TABLE. 
 
 NEW FORMULAS. 
 
 Na^ 23x2 = 46 K. 2 , 39X2=78 Ag 2 , 107.7x2=215.4 
 
 O, 16 O, 16 O, 16. 
 
 62 94 231.4 
 
 Inferences from the Tenth Table. From various facts already pre- 
 sented, the following two groups of formulas have been accepted : 
 
 Hydrochloric acid, HC1 Hydrogen oxide (water), H 2 O 
 
 Hydrobromic acid, HBr 
 
 Hydriodic acid, HI 
 
 Sodic chloride, NaCl Sodic oxide, Na 2 O 
 
 Sodic bromide, NaBr 
 
 Sodic iodide, Nal 
 
 Potassic chloride, KC1 Potassic oxide, K^O 
 
 Potassic bromide, KBr 
 
 Potassic iodide, KI 
 
 Argentic chloride, AgCl Argentic oxide, Ag. 2 O 
 
 Argentic bromide, AgBr 
 
 Argentic iodide, Agl 
 
 An inspection of these formulas recalls the striking and very important 
 suggestion that oxygen possesses a different numerical nature from chlo- 
 rine, bromine, and iodine, not only in that it has a different atomic weight 
 from theirs, but, further, in this, that while chlorine, bromine, and iodine 
 are satisfied to combine with hydrogen, sodium, potassium, and silver, atom 
 for atom, the atom of oxygen is only satisfied when it combines with two 
 atoms of the elements in question, 
 
222 ATOMIC WEIGHT. 
 
 This fact is emphasized by a great many other compounds, so much 
 so that oxygen is called a dyad, and the other elements mentioned are 
 called monads. Further, an atom of oxygen is said to have two points of 
 attraction, while an atom of hydrogen (and one atom of each of the other 
 substances mentioned) is said to have one point of attraction. 
 
 Yet further, the property of the elementary atoms by virtue of which 
 they attract different numbers of atoms is called equivalence or valence. 
 And an atom of hydrogen is accepted as the unit of valence. (See p. 223.) 
 
 III. 
 
 A STUDY OF SULPHUR AND SOME OF ITS COMPOUNDS. 
 
 Sulphur may be discussed very much as oxygen has 
 been. 
 
 (a) Experimental Pact. The density of sulphur vapor 
 is found to be about 32.2. 
 
 Inference. The atomic weight of sulphur is about 32. 
 
 (b) Experimental Fact. Sulphuretted hydrogen gas 
 is found to be composed of hydrogen and sulphur, and 
 to have the density about 17.2. 
 
 Inference I . Its molecular weight is 34. 
 
 Inference 2. It is probably made up of one atom of sulphur weighing 
 32, and two atoms of hydrogen weighing 2. 
 Inference 3. Its formula is probably H 2 S. 
 
 (c) Experimental Fact. Sulphuretted hydrogen gas 
 has been found to have the percentage composition : 
 
 Hydrogen . . . 5.88 parts by weight. 
 Sulphur . ... 94.12 " " 
 
 Total .... 100. 
 
 The ratios of these numbers are evidently as I : 16, 
 or as 2 : 32. 
 
ATOMIC WEIGHT. 223 
 
 Inference. These facts sustain the views already presented as to the 
 composition of sulphuretted hydrogen, and that the atomic weight of 
 sulphur is 32. 
 
 (d) Experimental Fact. The specific heat of sulphur 
 has been found to be .188. But ^ = about 33. 
 
 . loo 
 
 Inference. This affirms the selection of the number 32 as the atomic 
 weight of sulphur. 
 
 Experimental Fact. The most exact determinations 
 of the atomic weight of sulphur have been based upon 
 its combination with silver. The composition of sul- 
 phide of silver has been learned by experiment to be 
 
 Silver 87.07 per cent. 
 
 Sulphur 12:93 " 
 
 100. 
 
 Inference I. If the atomic weight previously found for silver is 107.7, 
 then the atomic weight of sulphur is 16, or some multiple of it. But the 
 results already stated suggest the number 32 as the proper atomic weight. 
 If this view is accepted, the following inference may be obtained. 
 
 Inference 2. The formula of sulphide of silver is Ag 2 S. Then 
 
 The molecular weight of Ag 2 = 215.4 (87.07 per cent) 
 The atomic weight of S = 32. (12.93 P er cent) 
 
 247.4 (100. per cent) 
 
 Inference 3. From these results sulphur is placed in the class of dyads, 
 as oxygen was. The notion of valence already reached is therefore sus- 
 tained by the studies of sulphur above described. 
 
 Inference 4. As previously intimated, it has been learned that certain 
 elements hydrogen, chlorine, bromine, iodine, sodium, potassium, silver 
 have the equivalence one, and certain elements oxygen, sulphur 
 have the equivalence two. The suggestion naturally arises that perhaps 
 other elements have the equivalence three or four, or indeed higher nurn- 
 
224 ATOMIC WEIGHT. 
 
 bers yet. It may be added that this suggestion is amply sustained by 
 facts, some of which will be presented soon. The general notion of 
 valence is adopted as a fundamental fact of chemistry. 
 
 NOTE. By gravimetric analysis of binary compounds 
 containing on the one hand elements whose atomic 
 weights have been provisionally adopted as just de- 
 scribed, and on the other hand other elements, atomic 
 weights of these latter elements may be secured sub- 
 ject, of course, to revision in the light of additional facts 
 such as have been already presented. 
 
CHAPTER XXII. 
 ATOMIC WEIGHT (continued}. 
 
 FIFTH STEP: CONFIRMATION OF THE ATOMIC 
 WEIGHTS CHOSEN. 
 
 THE preceding chapters on atomic weights have suf- 
 ficed to show how a few of these important constants 
 can be secured. The discussion has called attention to 
 certain methods pursued and certain precepts accepted. 
 
 Of course, when the atomic weight of a substance 
 is determined by a sufficient number of independent 
 methods, such weight may be used in fixing the molecu- 
 lar formula of compounds of the element. 
 
 On the other hand, it is a fact of deeper significance 
 that ivhen the molecular formula of a compound can be 
 determined by independent methods, this formula may 
 be of great service in securing atomic weights of ele- 
 ments, or in confirming those already secured. Every 
 effort is made, therefore, to determine molecular formulas 
 of elements and compounds. 
 
 I. 
 
 MOLECULAR FORMULA SECURED BY VOLUME COM- 
 POSITION. 
 
 In illustration, the four type-compounds of modern 
 chemistry may be referred to 
 
 225 
 
226 ATOMIC WEIGHT. 
 
 Hydrochloric Acid. First Fact. Hydrochloric acid 
 consists of hydrogen and chlorine, and nothing else. 
 
 Second Fact. Hydrochloric acid is a gas. 
 
 Third Fact. Two volumes of hydrochloric acid gas 
 yield by decomposition one volume of hydrogen and one 
 volume of chlorine. 
 
 Inference. The molecule of hydrochloric acid has some one of the 
 following formulas : 
 
 HC1, 
 
 H 2 C1 2 , 
 
 H 3 C1 3 , 
 
 HCl n . 
 
 On a previous page certain reasons have been assigned for adopting the 
 formula HC1. Yet other reasons will be assigned hereafter. (See p. 228.) 
 
 Fourth Fact. Hydrogen and chlorine do not form 
 any compound but this one, called hydrochloric acid. 
 
 Inference. Hydrochloric acid is the simplest possible compound of the 
 elements hydrogen and chlorine. Therefore it is a compound containing 
 one atom of each. 
 
 NOTE. This inference is not a particularly valid one, but it naturally 
 arises in cases of a single compound of two elementary substances. 
 
 In some cases it is distinctly misleading. Thus, previous to the recog- 
 nition of hydrogen dioxide, but one compound of hydrogen and oxygen 
 was known; viz. water. On this general ground the formula HO was 
 adopted. Subsequently the volume relations and other considerations 
 (some of them stated in this chapter) have led to the adoption of the 
 formula H 2 O. 
 
 Water. First Fact. Water consists of hydrogen and 
 oxygen, and nothing else. 
 
 Second Fact. Water may be changed into a vapor 
 and experimented upon in that form. 
 
 Third Fact. Two volumes of water vapor yield, by 
 
ATOMIC WEIGHT. 22/ 
 
 decomposition, two volumes of hydrogen gas and one 
 volume of oxygen gas. 
 
 Inference. The formula of water is probably one of the following : 
 H 2 0, 
 
 But reasons have been heretofore stated, favoring the view that the 
 Hereafter other reasons will be stated for this view. 
 
 Ammonia Gas. First Fact. Ammonia gas consists 
 of hydrogen and nitrogen, and nothing else. 
 
 Second Fact. The substance is a gas. 
 
 Third Fact. When two volumes of ammonia gas are 
 decomposed, they afford one volume of nitrogen gas 
 and three volumes of hydrogen gas. 
 
 Inference. The formula for ammonia gas is one of the following : 
 
 H 3 N, 
 H 6 N 2 , 
 
 H 9 N 3 , 
 
 H 3ll N B . 
 
 The simplest formula, H 3 N, may be accepted for the present, with the 
 intention of changing it if facts hereafter discovered demand such change. 
 
 Marsh Gas. First Fact. Marsh gas consists of car- 
 bon and hydrogen, and nothing else. 
 
 Second Fact. The substance is a gas. 
 
 Third Fact. Two volumes of marsh gas yield, by 
 decomposition, four volumes of hydrogen gas. (The 
 volume relations of the carbon cannot be stated, since 
 carbon cannot be obtained in a state of gas.) 
 
228 ATOMIC WEIGHT. 
 
 Inference, The formula of marsh gas is one of the following: 
 
 H 4 C, 
 H 8 C 2 , 
 
 H 12 C 3 , 
 
 H 4n C n . 
 
 The simplest formula H 4 C may be adopted at present, with the inten- 
 tion of changing it if facts subsequently discovered demand such change. 
 
 (Evidently the fact that carbon is not obtainable in a gaseous form 
 diminishes, to some extent, confidence in the formula adopted.) 
 
 II. 
 
 MOLECULAR FORMULA BASED UPON CHEMICAL SUBSTI- 
 TUTION. 
 
 Hydrochloric Acid. First Fact. Many chlorides may 
 be formed by substituting certain metals for the hydro- 
 gen in hydrochloric acid : such are the well-known chlo- 
 rides, sodium chloride, potassium chloride, silver chloride. 
 
 Second Fact. When such substitutions as those just 
 referred to are made, it is found that the whole of the 
 hydrogen may be replaced by a metal, but no fractional 
 part can be. Thus it is not possible to form a chloride 
 in which part of the hydrogen has been replaced by 
 potassium and part of the hydrogen remains unreplaced. 
 Replacement must be of the whole of the hydrogen or 
 of none at all. 
 
 Inference I. The amount of hydrogen in hydrochloric acid is chem- 
 ically indivisible. In other words, it is an atom. 
 
 NOTE. It is true that the chemist cannot so carry out his experiment 
 as to work upon a single molecule of hydrochloric acid. The smallest 
 quantity upon which he can experiment must necessarily contain millions 
 of molecules. But this does not in any way invalidate the conclusions 
 just reached. 
 
 If in a mass of hydrochloric acid containing (say) four molecules of 
 
ATOMIC WEIGHT. 22Q 
 
 hydrochloric acid, one-half of the hydrogen were replaced by potassium, 
 there might be a change somewhat as indicated below : 
 
 First Stage. Second Stage. 
 HC1, HC1, 
 
 HC1, HC1, 
 
 HC1, KC1, 
 
 HC1. KC1. 
 
 In a sense, however, all the hydrogen in the hydrochloric acid experi- 
 mented upon has been replaced by potassium. But the formulas given 
 represent simply an incomplete operation. They do not represent a 
 new and single compound containing part hydrogen and part potassium. 
 Instead, they represent a mixture of compounds, the one having all its 
 hydrogen replaced by potassium, as already intimated, the other having 
 none of its hydrogen yet replaced. 
 
 By a process of experiment and reasoning entirely similar to the fore- 
 going, it may be shown that the chlorine in hydrochloric acid may be 
 replaced by another element; for example, bromine or iodine. It is found, 
 however, that bromine and iodine respectively replace the whole of the 
 chlorine in the hydrochloric acid, or none at all. They cannot replace 
 the one-half or one-fourth, nor any other fractional part of the chlorine. 
 
 Inference 2. The chlorine in the molecule of hydrochloric acid is 
 practically indivisible. That is, it is a single atom. 
 
 Thus it seems to be proved that the formula for hydrochloric acid 
 is HC1. 
 
 Water. First Fact. Many compounds may be formed 
 by substituting proper metals for the hydrogen in water. 
 Thus the well-known compounds, potassic oxide, sodic 
 oxide, may be easily formed. 
 
 Second Fact. When such substitutions as those just 
 referred to are made, it is found that they may be accom- 
 plished in two different ways ; i.e. either the whole of the 
 hydrogen may be replaced by the potassium and sodium, 
 respectively, or the half of the hydrogen may be so 
 replaced. In these cases two entirely different substan- 
 ces are produced. When the whole of the hydrogen is 
 
23O ATOMIC WEIGHT. 
 
 replaced, the substance called potassic oxide is produced. 
 It contains 78 parts by weight of potassium, and 16 parts 
 by weight of oxygen. When the half of hydrogen is 
 replaced, a distinct and well-known compound is pro- 
 duced, called potassium hydroxide. It contains 39 parts 
 by weight of potassium, I part by weight of hydrogen, 
 1 6 parts by weight of oxygen. 
 
 Here, then, it is seen that potassium may replace the 
 half of the hydrogen in water or the whole of it. But 
 the displacement of hydrogen in water is not possible 
 in any other fractional zvay. 
 
 By a course of experiment and reasoning similar to 
 that pursued with hydrochloric acid, it may be shown 
 that the oxygen of water is not divisible; in other 
 words, is a single atom. 
 
 Inference I. It appears that the one-half of the hydrogen that 
 is in a molecule of water is the indivisible portion of hydrogen; i.e. is one 
 atom. In other words, it appears that the molecule of water has two 
 atoms of hydrogen. 
 
 Inference 2. It appears that the proper formula for water is H 2 O. 
 
 Ammonia Gas. By the use of undoubted facts of 
 observation and a method of reasoning similar to that 
 pursued in the two preceding illustrations, it may be 
 shown that the hydrogen of ammonia gas is divisible 
 into three parts, and that its nitrogen is not divisible. 
 
 Thus the inference is secured that the formula for ammonia gas is H 3 N. 
 
 Marsh Gas. In similar fashion, marsh gas may be 
 shown to have an amount of hydrogen that is divisible 
 into four parts and not into any other fractional amounts, 
 and that its carbon is indivisible. (See p. 168.) 
 
ATOMIC WEIGHT. 23! 
 
 Thus it is apparent that the formula H 4 C should be adopted for 
 marsh gas. 
 
 The foregoing discussion has sustained an opinion previously expressed 
 with reference to the valence of chlorine and oxygen; viz. chlorine has 
 been made out to be a monad, oxygen a dyad, and so also nitrogen and 
 carbon respectively a triad and a tetrad. 
 
 Many other facts of chemical substitution could be presented to sustain 
 the views just enunciated, and thence, of course, to sustain the formulas 
 already adopted. 
 
 III. 
 
 RELATION OF MOLECULAR WEIGHT TO MELTING-POINTS 
 OF SOLIDS AND BOILING-POINTS OF LIQUIDS. 
 
 Chemistry as a system cannot be complete until ele- 
 ments and compounds can be arranged in orderly lists, 
 showing regular progression of the various physical and 
 chemical properties of the substances. 
 
 Many small groups are recognized where such a prin- 
 ciple as this is successfully carried out. 
 
 There are certain marked cases of the simultaneous 
 advance of molecular weights and boiling-points in the 
 case of elementary substances. For example note the 
 following : 
 
 Name. Atomic Weight. Molecular Weight. Boiling-point. 
 
 Chlorine, 35.4 70.8 - 33.60 C. 
 
 Bromine, 79.8 T 59-6 59.27 C. 
 
 Iodine, 126.6 253.2 250 C. 
 
 Such advance, however, is by no means uniform. If the list of elemen- 
 tary substances be arranged in an order commencing with that of lowest 
 atomic weight, and ending with that of highest atomic weight, it will be 
 found that while there is a general tendency toward increase of melting- 
 point (for most of the elements are solid at ordinary temperature), this 
 increase is by no means uniform or even regular. 
 
232 ATOMIC WEIGHT. 
 
 The facts with respect to compound substances are so marked, however, 
 that the chances are that the order of arrangement of elements by atomic 
 weights is not quite correct; that some of the elements in the solid state 
 may have a larger number of atoms than others; and so the proper molec- 
 ular weight of elements (at present in most cases unknown) may be at 
 present recognized only in those few that can be given in regular order of 
 melting-points. 
 
 Here is a group showing such progress : 
 
 Name. Formula. Molecular Weight. Boiling-point. 
 
 Sulphur dioxide, SO 2 64 - 10 C. 
 
 Sulphur trioxide, SO 3 80 46 C. 
 
 In case, however, of the compounds of carbon and 
 hydrogen, several series can be constructed which ad- 
 vance with very striking regularity at once in molecular 
 weight and in boiling-point. 
 
 r zr 7 Molecular Boiling- 
 
 Name. Formula. , . ,, . . * 
 
 Weight. point. 
 
 Methane (marsh gas), CH 4 16 (g as ) 
 
 Ethane, C 2 H 6 30 (gas) 
 
 Propane, C 3 H 8 44 (gas) 
 
 Butane, C 4 H ]0 58 i C. 
 
 Pentane, C 5 H 12 72 38 C. 
 
 Hexane, C 6 H 14 86 71.5 C. 
 
 Heptane, C 7 H 16 100 98.4 C. 
 
 Octane, C 8 H 18 114 125.5 C. 
 
 A Study of Certain Nitrogen Compounds. One use 
 that can be made of boiling-point may be illustrated by 
 a short study of the compounds of nitrogen and oxygen. 
 
 These compounds are as follows : 
 
 Nitrogen monoxide, N 2 O; vapor density, 21.99; melting-point 99 C. 
 
 Nitrogen dioxide, N 2 O 2 or NO, not liquefied at 1 10 C. 
 
 Nitrogen trioxide, N 2 O 3 ; vapor density, 37.95; liquefies . . . 10 C. 
 
ATOMIC WEIGHT. 233 
 
 Nitrogen tetroxide, N 2 O 4 or NO 2 ; v. d. anomalous { solidifies at ~ 9 C. 
 
 I liquid boils at 22 C. 
 
 Nitrogen pentoxide, N 2 O 5 ; solid melts at 30 C. 
 
 The vapor density for nitrogen monoxide (about 22) points to a molec- 
 ular weight 44, and this corresponds to the requirements of the formula N 2 O. 
 The vapor density for nitrogen trioxide (about 38) points to the molecular 
 weight 76. This corresponds to the requirements of the formula N 2 O 3 . 
 
 There are reasons that need not be specified here for adopting for 
 nitrogen pentoxide the formula N 2 O 5 . 
 
 The following question then arises with respect to the 
 two compounds remaining : 
 
 Is the formula N 2 O 2 or the formula NO to be pre- 
 ferred for nitrogen dioxide ? Comparing the boiling- 
 points of this compound with the compound N 2 O, called 
 nitrogen protoxide, it is seen that the boiling-point of 
 N 2 O is very much lower. In accordance with a general 
 rule, substances having lower boiling-points should be of 
 simpler constitution. A substance having the formula 
 N 2 O 2 is of more complex constitution than a substance 
 having the formula N 2 O. If, however, we assign to the 
 substance designated as N 2 O 2 the formula NO, its con- 
 stitution becomes simpler than that of N 2 O ; it then 
 accords with the general rule. 
 
 There are certain facts with respect to the substance 
 designated as N 2 O 4 that lead to the conclusion that this 
 is the correct formula at low temperatures. It is thought 
 at high temperatures it dissociates, forming the simpler 
 molecule NO 2 . (See p. 144.) 
 
 NOTE. It appears probable that the following general law may be 
 safely accepted : 
 
 General Law. The more complex compounds of a series condense 
 more easily to liquids and solids and decompose more readily by heat than 
 the less complex compounds. 
 
234 
 
 ATOMIC WEIGHT. 
 
 IV. 
 
 RELATION OF MOLECULAR FORMULA TO CRYSTALLINE 
 
 FORM. 
 
 Mitscherlich's law of isomorphism may be stated as 
 follows : 
 
 FIG. 129. Mass of alum crystals. 
 
 In general, when two solid compounds are isomor- 
 phous that is, have the same crystalline form, they 
 have the same number of atoms and the same molecular 
 arrangement. 
 
 It hardly needs mention that in applying the law it 
 must be remembered that compound radicles, like am- 
 monium, methylamine, ethylamine, etc., must be counted 
 as elements. 
 
ATOMIC WEIGHT. 
 
 235 
 
 It must be admitted that there are well-recognized cases of substances 
 of similar crystalline form that are evidently not chemically analogous; 
 again, substances possessing most marked chemical resemblances are 
 known that solidify in differing crystalline forms. But while there are 
 many exceptions to the law, and it is not an authoritative guide, yet it is 
 of occasional value in confirming results obtained by other methods. 
 
 i. The law is well illustrated by the alums. A certain substance called 
 alum has been recognized with more or less distinctness for at least two 
 thousand years. During the last century its chemical composition has 
 been distinctly made out. It is usually expressed by the formula 
 
 K 2 S0 4 .A1 2 (S0 4 ) 3 .2 4 H. 2 0. 
 
 FIG. 130. Diagrams showing different forms assumed by crystals of the first or regular 
 
 system. 
 
 It crystallizes easily and distinctly in cubes or regular octohedrons, or some 
 simple modification of these belonging to the first or regular system. 
 
 Within a few years at least a dozen substances have been recognized 
 which bear such marked structural resemblance to ordinary alum that they 
 have all been called alums. 
 
 Examples : 
 
 Potassio-aluminic alum (ordinary alum), K 2 SO 4 A1 2 (SO 4 ) 3 24 H 2 O. 
 Sodio-aluminic alum, Na 2 SO 4 A1 2 (SO 4 ) 3 24 II,O. 
 
 Ammonio-aluminic alum, (NH 4 ) 2 SO 4 A1 2 (SO 4 ) 3 24 H 2 O. 
 
 Potassio-chromic alum, K 2 SO 4 Cr 2 (SO 4 ) 3 24JH 2 O. 
 
 Ammonio-ferric alum. (NH 4 ) 2 SO 4 Fe 2 (SO 4 ) 3 24 H 2 O. 
 
 The fact that these all crystallize in form similar to ordinary alum leads 
 chemists to confidently adopt for them the same general molecular formula 
 as that assigned to ordinary alum. Yet, further, two other inferences are 
 drawn; viz. that potassium, sod'ium, and ammonium are radicles of analo- 
 
236 ATOMIC WEIGHT. 
 
 gous character, and that aluminium, chromium, and iron are also of anal- 
 ogous chemical character. 
 
 2. The substances calcic carbonate, potassic nitrate, and sodic nitrate 
 have some marked crystalline resemblances. Thus a certain form of calcic 
 carbonate, found crystallized in nature and called arragonite, is recognized 
 as having the same crystalline form as potassic nitrate. Again, a slightly 
 different form of calcic carbonate, though of the same chemical composi- 
 tion, but found crystallized in nature in the form called calcspar, has a 
 crystalline form similar to that of common sodic nitrate. 
 
 Here, then, are three substances closely related as to crystalline forms. 
 Analysis has shown that possible formulas for these substances are 
 
 CaC0 3 , 
 KNO 3 , 
 NaNO 3 . 
 
 The crystalline resemblances then favor the adoption of these formulas. 
 It may be added that the formula CaCO 3 leads to the approval of the num- 
 ber 40 as the atomic weight of calcium, and the other formulas favor the 
 employment of the numbers 23 and 39, already well substantiated, for 
 sodium and potassium respectively. 
 
 V. 
 
 RELATION OF MOLECULAR FORMULA TO MOLECULAR 
 STABILITY. 
 
 The question has arisen whether the substance known 
 as nitrogen dioxide should have the formula N 2 O 2 , or 
 the formula NO. Facts already given, under the head 
 of boiling-points, indicate pretty clearly that the formula 
 should be NO. Certain facts with respect to the 
 decomposition of this substance as compared with the 
 decomposition of the substance nitrogen monoxide 
 (N 2 O) sustain this view. 
 
 Thus the general rule is that substances of more complex composition 
 possess less chemical stability than substances of less complex composition. 
 Now when a piece of glowing phosphorus is plunged into the gas nitrogen 
 monoxide (N 2 O), it readily decomposes the molecule, withdrawing oxygen; 
 
ATOMIC WEIGHT. 237 
 
 and the phosphorus continues to burn by combining with this oxygen. 
 But when burning phosphorus is introduced into the gas called nitrogen 
 dioxide (N 2 O 2 or NO), it is extinguished; that is, it does not withdraw the 
 oxygen. The conclusion is that the molecule of nitrogen dioxide is of 
 greater chemical stability than the molecule N. 2 O, and therefore is of simpler 
 constitution than the molecule N 2 O. This favors the assumption that the 
 formula of nitrogen dioxide should be taken as NO, and not as N 2 O 2 . For 
 if the formula were accepted as N 2 O 2 , we should have the more complex 
 molecule," having the greater chemical stability instead of the less, as the 
 general law demands. (See p. 232.) 
 
 VI. 
 
 MOLECULAR FORMULAS SUGGESTED BY RELATIONSHIP. 
 
 The substances marsh gas, methyl alcohol, and formic 
 acid are naturally related. The accepted formulas are 
 as follows : 
 
 Marsh gas . . . CH 4 , 
 
 Methyl alcohol . . CH 3 OH, 
 
 Formic acid . . . HOCHO, or HCOOH. 
 
 So the substances ethylene, ethyl alcohol, and acetic 
 acid are naturally related. The accepted formulas for 
 these are as follows : 
 
 Ethylene . . . C 2 H 4 , 
 
 Ethyl alcohol . . C,H 5 OH, 
 
 Acetic acid . . . HO(C 2 H 3 O), or CH 3 COOH. 
 
 Now the formulas chiefly in question are those of formic acid and 
 acetic acid. But the fact that the marsh gas and methyl alcohol, whos rt 
 formulas are established, have each one atom of carbon, favors the assump- 
 tion that formic acid contains one atom of carbon, and therefore that it has 
 the formula assigned. 
 
 Again, the fact that ethylene and ethyl alcohol, whose formulas are well 
 established, have each two atoms of carbon, favors the assumption that 
 acetic acid has two atoms of carbon, and therefore that it has the formula 
 assigned. 
 
238 ATOMIC WEIGHT. 
 
 VII. 
 
 MOLECULAR FORMULA SUGGESTED BY PRODUCTS OF 
 DECOMPOSITION. 
 
 When the two organic compounds, formic acid and 
 acetic acid, are made to combine with alkaline sub- 
 stances to produce respectively formates and acetates, 
 it is observed that 46 parts of formic acid do the same 
 work as 60 parts of acetic acid. These numbers may 
 then be taken temporarily as the molecular weights of 
 the two substances. Taken in conjunction with other 
 facts of analysis, the following statement may be pre- 
 pared : 
 
 Formic Acid. The formula HCOOH corresponds to 
 molecular weight 46. 
 
 Acetic Acid. The formula CH 3 COOH corresponds 
 to molecular weight 60. 
 
 But analysis has shown that 46 parts of formic acid 
 contain 12 parts of carbon, and that 60 parts of acetic 
 acid contain 24 parts of carbon. 
 
 Next consider the products of decomposition. 
 
 Experiment has shown that when these acids are subjected to the cur- 
 rent of the galvanic battery they are decomposed. Now it is observed 
 that from formic acid but one carbon compound is produced, i.e. carbon 
 dioxide. But from acetic acid two compounds of carbon are produced, 
 carbon monoxide and ethane. These facts suggest that the carbon in 
 formic acid acts somehow as a unit, while in acetic acid there is such a 
 difference in the condition of the carbon that it is easily susceptible of 
 division into at least two parts, the one part doing one thing, the other 
 part doing another thing. 
 
 These facts while not absolutely conclusive favor the opinion that 
 formic acid contains one atom of carbon, while acetic acid contains two 
 atoms of carbon. 
 
ATOMIC WEIGHT. 239 
 
 VIII. 
 
 THE ADOPTED MOLECULAR FORMULA SUPPORTED BY 
 CERTAIN EXCEPTIONAL COMPOUNDS. 
 
 Certain double salts, produced naturally or artificially, 
 may point out the existence of definite molecular 
 groups. 
 
 Thus in the Solvay soda works, at Syracuse, N.Y., a 
 salt was artificially formed in considerable quantity (al- 
 though decomposable by water) which had practically 
 the following formula: MgCO 3 Na 2 CO 3 NaCL This 
 combination was evidently of those single groups which 
 are accepted as molecules. Thus it seems to establish 
 the three formulas adopted for the three substances 
 taking part in it. 1 
 
 IX. 
 
 OTHER ILLUSTRATIONS OF THE CLOSE CONNECTION BE- 
 TWEEN THE PROPERTIES OF SUBSTANCES AND THEIR 
 MOLECULAR WEIGHTS. 
 
 (a) Density of Liquids as related to their Molecular 
 Structure, The following law, called Groshans's law, 
 has been enunciated : 
 
 At the temperature of ebullition, the density of compound bodies in the 
 liquid state is in proportion to the number of the atoms in their molecules. 
 
 (b) The Relation of the Physiological Action of Inorganic 
 Compounds with their Molecular Weights. In a study of 
 the influence of different salts in solution when intro- 
 duced into the blood of living animals, it has been 
 observed that the acid radicle of the salt has but little 
 
 1 Chemical News, Ivii. 3. 
 
24O ATOMIC WEIGHT. 
 
 influence. Any physiological action produced depends 
 almost entirely on the electro-positive component of the 
 salt, i.e. upon the metal. Again, it has been noted that 
 practically all the elements found in organized bodies 
 have atomic weights of less than 40. Thus it appears 
 that all the positive elements among them are monads 
 and dyads. Now, it has been noted that the physio- 
 logical action of substances increases from the monads 
 onward. 
 
 Dr. J. Blake, who has studied this subject, points out that the monads 
 tend to affect but one set of tissues or organs, the pulmonary arteries. 
 The dyads affect two or more, i.e. the centres of vomiting, the voluntary 
 and cardiac muscles, while with elements of higher equivalence the influ- 
 ence is more widespread and therefore more considerable : it extends to the 
 ganglia and even the brain itself. Again, experiments seem to show that 
 the physiological efficiency of substances belonging to one and the same 
 isomorphous group is directly proportional to the atomic weights; i.e. 
 the higher the atomic weight, the greater the action. The law has been 
 studied with respect to the following substances : first, lithium, sodium, 
 rubidium, thallium, silver; second, magnesium, iron, manganese, cobalt, 
 nickel, copper, zinc, cadmium; third, calcium, strontium, barium, lead; 
 fotirth, palladium, platinum, osmium, gold. 
 
 In case of chlorine, bromine, and iodine, however, the action seems to 
 vary inversely as the atomic weights. 
 
 In case of potassium and ammonium the influence is also partially 
 exceptional. 
 
 The whole study is an important one and may be looked upon as likely 
 to offer more valuable information in the future. Thus it may be that the 
 biological relations of chemical substances may assist in determining the 
 position of atoms and molecules in the chemical scale. 1 
 
 (c) The Magnetic Rotary Polarization of Compounds as 
 related to their Chemical Constitution. Chemists have 
 long felt assured that such rotation was dependent 
 
 1 Blake, J., Chemical News, xliii. 191 ; xlv. in ; Ivii. 194. 
 
ATOMIC WEIGHT. 
 
 241 
 
 upon th-i kind of molecules involved as well as their 
 number, yet difficulties in the way of demonstration 
 have prevented the attainment of any satisfactory con- 
 clusions. 
 
 Dr. Perk in has studied this subject very carefully. He has adopted a 
 new system of unit lengths for those portions of the substances experi- 
 mented upon; that is, he has employed such portions of liquid compounds 
 as would produce unit lengths of columns of vapor when in the latter con- 
 dition. As a result, it appears that certain definite relations do exist 
 between this magnetic rotary power and the molecular constitution of 
 bodies. It is not practicable to express these results in a few words. 
 Reference, therefore, must be made to Perkin's original paper. 1 
 
 (d) Freezing-points of Solutions as related to the 
 Molecular Weights of the Substances dissolved : Raoult's 
 Method. This method is of chief importance in case 
 of compounds which cannot be vaporized without dis- 
 sociation. It is based upon a principle described by 
 Coppet, that "salts of analogous constitution, dissolved 
 in quantities proportional to their molecular weights, 
 produce in their solution the same depression of the 
 freezing-point of the solvent." 
 
 Raoult has made a careful study of this subject, and as a result of his 
 work the law seems now to meet general acceptance. 
 
 If C represents the depression in degrees centigrade, P the number of 
 grammes of substance dissolved in 100 grammes of water, and M the 
 molecular weight of the substance, 
 
 = A, the coefficient of depression of the substance; 
 MA = 7\ the molecular depression of the solvent. 
 
 The constant T varies according to the substances used, and according 
 to the solvents employed. Of the latter, water, benzene, and acetic acid 
 have been found most useful. 
 
 i Perkin, W. H., Journal of the London Chemical Society, 1884, Transac- 
 tions, p. 421. 
 
242 ATOMIC WEIGHT. 
 
 This method is of especial value in its application to many organic 
 compounds of high molecular weight. 
 
 One general result of Raoult's studies may be presented in the follow- 
 ing general form : 
 
 While the freezing-point of a pure liquid is constant, every molecule of 
 foreign matter that dissolves occasions the same constant depression of that 
 point. In other words, in dilute solutions the depression of the freezing- 
 point of the solvent varies with the ratio between the numbers of molecules 
 of solvent and numbers of molecules of substance dissolved. 
 
 As respects vapor tension of the liquid, Raoult has shown that its 
 depression bears a relation to the percentage of foreign molecules which is 
 independent of temperature, but if certain proper distinctions are made, is 
 such that it represents a relative depression which may become a definite 
 constant for any substance which that particular liquid may dissolve. 
 
 Raoult's method has been applied to the determination of the molecular 
 weights of certain of the carbohydrates. As a result, the molecular weight 
 of dextrin was found to correspond approximately to the number 6480. 
 This points to the formula 20 (C 12 H 20 O ]0 ). In similar fashion the formula 
 for soluble starch has been suggested to be five times this value, or 
 5 (C 12 H 20 O 10 ) 20 . This would give as a molecular weight of soluble starch 
 about the number 32,4OO. 1 
 
 1 American Chemical Journal, xi. 67 ; also xii. 130 and 142. Chemical 
 News, Ix. 66. 
 
CHAPTER XXIII. 
 ATOMIC WEIGHT (continued). 
 
 SIXTH STEP: BRING ALL THE ATOMIC WEIGHTS INTO 
 ONE TABULAR STATEMENT. THE PERIODIC LAW. 
 
 WHEN the atomic weights of practically all the ele- 
 ments have been provisionally adopted, by methods in- 
 volving the principles already referred to (and perhaps 
 some others), then the "periodic law" may be consid- 
 ered. By its use certain numbers already adopted may 
 be confirmed ; perhaps, on the other hand, it may deter- 
 mine a new selection from the various possible atomic 
 weights. 
 
 The Work of Newlands and of Mendeleeff. The Eng- 
 lish chemist Newlands (in 186364) and the Russian 
 chemist Mendeleeff (in 1869-70) independently pub- 
 lished tables of the elements known at about those 
 dates. They first arranged all known elements in a 
 long list, commencing with that of the lowest atomic 
 weight, and advancing numerically to that of the high- 
 est. In considering this list, they noticed that the 
 elements formed several natural series, the members of 
 a given series showing a periodic progress in chemical 
 properties ; for example, in the kind and amount of 
 
 243 
 
244 ATOMIC WEIGHT. 
 
 atom-fixing power. Upon arranging the several series 
 one above another, it at once appeared that the corre- 
 sponding members of the several series formed natural 
 groups. For example, calcium, strontium, and barium 
 appeared in one group ; so also did phosphorus, arsenic, 
 and antimony ; and also sulphur, selenium, and tel- 
 lurium ; and also chlorine, bromine, and iodine. 
 
 Professor Crookes remarks : " Undoubtedly one of 
 the grandest steps taken in pure chemistry within our 
 epoch has been the discovery of the periodic law. This 
 generalization (as reference to Chemical News will show, 
 vols. vii., x., xii., and xiii.) was in the first place due to 
 Mr. J. A. R. Newlands. It was some time afterwards in- 
 dependently discovered by Mendeleeff, and since been 
 developed both by that eminent savant and by Meyer 
 and Carnelley." 
 
 The total result of the Mendeleeff classification is 
 now known as "the periodic system." It is presented 
 in the table (including 68 elements) found on the next 
 page. 
 
 As soon as the periodic table was adjusted, it sug- 
 gested the important truth that "the properties of an 
 element are a periodic function of its atomic weight." 
 It now appears, therefore, that when a new element is 
 studied and its properties are learned, these properties 
 determine its place in the periodic table. But the place 
 in the table at once suggests which, among several mul- 
 tiples, shall be accepted as the atomic weight. 
 
 The system has now secured very general adoption. Some of its merits 
 are the following : 
 
 FIRST. It is based on the atomic weights, constants which it must 
 be assumed are dependent upon some fundamental characteristics of 
 elements. 
 
ATOMIC WEIGHT. 
 
 245 
 
 
 
 O CO 
 
 00 <* 
 
 a> 01 
 
 rf^ CO 
 
 M 
 
 SERIES. 
 
 
 
 o 
 
 bd 
 
 
 
 
 
 
 
 
 I/I 
 
 
 
 
 
 
 
 H* 
 
 
 
 cr* 
 
 r^ 
 
 r 
 
 
 
 
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 1 1 
 
 Co O 
 Co 00 
 
 > 
 
 00 ON 
 L Oj 
 
 n 
 
 Co to 
 
 vO Co 
 
 a. 
 
 P-3 
 
 " 
 
 r-i 
 
 
 c 
 
 
 Oq 
 
 p 
 
 P 
 
 
 
 
 
 
 
 w 
 
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 P 
 
 
 
 
 
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 5s 
 
 oo ON 
 
 t* 
 
 bd 
 
 n> 
 
 ^'^ 1 
 
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 CfQ 
 
 
 n 
 
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 N 
 
 orcj 
 
 
 O" 
 
 
 
 
 cr 
 
 P 
 
 Kj 
 
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 w 
 
 
 
 
 ll 
 
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 <&~ 
 
 VOVO 
 
 
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 ^ 
 
 
 
 
 
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 ST 
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 S? 
 
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 4- to 
 
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 ro 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 4=> tO 
 
 vo ^r 
 
 Oi CO 
 
 . | . y3 y3 
 
 M * I-H 
 
 ^ 
 
 ^ 
 
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 4^ 01 
 
 NH . 
 
 4^ ' 
 
 
 
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 w 
 
 w 
 
 cr 
 
 Sr 
 
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 ON 
 
 Qb K 
 
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 SP 
 
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 L 
 
 
 g; 
 
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 ^^ 
 
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 4- 01 
 
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 25 
 
 
 
 
 
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 V.D 
 
 
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 ^ ^d /d 
 
 
 
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 ll 
 
 
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 ^ 
 
 
 
 
 
 3" 
 
 ^i 
 
 
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 NH 
 
 
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 Oi 
 
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 3" O 
 
 O ft O 
 
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 3-3 
 
 
 
246 ATOMIC WEIGHT. 
 
 SECOND. It throws most of the elements into groups and series which 
 accord with many of their undoubted geological, physical, and chemical 
 properties. 
 
 It cannot be denied that in this system some elements are brought 
 together that do not appear to be closely related. This is merely equiva- 
 lent to admitting that the system is not yet perfect. Probably, also, some 
 so-called elements are wrongly placed because of their peculiar compound 
 nature. 
 
 THIRD. It helps in the decision as to which of several combining num- 
 bers of an element shall be accepted as its atomic weight. Thus indium 
 might have the number 75.6 or the number 113.4 (one and a-half times 
 the former). The latter number is now chosen under guidance of the 
 periodic law. 
 
 P^OURTH. The periodic table has shown some gaps in the series of 
 numbers representing atomic weights. On this basis as long ago as 
 1871 Mendeleeff predicted the existence of two new elements, and more, 
 he stated their general range of properties. To one he gave the provisional 
 name eka-aluminium. Now in 1876 the element gallium was discovered, 
 and it proved to be the predicted eka-aluminium. So scandium, discovered 
 in 1879, proved to be Mendeleeff's predicted eka-boron. 
 
 The recently discovered element samarium fell into a place not previ- 
 ously occupied, thus contributing to support the system. 
 
 Algebraic Expression of the Periodic Law. Professor 
 Carnelley has recently studied the periodic law with a 
 view to expressing its numerical relations in the form 
 of an algebraic formula. For reasons which are given 
 in detail in the memoir, an expression of the form 
 
 A = c (m + Vz>) 
 
 is adopted, where A represents the atomic weight of 
 the element ; c, a constant ; m, a member of a series in 
 arithmetical progression, depending upon the horizontal 
 series in the periodic table to which the element be- 
 longs ; and v, the maximum valence, or the number of 
 the vertical group of which the element is a member. 
 
ATOMIC WEIGHT. 247 
 
 From a number of approximations Professor Carnelley finds that m is 
 best represented by the value o in the lithium-beryllium-boron, etc., hori- 
 zontal series; by 2], in the sodium series; 5, in the potassium series; and 
 8% 12, 15', 19, 22], etc., in the subsequent series. Thus m is a member 
 of an arithmetical series of which the common difference is 2} for the first 
 three members and 3] for all the rest. On calculating the values of the 
 constant c from the equation 
 
 A 
 
 for 55 of the elements, the numbers are all found to lie between 6.0 and 
 7.2, with a mean value of 6.6. In by far the majority of cases the value is 
 much closer to the mean 6.6 than is represented by the two extreme limits; 
 thus in 35 cases the values lie between 6.45 and 6.75. If the number 6.6, 
 therefore, is adopted as the value of <r, and the atomic weights of the ele- 
 ments are then calculated from the formula 
 
 the calculated atomic weights thus obtained approximate much more closely 
 to the experimental atomic weights than do the numbers derived from an 
 application of the atomic heat approximation of Dulong and Petit. The 
 number 6.6 at once strikes one as being remarkably near to the celebrated 
 6.4 of Dulong and Petit, and Professor Carnelley draws the conclusion that 
 there must be a connection between the two. This assumption appears to 
 be supported by several interesting facts. 1 
 
 Prout's Hypothesis. -As early as 1816 the theory was 
 suggested that the atomic, weights may be represented 
 by numbers that are exact multiples of that of hydro- 
 gen. This led to the further suggestion that possibly 
 hydrogen is a sort of " primordial matter which forms 
 the other elements by successive condensations of it- 
 self." 
 
 The most critical determinations of the atomic weights 
 seem at present to afford numbers that are not integral 
 
 1 Nature, xli. 304. 
 
248 ATOMIC WEIGHT. 
 
 multiples of that for hydrogen. But in most cases the 
 variations are but slight, and it cannot be declared with 
 certainty that the atomic weights at present held may 
 not be subject to corrections such as will in future 
 afford numbers sustaining Prout's proposition. 
 
 Thus recent and careful recalculations of the atomic weights show that 
 " thirty-nine out of sixty-five elements have weights varying each by less 
 than the tenth of a unit from even multiples of the atomic weight of 
 hydrogen." Of the remaining elements, twenty-six have atomic weights 
 that are known to be defectively determined. Thus Prout's hypothesis 
 acquires new interest. 
 
CHAPTER XXIV. 
 
 ATOMIC WEIGHT (continued}. 
 
 ELEMENTARY SUBSTANCES AS MOLECULAR. 
 
 IT is believed that in most cases elementary sub- 
 stances are arranged in groups which may be properly 
 called molecules. 
 
 FIRST. This view is sustained by the volume condi- 
 tions of certain chemical unions. 
 
 Example a. Two volumes of hydrogen and two 
 volumes of chlorine combine to form four volumes of 
 hydrochloric acid gas. In accordance with Avogadro's 
 law, two volumes of hydrogen may be considered as 
 having n molecules of hydrogen, and two volumes of 
 chlorine n molecules of chlorine. Then four volumes 
 of hydrochloric acid gas must have 2n molecules of the 
 substance represented by HC1. But each of these 2n 
 molecules contains at least one atom of hydrogen, a 
 total of 2n atoms of hydrogen. But these 2n atoms 
 of hydrogen are derived from n molecules of hydrogen. 
 Therefore each molecule of hydrogen has two atoms of 
 hydrogen. By a similar process of reasoning it is appar- 
 ent that each molecule of chlorine contains two atoms 
 of chlorine. 
 
 In certain other cases, elements may be shown to contain four atoms in 
 the molecule. 
 
 In some cases, it is true, the number of atoms in a molecule cannot be 
 
 readily determined. 
 
 249 
 
2$o ATOMIC WEIGHT. 
 
 Example b. When sulphur burns in oxygen gas to form the molecule 
 SO 2 , there is neither increase nor decrease of volume; i.e. apparently one 
 molecule of oxygen gas has furnished one molecule of sulphur dioxide, SO 2 . 
 This favors the theory that the amount of oxygen gas in the molecule SO 2 
 is the same as the amount of oxygen gas in one molecule of oxygen. 
 Whence one molecule of oxygen gas appears to consist of two atoms. 
 
 When carbon burns in oxygen gas to form the molecule CO 2 , there is 
 neither increase nor decrease in volume. By a course of reasoning similar 
 to that just employed with respect to sulphur dioxide, the view then brought 
 forward is sustained by the burning of carbon. 
 
 SECOND. This view harmonizes with certain facts of 
 thermo-chem istry. 
 
 Example a. Phosphorus burns in the gas nitrogen 
 monoxide, N 2 O. It also burns in oxygen gas. Carbon, 
 also, will burn in either of these gases. Now it is ob- 
 served that more heat is evolved when the combustible 
 burns in nitrogen monoxide than when it burns in 
 oxygen. 
 
 According to the molecular theory, oxygen has mole- 
 cules as well as nitrogen monoxide. The heat afforded 
 by the combustion represents a difference between the 
 true amount of heat evolved by the chemical action and 
 the amount of heat absorbed in decomposing the mole- 
 cule containing oxygen. The less amount of heat given 
 off by the combustion in pure oxygen suggests that 
 when the phosphorus withdraws an atom of oxygen 
 from its companion atom, more energy is expended than 
 when it draws the oxygen from the companion nitrogen. 
 
 Example b. When the very explosive compound called chloride of 
 nitrogen decomposes, the reaction is as follows : 
 
 2 NC1 3 exploded = N 2 + 3 C1 2 . 
 
 Now when this operation goes on, 38,100 units of heat are liberated. This 
 extraordinary result is believed to be explainable, on the molecular theory, 
 as due to the fact that the affinity of an atom of nitrogen for another atom 
 
ATOMIC WEIGHT. 
 
 of nitrogen, and the affinity of an atom of chlorine for another atom of 
 chlorine, are far greater than is the affinity of the atom of nitrogen for the 
 atoms of chlorine with which it was combined in the explosive substance. 
 
 It is true that nitrogen as an elementary gas, and under certain other 
 circumstances, is very inert. But nitrogen is found as a constituent of a 
 very large number of compounds. Some of these are very stable, and the 
 nitrogen holds to the other constituents with great affinity. Ammonia gas 
 (NH 3 ) is an example. This shows no fundamental lack of affinity in the 
 nitrogen atom. Perhaps the very inertness of the free nitrogen molecule 
 may be due to the affinity binding the two atoms of this molecule together. 
 
 THIRD. The f acts of allotropism accord with this 
 theory. 
 
 Some elementary substances are capable of undergoing 
 a great change in their properties without any change 
 of weight, and without any addition or withdrawal of 
 other kinds of matter. Thus a given portion of ordi- 
 nary oxygen may be changed at least partly into 
 ozone and back again. So ordinary phosphorus may 
 be changed into red phosphorus and back again. One 
 of the forms of such substances is called the allotropic 
 form of it, and this general property of bodies is termed 
 allotropism. The view now held is that such modifica- 
 tions represent some change in the number of atoms in 
 the molecule of the element, or some change in their 
 arrangement within the molecule. 
 
 Occasionally the term allotropism is applied to compounds to describe a 
 kind of physical isomerism. Indeed, the three states of aggregation in 
 which a compound may exist may be considered as representing a sort of 
 allotropism. 
 
 Many natural minerals display a kind of allotropism in their differences 
 from the same substances artificially produced. 
 
 The properties of ozone support the theory in ques- 
 tion. When oxygen changes to ozone, condensation 
 occurs without loss of matter; and when ozone changes 
 
2 $2 ATOMIC WEIGHT. 
 
 to oxygen, corresponding expansion occurs. Ozone is 
 i^ times as heavy as oxygen ; in other words, ozone 
 has the density 24, and oxygen has the density 16. 
 According to Avogadro's law, and the molecular theory 
 of elements, ozone should have the molecular weight 48, 
 and oxygen the molecular weight 32. This means, then, 
 that the molecule of ozone should have three atoms of 
 oxygen, and a molecule of ordinary oxygen should have 
 two atoms of oxygen. 
 
 Now ozone is characterized by two striking properties at first incon- 
 sistent; i.e. it readily oxidizes certain substances, like metallic silver, add- 
 ing oxygen to the silver. Again, it readily reduces certain substances like 
 oxide of silver; i.e. withdrawing oxygen from the silver. But the incon- 
 sistency is immediately explained by the molecular theory. Ozone (O 3 ), 
 by imparting oxygen to silver, is itself changed to the more stable form 
 O. 2 ; i.e. ordinary oxygen. Again, ozone (O 3 ), in withdrawing oxygen 
 from oxide of silver, at once gives rise to 2 O 2 ; i.e. to two molecules of the 
 stable form of oxygen called ordinary oxygen. 
 
 FOURTH. This theory affords the best explanation 
 of the properties of certain compounds like hydrogen 
 dioxide (H 2 O 2 ). Like ozone, it readily withdraws oxygen 
 from certain substances and it readily adds oxygen to 
 certain substances. The apparent inconsistency is at 
 once explained by the molecular theory. By adding 
 oxygen to certain substances the unstable molecule 
 H 2 O 2 is changed to the stable molecule H 2 O. On the 
 other hand, in withdrawing oxygen from certain sub- 
 stances, one atom of oxygen withdrawn unites with one 
 atom of oxygen from the H 2 O 2 , thus producing the sta- 
 ble molecule O 2 . But the existence of such a molecule 
 as O 2 is the very point that this discussion is now 
 intended to support. 
 
 FIFTH. The superior activity of substances in the 
 
ATOMIC WEIGHT. 253 
 
 nascent state is best explained by this theory. It is 
 supposed that atoms of a gas when freshly liberated, 
 in other words, when in the nascent state, have not yet 
 combined to form molecules of that particular element. 
 They are then more ready to combine with other sub- 
 stances than when, as in their ordinary condition, they 
 have to leave companion atoms to do so. 
 
 SIXTH. The theory accords with the facts enunciated 
 in the law of Charles. 
 
 If the expansion of compound gases by increase of heat 
 is due to a forcing apart of a certain number of mole- 
 cules, the fact that the elementary gases obey the same 
 law shows that they must have molecules of the same 
 size as those of compound gases. 
 
 If elementary gaseous substances were composed of 
 single atoms of smaller size, they should be expected to 
 expand at least twice as much as compound gases by 
 accession of heat. 
 
 It must not be supposed that all the substances known 
 to chemists afford satisfactory information as to their 
 molecular structure. This is true of elementary sub- 
 stances, and of compound substances as well. Chemists 
 are still in the dark with respect to the proper molec- 
 ular formulas of certain elements and compounds. For 
 the present, molecular formulas as well as atomic weights 
 are adopted that are recognized as only approximate, and 
 that are likely to demand revision hereafter. 
 
INDEX. 
 
 [THE NUMBERS REFER TO PAGES.] 
 
 Acid, acetic, 237, 238. 
 
 formic, 237, 238. 
 
 hydrochloric, 77, 226, 228. 
 Adhesion between gases and gases, 136. 
 
 between liquids and gases, 134. 
 
 between liquids and liquids, 131. 
 
 between solids and gases, 130. 
 
 between solids and liquids, 114. 
 
 between solids and solids, 114. 
 Affinity, chemical, 3, 141. 
 Air, atmospheric, 139. 
 Allotropism, 251. 
 Alloys, 128. 
 
 fusible, 49. 
 Alum, 126. 
 
 crystalline form of, 235. 
 Amalgams, 129. 
 Ammonia fountain, 134. 
 
 gas, 227, 230. 
 
 gas, liquefaction of, 59. 
 
 gas, thermolysis of, 146. 
 Ammonium, 27. 
 
 Ampere, A. M., 78, 80, 188, 190. 
 Analysis, 26. 
 
 Andrews, Thos., 20, 35, 58. 
 Antiseptics, 165. 
 Arragonite, 236. 
 Atmosphere, terrestrial, 137. 
 Atoms, 3, 14, 15, 29, 30, 31, 32. 
 Attraction, chemical, 168, 170, 187, 188, l 
 Avogadro, 12, 78. 
 
 Bacteria, 156, 158, 165, 166. 
 
 Balance, 7, 12, 138. 
 
 Barium, 3. 
 
 Barometer, 8, 9, 10, 69. 
 
 Becquerel, 188. 
 
 Benzol, 32. 
 
 Berthelot, M., 176, 177. 
 
 Berthollet, 188. 
 
 Berzelius, J. J., 12, 188, 194, 195, 200. 
 
 Bismuth crystals, 108. 
 
 Botany, i. 
 
 Boyle, 63, 64, 65, 67. 
 
 Brodie, 29. 
 
 Bromine, 217, 231. 
 
 Bunsen, ice calorimeter, 214. 
 
 Cadmium, 3. 
 Cailletet's apparatus, 61. 
 
 254 
 
 Calcspar, 236. 
 Calorimetry, 178, 181. 
 Calory, 177. 
 Cane sugar, 124. 
 Capillarity, 115, 116. 
 Carbon, 216. 
 
 dioxide, 139. 
 
 Carnelley, Thos., 29, 244, 246. 
 Carre, ice-machine, 51. 
 Cathctometer, 69. 
 Charles, J. A. C., 65, 66, 68. 
 Chemistry, 2. 
 Chlorine, 216, 231. 
 Cleavage, 103. 
 Cohesion, 101, 102. 
 Colloids, 134. 
 
 Compound radicles, 27, 34. 
 Compounds, specific heat of, 215. 
 Cooke, J. P., 194. 
 Corn starch, 153. 
 Corrosive sublimate, 165. 
 Critical point, 58, 63, 64. 
 Crookes, Wm., 30, 38, 39, 41, 194, 24 
 Cryohydrates, 128. 
 Crystals, IOKJ, no. 
 Crystallization, 103, 104, 108. 
 
 Dalton, John, 6, 29, 196, 200. 
 Davy, Sir H., 188, 189. 
 Deleuil, apparatus, 58. 
 Deliquescence, 122. 
 Deville, H. St. C., 145. 
 Dialysis, 132, 133. 
 Diathermancy, 19. 
 Diffusipmeter, 68. 
 Dissociation, 49, 50. 
 Dobereiner's lamp, 131. 
 Dulong, 213. 
 Dumas, J. B. A., 12, 188. 
 
 Efflorescence, 128. 
 Eka-aluminium, 246. 
 Eka-boron, 246. 
 Electricity, 93, 94, 96, 97. 
 
 related to chemical change, 147. 
 Electric arc, 96, 147. 
 
 spark, 148. 
 Electrolysis, 24, 147. 
 Electroplating, 97, 147. 
 Elements and compounds, 5. 
 
INDEX. 
 
 255 
 
 Elements considered molecular, 249. 
 
 atomic heats of, 215. 
 
 specific heats of, 212. 
 Energy, 82, 172. 
 Ether, the, 83. 
 Ethylene, 237. 
 Eudiometer, 12. 
 Eutexia, 49, 123. 
 Evaporation, 54. 
 Expansion, 85. 
 
 Faraday, M., 29. 
 
 Formula, molecular, 225, 234, 236, 237, 238. 
 
 Freezing mixtures, 123. 
 
 Gas, 23, 36, 57, 67, 209, 212. 
 Gay-Lussac, 75, 76. 
 Geissler tubes, 24, 25, 91. 
 Geology, i. 
 Gerhardt, 12. 
 Germicides, 165. 
 Gladstone, J. H., 29. 
 Gmelin, L., 188. 
 Goniometer, 105. 
 Graham, Thos., 29, 67, 130. 
 Granite, 114. 
 Gravitation, 99. 
 Gunpowder, 171. 
 Guthrie, F., 127. 
 
 Hannay, 131. 
 Heat, 42, 82, 83, 144. 
 
 latent, 53, 54, 57. 
 
 specific, 45, 46. 
 
 from chemical combination, 184, 185. 
 Heterogeneity, 17, 18, 21, 23, 25. 
 History, natural, i. 
 Hoff, J. H. van 't, 118. 
 Hofmann, A. W., 27, 28. 
 Hogarth, 131. 
 Holtz machine, 148. 
 Hydrargyrum, 3. 
 Hydrogen, 70. 
 
 dioxide, 252. 
 
 Ice calory, 177. 
 Iceland spar, 21. 
 Ice-machine, 51, 52, 53, 56. 
 Infusoria, 157. 
 Iodine, 183, 217, 231. 
 
 Kekule, 32. 
 Koch, 155, 156. 
 
 Latent heat, 53, 54, 57. 
 
 Laurent, 12. 
 
 Lavoisier, A. L., 6, 8. 
 
 Laws, of definite proportions, 12, 72. 
 
 of Charles, 65. 
 
 of Dulong and Petit, 213. 
 
 of Gay-Lussac, 75. 
 
 of Henry and Dalton, 71. 
 
 of Groshans, 239. 
 
 of insolubility, 171. 
 
 of isomorphism, 104, 234. 
 
 of Mariotte, 63. 
 
 Laws, of Mitscherlich, 234. 
 
 of multiple proportions, 12, 73. 
 
 of maximum work, 177. 
 
 of volatility, 172. 
 Lead, 128. 
 
 boro-silicate of, 23. 
 Leucomaines, 166. 
 Light, 20, 22, 86, 87, 88, 146. 
 Liquid, 36, 52, 59, 60, 132, 134, 239. 
 Liquefaction, 47, 48, 121. 
 Lockyer, J. N., 29, 92, 149. 
 
 Mallet, J. W., 194. 
 
 Mariotte, law, 63. 
 
 Marsh gas, 227, 230, 232. 
 
 Masses, 3, 6, 99. 
 
 Matter, 3, 7, 15, 35, 38, 63, 172. 
 
 Matthieson, 130. 
 
 Maxwell, Clerk, 176. 
 
 Mechanics, 2. 
 
 Mercury, 3. 
 
 as a solvent, 119. 
 Melting, 43, 46, 47, 48. 
 Mendeleeff, D. I., 29, 197, 243. 
 Methyl alcohol, 237. 
 Meyer, L., 29, 244. 
 Meyer, V., 146. 
 Microbes, 150, 155, 157, 159. 
 Microcrith, 3, 24, 75, 199. 
 Mills, 29. 
 
 Mitscherlich, 104, 234. 
 Moistening, 115. 
 Molecule, 3, 4, 15, 28, 78, 101. 
 Muir, M. M. P., 176, 177. 
 Mycoderma, aceti, 158, 159. 
 
 vini, 158. 
 
 Nascent state, 253. 
 
 Naumann, A., 176. 
 
 Newlands, J. A. R., 29, 197, 243. 
 
 Newton, Sir Isaac, 188. 
 
 Nitrogen, oxides of, 73, 74, 232, 233, 236 
 
 Occlusion, 130. 
 
 Organic compounds, 151, 153. 
 
 Organized bodies, 105, 154. 
 
 Osmose, 132. 
 
 Oxygen, study of, 219. 
 
 unit of atomic weight, 200, 202. 
 Ozone, 5, 23, 251, 252. 
 
 Palladium, 130. 
 
 Pasteur, L., 34, 155, 160, 161, 163. 
 
 Pattinson, 128. 
 
 Periodic law, 243, 246. 
 
 Perkin, W. H., 241. 
 
 Petit, 213. 
 
 Philosophy, natural, 2. 
 
 Physics, 2. 
 
 Platinum, 130. 
 
 Polariscope, 23. 
 
 Polarity, 102. 
 
 Polarization, 22, 240. 
 
 Potassium, 205, 218. 
 
 Potassic nitrate, no. 
 
 Potato starch, 152. 
 
2 5 6 
 
 INDEX. 
 
 Pressure, in liquefaction of gases, 58. 
 
 Protagon, 5. 
 
 Protyle, 30. 
 
 Front's hypothesis, 247. 
 
 Ptomaines, 166. 
 
 Quartz, 112. 
 
 Radicles, compound, 27, 234. 
 Radiometer, 40. 
 Raoult, 38, 118, 241. 
 Rayleigh, 194. 
 Refraction, 21, 22. 
 Regnault, H. V., 211. 
 Ruhmkorff, 148, 150. 
 
 Saccharimeter, 23. 
 Sodium, 205, 217. 
 
 chloride, 26. 
 
 sulphate, 126. 
 Salt, crystals of, in. 
 Saturation, 122. 
 Science, i. 
 
 Silver, 127, 136, 218, 223. 
 Slit, for spectroscope, 89. 
 Solid, 36, 124. 
 Solubility, of gases, 71, 134, 135. 
 
 of solids, 118. 
 
 Specific heat, 214, 216, 247. 
 Spectroscope, 86, 87, 89, 90, 91. 
 Spencer, Herbert, 29. 
 Spheroidal state, 116. 
 Starch, 5, 152, 153. 
 Stas, 194. 
 
 Steam, latent heat of, 53. 
 Sterilizing apparatus, 160, 162. 
 Stokes, 29. 
 Sugar, no, 125. 
 Sulphur, a study of, 222. 
 
 crystals, 109. 
 
 dioxide, 56, 60, 232. 
 
 density of vapor, 146, 222. 
 
 specific heat of, 223. 
 
 Sulphur trioxide, 232. 
 
 use as disinfectant, 164. 
 Sulphuretted hydrogen, 222. 
 Synthesis, 26. 
 Syphon, 71. 
 Systems, of crystals, 105. 
 
 Temperature, 58, 84. 
 Theory, atomic, 6. 
 Thermal units, 177. 
 Thermo-chemistry, 183. 
 Thermo-dynamics, laws of, 176. 
 Thermometer, 65, 67, 84, 85. 
 Thermolysis, 144. 
 Thomson, Sir Wm., 31. 
 Thomsen, Julius, 38, 176, 177. 
 Type compounds, 202, 225. 
 
 Uranium, 200. 
 
 Vapor, 37. 
 
 density, 211, 213. 
 
 Pressure, 52. 
 processes, 150. 
 
 Water, 226, 229. 
 
 as a solvent, 119. 
 
 calory, 177. 
 
 electrolysis of, 148. 
 
 latent heat of, 47. 
 
 spheroidal state of, 117. 
 
 vapor, 77, 219. 
 Weight, atomic, 193, 196, 199, 204, 209, 245. 
 
 molecular, 231, 239, 241. 
 Wheat starch, 152. 
 Wislicenus, 34. 
 Work, 191. 
 
 Yeast plant, 152. 
 
 Zero, absolute, 85. 
 Zoology, i. 
 Zinc, 3, 120, 168. 
 
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