UC-NRLF 
 
 D E SE5 bl3 
 
 
 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE. 
 
 1034 - 
 
 Atmospheric Actino'^.etry 
 
 AND THE 
 
 ACTINIC CONSTITUTION OF THE ATMOSPHERE. 
 
 E. DUCLAUX, 
 
 Professor of Physics in the Agronomical Insli/iiU, Paris. 
 
 CITY OF WASHINGTON : 
 PUBLISHED BY THE SMITHSONIAN INSTITUTION. 
 
 1896. 
 
SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE. 
 1034 
 
 Ibobghins jfunb. 
 
 Atmospheric Actinometry 
 
 AND THE 
 
 ACTINIC CONSTITUTION OF THE ATMOSPHERE. 
 
 BY 
 
 E. DUCLAUX, 
 
 Professor of Physics in the Agronomical Institute^ Paris. 
 
 • • • « • • 
 
 • • • • « « • 
 « « •• • «• 
 
 CITY OF WASHINGTON : 
 PUBLISHED BY THE SMITHSONIAN INSTITUTION. 
 
 1896. 
 
Ube 'Rntcherbocber prese, Dew ]|2orlt 
 
ADVERTISEMENT. 
 
 The present memoir is a translation of the treatise entitled " Sur 
 I'actinom^trie atmosph^rique et sur la constitution actinique de I'atmos- 
 phere," submitted by Professor Emile Duclaux, in competition for one of 
 the Hodgkins Fund prizes offered by the Smithsonian Institution in a 
 circular dated March 31, 1893. The competition closed December 31, 
 1894; and on August 9, 1895, the Award Committee, having completed 
 its examination of the 218 papers submitted by contestants, granted 
 honorable mention to Professor Duclaux and recommended his memoir 
 for publication by the Smithsonian Institution. 
 
 The Committee was composed of the following members : the Secre- 
 tary of the Institution S. P. Langley, Chairman, ex-officio : Doctor Gr. 
 Brown Goode, appointed by the Secretary of the Smithsonian Institu- 
 tion ; Assistant Surgeon-General John S. Billings, appointed by the 
 President of the National Academy of Sciences ; and Professor M. W. 
 Harrington, appointed by the President of the American Association for 
 the Advancement of Science. The Foreign Advisory Committee, as first 
 constituted, was represented by Monsieur J. Janssen, Professor T. H. 
 Huxley, and Professor von Helmholtz ; and after the death of the latter, 
 Doctor W. von Bezold was added. 
 
 S. P. LANGLEY, 
 
 SECBETABY. 
 
 Washington City, May, 1896. 
 
 1283: 
 
Digitized by the Internet Archive 
 
 in 2007 with funding from 
 
 IVIicrosoft Corporation 
 
 http://www.archive.org/details/atmosphericactinOOduclrich 
 
Atmospheric Actinometry and the Actinic Constitution 
 
 OF THE Atmosphere. 
 
 By E. Duclaux, 
 
 Professor of Physics in the Agronomical Institute^ Paris. 
 
 ATMOSPHERIC ACTINOMETRY. 
 
 The progress made by science leads us more and more to attribute to chemical 
 rays a special action, which is different from and, to a certain extent, independent 
 of that of the calorific and luminous rays. The chemical radiations of the sun, 
 reaching the limits of our atmosphere, become modified while passing through it, 
 according to a law which is peculiar to them ; and, so far as can be seen in so new 
 a subject, their absorption is not the same as that of the calorific or luminous parts 
 of the spectrum. 
 
 Photographers, especially those who take landscapes, well know that days 
 which are equally warm or equally luminous do not always give the same results 
 for the same length of exposure, and that there are days when, for some unknown 
 reason, the chemical impression is much slower than on others. 
 
 Another argument may be drawn from what often happens in northern lands, 
 where vegetation, which is well known to be specially susceptible to the power of 
 chemical rays, makes much more rapid progress than in temperate I'egions, notwith- 
 standing the fainter light and the lower temperature. 
 
 To what are such differences due ? What law does the chemical absorption 
 of the atmosphere obey, and on what does it depend ? Ought we to attribute it to 
 its normal elements: oxygen, nitrogen, carbonic acid, and water vapor? Then it 
 should have some general uniformity. Or ought we to see in it, on the con- 
 trary, the action of solid or volatile elements, which incessantly reach it from the 
 bare or from the cultivated soil ? Then it should have a local character, leading 
 to a multiplicity of chemical climates. These are very importatft questions, for 
 
2 ATMOSPHERIC ACTINOMETRY 
 
 which science has as yet no answer ; not that the subject has not already been 
 thoroughly investigated, but because in all the actinometric inquiries proposed so 
 fai', sufficient care has not been bestowed upon the separation of chemical action 
 from luminous and calorific effects. 
 
 The process which best shows the incorrectness of the methods employed 
 heretofore is that of Messi's. Bunsen and Roscoe, which depends upon a mixture 
 of chlorine and of hydrogen, exposed to the light. The intensity of the chemical 
 action is then estimated by the quantity of hydrochloric acid formed in a given time, 
 or rather by the diminution of volume which necessarily follows. This method 
 has two grave defects. One is that reaction may take place from the effect of heat 
 quite as well as from that of chemical rays, and that consequently it does not 
 separate the two actions which it is important to isolate. The second, much more 
 sei'ious, defect is this, that the reaction is extremely exothermic and continues, 
 when once begun, under the influence of the heat which it develops. There is, 
 therefore, no proportionality between the active cause and the effect it produces. 
 The cause is simply provocative and starts a mechanism, which continues to work 
 independently. It is true that an effort is made to reduce to a minimum the work 
 of this mechanism, by operating only with very small quantities of gas and by 
 multiplying the cooling surfaces, in such a way that the phenomenon constantly 
 requires a new excitation in order to continue. But this is not suflScient to relieve 
 the method of the charge of lacking proportionality between cause and effect, which 
 renders the measurements almost illusoiy, in spite of the care taken by Messrs. 
 Bunsen and Roscoe to discuss them. 
 
 We find the same defects, though perhaps a little less seriously, in the often 
 employed method which depends upon the reduction of ferric oxalate by light. 
 Since the first observation by Dobereiner, H. Draper, Marchand, and G. Lemoine 
 have studied this reaction. As in the preceding case the oxalate, or the 
 equivalent mixture of feri'ic chloride and oxalic acid, is reduced by the action of 
 heat alone, and although this reduction is slow, it operates as a source of error. 
 Moreover, the liquid is colored and loses its color in proportion as the pi'ocess 
 continues. Hence the conditions of absorption are modified during the process 
 and this by a phenomenon which is to a certain degree external. Finally, the re- 
 action is still sufficiently exothermic to require that this property should be taken 
 into account. All these defects have been corrected, so far as possible, by M. G. 
 Lemoine, who has for some time been making a careful study of the process, but 
 the method loses thus that neatness and that simplicity w^hich are so desirable. 
 
 The ideal would be attained by the discoveiy of a limpid and transparent 
 liquid which would not change while the reaction went on, becoming the seat of 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 3 
 
 an easily measurable, chemical phenomenon, which could be the result of no action 
 except that of heat. Can we go any faither in our demands and require that it 
 should not be exothermic in any degree ? M. Berthelot does not think so, and be- 
 lieves that the addition of enei'gy resulting from the absoi'ption of calorific, lumi- 
 nous, or chemical radiation would not be sufficient to produce a chemical phenomenon 
 which, while going on, would not give rise to a small amount of heat. . I do not 
 very well see why a calorific absorption of solar radiation might not compensate 
 for some heat of combination, and even permit a slightly endothermic reaction to 
 appeal'. I have endeavored to discover some simple method, which, complying 
 with this programme, could be interpreted without ambiguity, but I have not 
 succeeded. I have been compelled to content myself with an old, well known 
 reaction — the oxidation which weak solutions of oxalic acid undergo upon exposure 
 to light. 
 
 These solutions are and remain transparent. The oxalic acid in them is ti-ans- 
 formed into carbonic acid, which disappears by diffusion, so that the oxidation 
 which it has undergone can be easily ascertained by an acidimetric determination- 
 made before and after its exposure to light. The reaction thus produced is faintly 
 exothermic, to be sure, but as only very weak solutions are taken, for it is well not 
 to exceed 2 or 3 grammes of crystallized oxalic acid per litre, thei-e is no reason 
 why we should be troubled about the error which arises from this fact. Moreover, 
 the liberation of heat, which results from combustion, even if it should be percep- 
 tible, would i-emain without effect, for oxalic acid oxidizes onl}^ with extreme slow- 
 ness under the influence of heat alone. 
 
 Exp. — 10 c. c. of a solution of oxalic acid, titrating 19 c. c. of lime-water per litre, 
 
 was heated on a water bath to nearly 95°, in a flask of 125 c. c. 
 
 After heating 4 hours, the titre is 18.5 c. c. Loss 2.6 per cent. 
 " " 8 " " " 18.0 " " 5.2 " 
 
 Exp. — 10 c. c. of another solution, titrating 16.6 c. c. of lime-water per litre, was 
 
 heated an hour and a half to 115°. 
 
 The titre falls only to 16.2 c. c. and 16.1 c. c. 
 During the days of greatest heat, the temperature hardly exceeds 50° in the 
 shallow vessels, in which the liquid is exposed to the sun. It may, therefore, be 
 assumed that neither the solar heat nor the heat produced by combustion have any 
 percej)tible effect upon the transformation of the oxalic acid, which may, on fine 
 days, reach or even exceed 50 per cent of the acid contained in the solution. 
 
 As the calorific rays have hardly any power, it would be desirable to eliminate 
 the action of the luminous rays also, but the chemical radiations are so closely in- 
 termingled with the latter that it is difficult to sepai-ate them. Let us be content, 
 
4 - ATMOSPHERIC ACTINOMETRY 
 
 therefore, for tlie present, to know that our solution of oxalic acid is peculiarly 
 affected by the action of the luminous and the chemical parts of the spectrum. 
 We shall soon find reasons to believe that it is the chemical part alone which acts. 
 But we have first to investigate the manner by which combustion is produced 
 before we can determine what influences cause it. 
 
 Study of the Pkocess. 
 
 Oxalic acid, dissolved in water and exposed to light, absorbs oxygen, and 
 changes almost entirely into carbonic acid. There appears also a little formic acid, 
 but in almost infinitesimal quantities. Hence it follows, as we have seen, that we 
 can ascei'tain the quantity of oxidized acid by a simple acidimetric detei-mination. 
 
 INl'LUENCE OF CONCENTRATION. 
 
 In order to study the actinometric process, the first thing to discover is the 
 degree of concentration which gives the lai'gest amount of sensibility. In order to 
 know this I exposed to the sun, under precisely the same conditions, during three 
 fine days from June 4th to June 6th, including about 36 houi's of insolation, four 
 liquids, containing, respectively, per litre : 
 
 ^rm. 
 
 grin. 
 
 grm. 
 
 grm. 
 
 63 
 
 31-5 
 
 12.6 
 
 6.3 of oxalic acid ; 
 
 I 
 
 4 
 
 i 
 
 t'^ of an equivalent, 
 
 that is, 
 
 per litre. At the end of this time, an acidimetric analysis gave nie the quantities 
 
 of acid which had been burnt, and I computed from this the piopoi-tion of acid 
 
 which had disappeared from each of the vessels. 
 
 The figures wei-e the following, counted in milligrammes per litre : 
 
 1 Equiv. I Equiv. ^ Equiv. -jV Equiv. 
 
 Quantity of acid burnt, 2,500 2,800 4,700 3,300 
 
 Proportion, 4^ 9^ 38^ 52^ 
 
 To reach the maximum in the absolute quantity of acid consumed we nuist, 
 therefoi-e, operate with solutions neither too concentrated nor too weak. Solutions 
 which are too concentrated oxidize slowly, and the variation in chemical value is 
 often noted with difficulty. It is, on the other hand, very easy to measure this 
 variation with solutions which are rather weak, because it represents a notable 
 fraction of the primitive value. But, on the other hand, when the liquid has been 
 weakened by the sun, the last portions burn quite slowly. There aie, therefore, 
 two dangers to avoid. 
 
 After various trials I decided upon a solution whose variation of titre during 
 the most favorable days should not exceed one half of the initial value. This is a 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 5 
 
 solution containing about ^ an equivalent or about 3 grammes of oxalic acid per 
 litre. Ten cubic centimetres of this liquid are saturated by about an equal volume 
 of common lime-water, so that the daily variation of the titre amounts to 4 or 5 c. c. 
 of lime-water, a quantity which can be measured down to j^^ by means of a 
 burette. The accuracy which we thus obtain is moi'e than sufficient, as we shall 
 presently see. 
 
 rNFLUENCE OF THE DEPTH OF SOLUTION. 
 
 In discussing the question of oxidation, we must consider the part played by 
 the ease with which oxygen penetrates into the depths of the solution. We can 
 easily calculate that the 10 c. c. of oxalic solution, which are used in each one of 
 the experiments, require for complete combustion about 3 c. c. of oxygen, a quantity 
 greatly superior to that which is alieady disvsolved. Whatever the facility may be 
 with which this gas penetrates into a liquid, which is subjected for 8 to 10 hours 
 to insolation in free contact with the air, we may well ask if a solution of oxalic 
 acid oxidizes in the sun in the same way in a vessel of shallow depth, in which its 
 thickness is small, as in a cone-shaped glass or in a round tube. The following 
 experiment fui-nishes an answer to this question : 
 
 Mep. — On August 16th, 17th, and 18th, I exposed to the sun 10 c. c. of a -^\- 
 normal solution of oxalic acid, as follows : 
 
 (a) In a cone-shaped glass, 
 
 (b) In an ordinary test tube, 
 
 (c) In a Bohemian glass matti'ass with flat bottom. 
 
 To secure uniformity of tempei'ature, the cylindrical tube b was placed 
 upright in the mattrass c ; the exposure continued from 8 o'clock a.m. till 
 3.30 in the evening. The following proportions of acid were consumed : 
 
 
 a 
 
 b 
 
 c 
 
 August i6th, 
 
 29^ 
 
 -% 
 
 65 
 
 " lyth, 
 
 34 
 
 14 
 
 97 
 
 " 1 8th, 
 
 34 
 
 '3 
 
 84 
 
 " 19th, 
 
 31 
 
 14 
 
 87 
 
 Thus, eveiything else being equal, the proportion of acid consumed is nuich 
 greater in a vessel with a flat bottom than in a cylindiical tube. The difference is 
 indeed so very striking, that the difficulty with which oxygen penetrates the solu- 
 tion does not suffice to explain it. 
 
 A combustion of 13 per cent, produced in 7 hours in the 10 c. c. of liquid, 
 contained in the mattrass b, has not required more than 0.4 c. c. of oxygen ; in 
 other words, about 6 times the normal quantity dissolved in the solution. When 
 
6 ATMOSPHERIC ACTINOMETRY 
 
 we think of the rapidity witli vvhicli de-aerated water aerates itself anew, it is hard 
 to believe that it was the oxygen wliich was wanting, and we are thus led to be- 
 lieve that the chemical action was at fault. If the incident ray does not bring with 
 it an excess of chemical enei'gy, the superficial la3^ers absorb as much as is available, 
 and thei'e is none left for the lower strata, even though all the needful oxygen 
 should be at hand to burn the acid which is present. 
 
 The question is of some importance, because it teaches us the quantum of 
 chemical action which may be expected from light in the vicinity of the soil, and 
 consequently, also, the degree of atmospheric absorption. In oi'der to get informa- 
 tion on this subject, let us operate with shallow, cylindrical vessels, which are at 
 most a centimetre high at the rim, so that there can be no stagnation of air above the 
 liqiiid, and that the oxygen always has easy access to the latter. If the actinic 
 influence is deficient in the incident light, we must be able to put in evidence the 
 influence of the surface and the depth of the liquid. For equal depths the com- 
 bustion will have to be propoi-tional to the surface. Foi' equal surfaces with dif- 
 ferent depths, combustion, if limited to the superficial layeis, should not increase 
 with the volume and the depth of the solution, or at least not inci-ease so rapidly. 
 This is exactly what experience shows. 
 
 Exp. — ^Into two cylindrical, very shallow vessels, having the same surface, I poured 
 10 and 20 c. c. of a half-deci-normal solution of oxalic acid. After a rather 
 dark and somewhat stormy day, I find that 28 per cent, of the acid has 
 been burnt in the vessel that held 10 c. c, and only 23 per cent, in the other. 
 As it held twice as much liquid as the other, the absolute quantities of 
 acid burnt are relatively 28 and 46, while the depths of solution were in 
 the ratio of 1 to 2. Combustion, therefore, increases less quickly than 
 depth. As the latter has not exceeded a centimetre in the vessel in 
 which it was greatest, and as, moreover, the total combustion was very 
 slight, we cannot admit that oxygen was wanting. But the solar rays, 
 deprived of their chemical radiations, which were rendered active by their 
 passage thiough the superficial strata, reached the lower layers very nuich 
 weakened, although the luminous transparency of the two liquids was 
 
 perfect. 
 There exists then a kind of shifting of the actinic rays during the passage of 
 the light through the first layers which it encounters; and, whether these rays are 
 not abundant or whether the absorption be very efficient and the medium very 
 opaque for them, the weakening process is very rapid. In return, when the surface 
 alone is allowed to vary, while the height of the liquid remains unchanged, the 
 effect of combustion is proportional to the surface, and consequently to the volume. 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 7 
 
 Myp. — I procured two cylindrical vessels with flat bottoms, of Bohemian glass, 
 the bottom surfaces of which were as 1 to 2. I exposed them to the sun, 
 one with 10 c, c, the other with 20 c. c. of one and the same half-deci-norraal 
 solution of oxalic acid. The quantities of burnt acid have always been in 
 the propoi-tion of 1 to 2, in a long sei-ies of experiments, with an approxi- 
 mation equal to that which the process of analysis should demand. 
 We shall have to avail ourselves of all these results when we try to ascertain 
 the cause of atmospheric absorption. Let us be content, for the time being, with 
 drawing a practical conclusion from them, namely, that it is desirable always to 
 work with vessels of the same dimensions, and with e<pial quantities of solution, if 
 we wish to obtain figures that can be compared with each other. 
 
 I employ small blown vessels of Bohemian glass, with flat bottoms, such as are 
 found in trade. I choose tliem of the same dimensions, or nearly so, which can 
 easily be ascertained by fitting together their edges and noting whether they have 
 nearly the same external diameter. It is not necessary to carry accuracy any 
 fai-ther, considering all the inevitable irregularities connected with measurement. 
 
 Those which I have used measured about 4.5 centimetres in diaineter, and 
 10 c. c. of liquid had there a thickness of about 6 millimetres. When I was at work 
 in the country, in the Cantal or in the Puy-de-Dome, where the clouds of atmos- 
 pheric dust are not calcareous, I left them freely exposed to the air upon a small 
 table, so placed as to face the south and to expose them to the sun all day long. 
 The heating which takes place in them is never very great, as the following 
 experiments show ; although they were not made in a flat vessel, but in a cone- 
 shaped glass with a foot. The heating is less by 4° or 5° C. when working with 
 a flat vessel. 
 
 Mi^. — The same glass with a foot and containing 10 c. c. of a half-deci-normal 
 solution of oxalic acid was exposed from the 15th to the 27th of August, 
 1885, daily to the sun. Every day the mean pressure, the maximum 
 temperature of the liquid, and the aspect of the sky were carefully noted. 
 Here follow the proportions of acid burnt on the different days, during 
 which the weather was very fine. The experiments were made at Fau, in 
 the Cantal, at an altitude of about 700 metres. 
 
ATMOSPHERIC ACTINOMETRY 
 
 Date. 
 
 Barom. 
 
 Pressure. 
 
 Max. Temper. 
 
 Combustion. 
 
 Condition of the Sky. 
 
 August 15. 
 
 712 
 
 mm. 
 
 36°. 8 
 
 21 i 
 
 Fine. Slight cirrus. 
 
 " 16. 
 
 712 
 
 t< 
 
 37°-S 
 
 29 i 
 
 Fine. 
 
 'T- 
 
 710 
 
 
 37°.5 
 
 34 i 
 
 
 IS. 
 
 710 
 
 
 37°-S 
 
 32^ 
 
 
 " 19- 
 
 70s 
 
 
 3S°-9 
 
 32 i> 
 
 East wind. Clear sky. 
 
 " 2C. 
 
 705 
 
 
 3S''.2 
 
 30 i 
 
 t< tt U (( 
 
 " 21. 
 
 70s 
 
 
 30° 
 
 24 i 
 
 Cirro-cumulus. Fresh weather. 
 
 " 22. 
 
 70s 
 
 
 25° 
 
 24 ^ 
 
 Sky clouded in the morning. 
 
 " 23- 
 
 705 
 
 
 22° 
 
 22 <f, 
 
 Cumulus. 
 
 24. 
 
 710 
 
 
 
 
 30 
 
 27 i 
 
 Sky overcast. 
 
 " 25. 
 
 710 
 
 
 2S° 
 
 25 i 
 
 Stormy weather. Rain at night. 
 
 26. 
 
 705 
 
 
 30° 
 
 30 i 
 
 Cumulus. 
 
 " 27- 
 
 700 
 
 
 29" 
 
 26 ^ 
 
 
 The most active combustions correspond to the highest maximum temperatures, 
 but only because both of them indicate, each in its own way, the presence of a 
 livelier and more active sunlight. When the sky is overcast or shows cumuli, 
 the solar combustion may be more powerful than when there are cirri, even though 
 the maximum temperature should be lower. 
 
 In no case, as will be seen, has the temperature of the solution risen to a suffi- 
 ciently high level to affect the chemical combustion which takes place there. We 
 may, however, if we wish it, secure ourselves against this cause of eri'or by causing 
 the light vessels which contain the solution of oxalic acid, to float on a water-bath. 
 They will then, during the day on which they are exposed to the sun, be heated a 
 few degrees only. This is a method which I have adopted only during the hottest 
 and driest days. The water-bath served as much to resti'ain evaporation as to 
 prevent heating of the solution. 
 
 INFLUENCE OF THE AGE OF THE SOLUTION. 
 
 We now reach an unexpected fact, namely : that a fresh solution of oxalic 
 acid does not behave like an older solution of the same strength, and appears much 
 more refractory to the action of the sun. It becomes sensitive only very slowly, 
 and it requires even several weeks for that end, when it is kept in diffused light. 
 Etcp. — On September 5, 1885, I compared an old solution of oxalic acid, con- 
 taining -^ equivalent (1.575 grm.) of this acid per litre, with another 
 liquid, which I prepared at that moment, of the same sti'ength. The com- 
 mon titre of these two solutions amounted to 22.8 c. c. of lime-water 
 for 20 cubic centimetres. 
 
 At the close of the day (September 5th), which had been rather 
 foggy, two insolated vessels containing the older liquid titrated togethei- 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 9 
 
 16.2 c. c. of the same lime-water and had consequently lost 6.6 c. c. of 
 tlieir original strength. Two vessels with the new solution titrated to- 
 gether 21.7 c. c. and had consequently lost not more than 1.1 c. c. The 
 new solution is therefore nearly 6 times less sensitive than the other. 
 
 The next day, the weather being fine, the losses amounted to 1.5 c. c. 
 for the new solution and to 8.5 c. c. for the old. This is about the same 
 ratio as on the day before. 
 
 On September 12th, after a fine day, four trials gave me the same 
 results, losses of 9.3 c. c. for the old solution, and of 5.9 c. c. foi- the newly 
 made. The difference in sensitiveness was less marked than six days 
 previously. 
 
 On September 25th, twenty days latei-, the losses became 8.6 c. c. for 
 
 the first liquid, and 7.7 c. c. for the second. This is not yet equality, 
 
 which was reached only in the month of October, after a little more than 
 
 a month. 
 
 The fact that two liquids of different ages reach at the end of some time the 
 
 same degree of sensitiveness, proves that thei-e must be a maximum. We shall, 
 
 however, soon see that this is not a maximum mcuximcyrnm. However this may be, 
 
 if the solution of oxalic acid has once reached this maximum, it differs in no way 
 
 from what it was at first, neither from a chemical nor from a physical point of 
 
 view ; it gives by evaporation the same crystallized acid, and its acidiraetric value 
 
 is unchanged. A molecular activity, however, has been at work, upon which I 
 
 shall not dwell just here. I will state now only two important facts concerning it: 
 
 one is that it requires time for its completion, and the other that it betrays itself 
 
 by easier oxidizability under the influence of solar radiations. 
 
 The only phenomenon which in our present state of knowledge may be com- 
 pared to tliat which we have just discovered, is the increase of sensitiveness 
 observed in sensitized collodion which has been allowed to rest and to grow old 
 for a few days. This fact is well known to photographers. It may make us think 
 also, by analogy, of the valuations in the rotatory power of sugar solutions, some 
 hours after their preparation up to the moment when they become stable. It is 
 admitted that these few hours are necessary to enable the sugar molecules to spread 
 uniformly throughout the solution and to assume the orientation necessary to 
 stable equilibrium. But all these analogies are remote. The phenomenon deserves 
 being investigated by itself, and we have here only to face its practical conse- 
 quences. 
 
 These may be summed up in a few words : that it is desirable to allow the oxalic 
 acid solution to acquire such sensitiveness before using. This is all the easier since 
 
10 ATMOSPHERIC ACTINOMETRY 
 
 these solutions can become sensitive in a concentrated state and preserve this sensi- 
 tiveness even after being diluted. One can then provide a mother-liquor, so to 
 speak, which may be made sensitive and w^hich afterwards may be diluted as 
 necessity arises. Ordinarily I used to prepare a normal solution to the amount of 
 several litres, containing 13 grammes per litre, which I kept for some weeks under 
 diffused light, and subsequently diluted, in fractions, to the twentieth or fortieth 
 degree. One litre of this mothei'-liquid, rendered duly sensitive, may thus serve 
 for 2000 tests. In all the comparative experiments which will be mentioned in 
 this memoir, I have always taken pains to work with identical liquids and such as 
 had the same sensitiveness. 
 
 We are now possessed of oui" actual working process, which amounts to 
 this : To expose to the sun during the day a shallow dish, containing 20 c. c. of a 
 half-deci-normal solution of oxalic acid, which has become sensitive by time, and to 
 measure at the close of the day, by a titration with lime-water, the quantity of acid 
 which has disappeared by oxidation. 
 
 Let us now see what results have been obtained by this process. 
 
 ACTINOMETRIC MEASUREMENTS. 
 
 Since the year 1885 I have made several series of actinometric measurements, 
 especially dui'ing fine weather and at times when I was sufficiently master of my 
 own time to secure to them the regularity which they require. All these experi- 
 ments, made at different times and at different places, are not absolutely alike, since 
 the solutions used might have undergone some change. But such variations amount 
 to little from one year to another, and to almost nothing in the course of the 
 same year, as I have been able to determine repeatedly ; for every time when I 
 changed the solution, I exposed simultaneously two or more vessels with old and 
 with new material, and I always found that the solar combustion was the same for 
 both, up to that degree of approximation which the measurements demand. 
 
 While operating with two or more vessels containing one and the same liquid, 
 it does not always happen that we find the same result for all at the close of the 
 day. There are irregularities in the process, some of which will be explained 
 presently, while the others have until now defied all efforts at explanation, so sud- 
 den are they and so exceptional. There is no other remedy for this than to elimi- 
 nate such out-of-the-way cases, which are always rare, making every day a trial with 
 3 or 4 vessels and keeping, of the figures thus obtained, only those which are con- 
 cordant. 
 
 It is in this way that the following observations have been made. For each of 
 them a record has been kept of the proportion of oxalic acid burned in 10 c. c. of a 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. H 
 
 half-deci-normal solution exposed to the sun. Fuitliermore the state of the sky has 
 been recorded and the principal incidents of the day of insolation. 
 
 Special attention has been paid to verifying the solar and antisolar lights ' 
 which were very fi-equent during the first years in which these observations were 
 made and which have never been absent since that time. I have desciibed the 
 aspect they assumed in the countries in which I began my observations. The more 
 1 study them, the more I consider them as solely due to the presence of aqueous 
 vapor at very great heights in the atmosphere. We shall have to examine, from 
 this point of view, their influence on the phenomena of solar combustion. 
 
 The tables which follow are also intended to show the very considerable 
 vai'iation which the quantities of oxalic acid consumed present from day to day. 
 The combustion, which is almost completely absent on cloudy or rainy days, may 
 reach or even exceed 50 per cent of the acid during bright and luminous days. But 
 there are also some very bright days, during which combustion is feeble, and twice 
 it has happened that I was unable to take photographs for want of proper light, 
 being deceived by the apparent biightness of the day on which I was working. 
 
 I shall quote ray observations very nearly in the order in which I made them, 
 from the moment when I had legulated the process of measurement ; and in con- 
 nection with each one of these sets of observations I shall cite the facts which 
 they have revealed to me and which subsequent observations have only confirmed. 
 
 OBSERVATIONS OF THE YEAR 1885. 
 
 Made at Fau (Cantal). Altitude 800 metres. Country of meadows and of 
 woods. Volcanic soil. (Andesite and basalt of the plateau.) 
 
 I have inserted above (page 8) some observations which I made at the end of 
 August with the solution in a conical glass. The combustion is a little less rapid 
 than in the vessels of Bohemian glass which I used in experiments of a later date. 
 Here follow those made in September and October. S. and A. S. repiesent solar 
 lights in the west and anti-solar lights. 
 
 ' See, on this subject, a note inserted in W\e.Comptes Rendus de I' Academie des Sciences, yo\. 
 xcix, p. 714. 
 
12 
 
 ATMOSPHERIC ACTINOMETRY 
 
 Date. 
 
 Combustion. 
 
 Remarks. 
 
 September 2 
 
 32^ 
 
 Rain the night before. Clear weather. S. faint. 
 
 a 
 
 3 
 
 0^ 
 
 Rainy and stormy day. No suft. 
 
 a 
 
 4 
 
 li 
 
 Glimpses of the sun. Barometer rises again. 
 
 (( 
 
 S 
 
 13^ 
 
 Day jjartly sunny, partly rainy. 
 
 C( 
 
 6 
 
 7^ 
 
 Rain in the morning ; a little sun in the evening. 
 
 (( 
 
 7 
 8 
 
 7^ 
 28^ 
 
 t( (( ^^ (( »< a i*. (t <t ti 
 
 u 
 
 Day in appearance similar to the two preceding. 
 
 it 
 
 9 
 
 Ti 
 
 Fog and rain in the morning ; sun in the evening. S. and A. S. con- 
 tinuous during more than an hour. 
 
 it 
 
 10 
 
 35^ 
 
 Very fine day. S. and A. S. 
 
 ti 
 
 II 
 
 28^ 
 
 Rainy in the morning. Clearing in the evening. S. and A. S. very 
 bright. 
 
 i( 
 
 12 
 
 36^ 
 
 Fine day, corona around the moon. S. and A. S. 
 
 if 
 
 13 
 
 22 <^ 
 
 A fine day. Cirrus. In the evening, at sunset clouds in the west 
 project their shadow in the east upon anti-solar lights. There are 
 besides, in the neighborhood of this light, small cloudlets, the violet- 
 red color of which is exactly the same, except as regards intensity, 
 as that of the anti-solar light. An irregular corona around the 
 moon, fringed and elongated in certain directions by cirrus-streamers 
 to the four points. 
 
 it 
 
 14 
 
 21^ 
 
 A very fine and very hot day. S. and A. S. very fine. 
 
 fi 
 
 15 
 
 27^ 
 
 A very fine and very hot day. S. and A. S feeble. 
 
 it 
 
 16 
 
 35^ 
 
 " day and slight cirrus. No S. and A. S. 
 
 (( 
 
 ■n 
 
 21 ^ 
 
 " cirrus and cirro-cumulus. At night storm. 
 
 n 
 
 18 
 
 17^ 
 
 Quite fine in the morning. Cloudy in the evening. Rain and storm 
 at 6 o'clock P.M. 
 
 ti 
 
 19 
 
 17^ 
 
 A day divided between sun and clouds. 
 
 ti 
 
 20 
 
 34^ 
 
 A fine day. S. and A. S. 
 
 a 
 
 21 
 
 29^ 
 
 " " Cirrus and cirro-cumulus. 
 
 li 
 
 22 
 
 33^ 
 
 Fine day without clouds. S. and A. S. fine but short-lived. 
 
 
 23 
 
 32^ 
 
 Fine day without clouds. Fine but short-lived S. and .A. S. 
 
 n 
 
 24 
 
 29^ 
 
 Numerous cirri in the evening. Cumulus and storm. Barometer 
 falls. 
 
 a 
 
 25 
 
 1^ 
 
 Dark and cold day. Lunar halo of 22°. 
 
 n 
 
 26 
 
 13^ 
 
 Middling day. Some glimpses of the sun. 
 
 ti 
 
 27 
 
 oi 
 
 Rainy weather in the morning. Dark and cold all day. 
 
 it 
 
 28 
 
 4^ 
 
 A few glimpses of the sun. 
 
 u 
 
 29 
 
 3^ 
 
 Fog and rain all day. 
 
 it 
 
 30 
 
 2^ 
 
 Rain in the morning, in the evening fog. No sun. 
 
 October 
 
 I 
 
 3^ 
 
 Rain in the morning ; very little sun in the evening. S. and A. S. 
 Violet mist in the valley. 
 
 (1 
 
 2 
 
 12^ 
 
 Quite a fine day. In the evening violet lights very perceptible in the 
 neighborhood of Venus, whose brilliancy is very great. 
 
 t< 
 
 3 
 
 20^ 
 
 Sky cloudy all day. 
 
 it 
 
 4 
 
 II ^ 
 
 Fine in the morning ; in the evening dark. 
 
 tt 
 
 5 
 
 24^ 
 
 A fine day. Very few clouds. S. and A. S. 
 
 tt 
 
 6 
 
 12^ 
 
 Foggy day. 
 
 tt 
 
 7 
 
 1^ 
 
 Rain all day. 
 
 it 
 
 8 
 
 24^ 
 
 Fine day, autumn like. Sun rather veiled. 
 
 tt 
 
 9 
 
 13^ 
 
 Rain in the morning ; sun in the evening. 
 
 it 
 
 10 
 
 6^ 
 
 Rain all day. Rare glimpses of the sun. 
 
 it 
 
 II 
 
 ni 
 
 Rain in the morning ; a little sun in the evening. 
 
 it 
 
 12 
 
 12^ 
 
 Covered sky ; rare glimpses of the sun. 
 
 it 
 
 13 
 
 7^ 
 
 Sky overcast ; cold weather. North wind. Frost at night. 
 
 It 
 
 14 
 
 22 ^ 
 
 Fine in the morning, overcast in the evening. 
 
 it 
 
 15 
 
 Ai 
 
 Dark and rainy day. 
 
 ti 
 
 16 
 
 13^ 
 
 Rainy in the morning ; in the evening breaks in the clouds. 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 
 
 13 
 
 Date. 
 
 Combustion. 
 
 Remarks. 
 
 October 
 
 >7 
 
 19^ 
 
 Very fine day, from beginning to end. 
 
 
 i8 
 
 t8^ 
 
 " " " In the evening cirrus and halo of 22°. Barometer falls. 
 
 
 >9 
 
 18^ 
 
 Dark in the morning, a little sun in the evening. Incessant rain after 
 3 o'clock P.M. 
 
 
 20 
 
 5^ 
 
 Rainy day. No sun. 
 
 
 21 
 
 9^ 
 
 ii i< 
 
 
 22 
 
 4^ 
 
 it it 
 
 
 23 
 
 24^ 
 
 Quite a fine day, in spite of east wind which rose very high after 
 9 o'clock. 
 
 
 24 
 
 3i 
 
 Rainy day from beginning to end. 
 
 
 25 
 
 14^ 
 
 Partly sun and partly rain. Two currents in the air, one from the 
 south, superior, carrying off cirri ; the other from the north, inferior, 
 with clouds. The latter finally dominates and after having brought 
 up intermittent and slight rains, it gives a cool night. 
 
 tt 
 
 26 
 
 2^ 
 
 A rainy day. 
 
 i( 
 
 27 
 
 -5^ 
 
 Stormy at night. Day quite fine. 
 
 a 
 
 28 
 
 3J^ 
 
 Rainy day. 
 
 a 
 
 29 
 
 5^ 
 
 " Rare glimpses of light. 
 
 These two months of iininteirupted obsei-vation prove already that tlie solar 
 combustion passes through very different values within 24 hours. These changes 
 are sometimes very sudden and exceed especially those of the thermometer, the 
 bai'ometer, and even those of the avei-age brightness of the day. The actinometric 
 effect does not show, therefore, that approximate constanc}% which makes it rela- 
 tively so easy to measure the other effects of solar I'adiation ; it requires a veiy 
 close and minute investigation. 
 
 While it amounts to little or nothing at all in overcast and rainy weather, it 
 rises very perceptibly during fine, sunny days; but it seems to be subject to other 
 influences yet beside those which we have mentioned when we spoke of "a fine 
 day," " fine weather," etc. If we find, in fact, that the days from tiie 20th to the 
 24th September i-esemble each other very closely, as far as their external physi- 
 ognomy is concerned, and are also very much alike in point of actinoiuetry, we 
 have on the other hand the example of October 17th, 18th, and 19th, during 
 which the degree of combustion was the same, and this although the weather had 
 been very fine during the first two days and very indifferent during the last. An 
 instance of the opposite nature is offered to us by the 6th, 7th, and 8th of Septem- 
 ber, which diffeied veiy much in their actinometric aspect, whilst they lesembled 
 each other so far as their external physiognomy was concerned. 
 
 It would be interesting to find out under what influences these variations are 
 produced. In the meantime, until we reach that point, let us notice that the com- 
 bustion on the finest days in October does not amount to as much as that obtained 
 on the finest days in September, and that the latter again do not equal the fine 
 
14 ATMOSPHERIC ACTINOMETRY 
 
 days in August, mentioned on page 8, if we bear in mind that the expei'iments 
 mentioned on that page were made in a cone-shaped glass, capable of holding the 
 bulb of a thermometer and not, like those made in September, with shallow vessels 
 in which the figures would have been much higher. 
 
 One might be tempted to see here the effect of the lessened length of days. 
 But, in order to avoid this influence, the length of exposure has been everywhere 
 precisely the same : from 8.30 a.m. to 4.30 p.m. There is, therefore, an influence 
 due to the seasons, which we must also endeavor to trace back to its true cause. 
 For this purpose we can only collect the gi-eatest possible amount of evidence. 
 
 RESULTS OF OBSERVATIONS MADE IN 1886 AND 1887. 
 
 I made for this end several series of experiments in 1886 and 1887, at Paris, 
 in the Cantal, and at Orcines, at the foot of the Puy-de-Dome. Ilnfoi'tunately I 
 cannot report them here in detail, having mislaid the papers which contained the 
 record. I can only indicate the general results which I have retained in my 
 memory, because they served as a starting-point for new investigations. In the 
 first place I again found evidence of the almost perfect independence between the 
 degree of solar combustion of oxalic acid, and the occurrence of solar and anti-solar 
 lights. If there are any examples of coincidence between an active combustion 
 and the presence of such lights, it is because these lights appear only in fine 
 weather. But there are also other cases in which combustion is very rapid and 
 when those lights are altogether missing. If they play any part at all, it seems to 
 be one quite secondary, and this view agrees very fairly with the hypothesis that 
 the phenomenon is due to the presence of aqueous vapors in the upper regions of 
 the "atmosphere. It is, of course, well known that liquid water or water in the 
 form of steam influences the activity of actinic combustion very little. 
 
 Another very important result is this : that the maxima of the figures of com- 
 bustion during the finest days are higher in spring than in summer. The difference 
 did not stiike me as quite so marked as between summer and autumn. As to the 
 rriaximum in spring I have always found it very clearly marked during observa- 
 tions carried on for four years, and an example of this will be found, unfortunately 
 too limited in its nature, when I shall speak of my experiments of 1888. 
 
 The maximum in spring appears alike in Paris and in the country. But I 
 have also found that solar combustion was less intense in Paiis than in the Cantal 
 or in the Puy-de-Dome; this difference appears not only in the high figures 
 connected with the oxalic acid, but I have found it also in a long series of 
 experiments, which I had undertaken in order to study the transformations which 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 15 
 
 several organic substances undergo in the solar light, and which offer a greater 
 resistance than oxalic acid. These I had been compelled to leave in the light for 
 weeks and for months befoie the attack was complete. They thus summed up the 
 influence to which they had been subjected during the length of the exposure. 
 Now this phenomenon required, generally, for its termination, much more time, 
 sometimes three oi- four times more, in Paris than in the country. 
 
 Among the facts of this kind I can only quote one which I find I had by 
 chance inserted in a work intended for the examination of another question. A 
 deci-noi'mal solution of tai'taric acid which was every day exposed from 10 a.m. till 
 2 P.M. to the sun in Paris, had lost by combustion in seven months and a half only 
 10 per cent of its acid, while in the Cantal an identical solution had lost in two 
 months 47 per cent. This involves a combustion about fifteen times more rapid, 
 and although the length of exposure was a little, greater every day in the Cantal 
 than in Paris, and although the quantum of solar combustion increases moi-e rapidly 
 than the length of exposure, this is not enough to make up for the difference. In 
 another case involving the combustion of glucose in an alkaline liquid, I found 
 that to take two yeai's in Paris which had required only three months in the 
 Cantal. 
 
 Finally, this experiment teaches us also that average years do not resemble 
 each other and that, if thei-e are some which are rich in chemical radiations, there 
 are poor ones also. These differences between one year and another, from 
 this point of view, appeared to me more marked than in any other respect. We 
 have shown above the most striking inequalities between consecutive days of the 
 same season. They recur, less markedly, for consecutive years. 
 
 These statements which, I repeat, I regret not being able to support by 
 figures, suggest a number of problems for which I have begun to seek a solution. 
 
 In the first place, the fact that active combustion is stronger on fine days in 
 spring than in summer and in autumn, shows that there must be another cause of 
 action than the influence of temperature, or the height of the sun above the horizon. 
 We are naturally led to think of the influence of the volatile oi'ganic products 
 which vegetation scatters in the air during summer, and which, if they are capable 
 of being oxidized, absorb and utilize for their own benefit the chemical radiations 
 of the light which passes through the atmosphere, preventing them from reaching 
 the soil. We are confirmed in this view by what has been said before (page 6) 
 concerning the relative poverty of solar light, at the time when it reaches us, in 
 radiations able to oxidize oxalic acid. 
 
 In the second place, the difference between the sum total of the annual radia- 
 tions at Paris and at the Cantal, or on the high table-lands of the Puy-de-Dorae, 
 
16 ATMOSPHERIC ACTINOMETRY 
 
 leads us to ask if the question of altitude may not perhaps be of importance. Two 
 identical vessels, containing the same solution and exposed during the same time 
 at different heights in the atmosphere — will they^or will they not undergo the 
 same degree of oxidation ? 
 
 INFLUENCE OF ALTITUDE. 
 
 I begin with the last question, because the documents which helped me to 
 solve it were lost, together with those which gave the results of the experiments 
 already mentioned, and I must therefore be very brief in my treatment. 
 
 In order to solve this pi'oblem I installed myself at the foot of the Pny-de- 
 Dome in the little village of Orcines, and I made a number of expeiiments simul- 
 taneously in the garden of the house in which we lived, and on the terrace of the 
 observatory on the Puy-de-Dorae, where M. Humandon kindly undertook to expose 
 and to remove again at certain fixed houi-s the vessels containing the oxalic acid 
 which had been rendered sensitive. ,The two stations are distant from each other 
 4 kilometres in a dii'ect line. The vertical difference amounts to about 400 metres. 
 The incline, therefore, between the two stations does not count foi- much, and they 
 cannot be considered as being upon the same vertical line. Experiments made on 
 the toj) and at the foot of the Eiffel tower would have been more satisfactory in 
 this respect. But at the Eift'el tower I should have had to appi'ehend encounter- 
 ing difficulties of another kind, especially the want of homogeneousness between 
 the layers of the atmosphere. For the lower ones which had swept populous parts 
 of the city could, in that amount, no longer be considered equivalent to the upper 
 parts. What tempted me to choose the station of the Puy-de-Dome was exactly 
 this homogeneousness of the whole region so far as its vegetation was concei-ned. 
 The Puy-de-I)ome is suri-ounded to a great distance by a dry, almost deserted 
 countiy, covered with woods and lai'gely with heathei-, while some portions are 
 absolutely bare in places where pozzolanes crop out of the soil or in those immense 
 overflows of lava of recent date, which are called " cheires," and which defy any 
 attempt at cultivation. One of these "cheires" cropped out close to the house in 
 which I dwelt, and I imagine that, on the whole, there was no reason why the air 
 on the top and that of the table-land from which the mountain I'ises, should be 
 heterogeneous. In spite of these favorable circumstances I did not find that the 
 combustion at the observatory was very different from that at Orcines. It exceeded 
 the latter a little, on an average, but with exceptional results in one or the other 
 direction, so as to prevent any positive conclusion. I remember that my estimates 
 showed an increase of altitude accompanying an increase of actinic intensity, but 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 17 
 
 that they were not such as to pi-ove it. This conclusion diminishes my regret at 
 having mislaid the data. 
 
 INFLUENCE OF OXIDIZABLE SUBSTANCES SUSPENDED IN THE AIR. 
 
 With this second question I have been more successful, since comparative ex- 
 periments can here be made with far more precision than elsewhere. All that is 
 necessary is to expose in one and the same place two vessels containing the same 
 oxalic solution ; one being made to float upon water contained in a deep crystal- 
 lizing pan in such a way tliat a layer of stagnant aqueous vapor may be kept 
 above the surface of the liquid which it contains. The other vessel floats in like 
 manner, and under the same conditions, on the surface of some turpentine or of 
 any other essential oil. It is always found that combustion is far less advanced in 
 the second than in the first vessel. As I said before, I lost the relative figures of 
 the results obtained by the experiments made in 1886 and 1887. But Mr. Elf- 
 ving, professor at the University of Helsingfors (Finland), to whom I had men- 
 tioned the results thus obtained, began once more to experiment with essence of 
 turpentine, and I will here quote the results as he reported them to me in a letter 
 which I have fortunately preserved : 
 
 "I have repeated and confirmed your experiments on August 30, 1888, from 
 8 A.M. to 4 P.M. with a clear sky. There wei-e 53 per cent of the oxalic acid burnt 
 above the water, and 39 per cent above a bath of essence of turpentine. The next 
 day, which remained clear from 9 a.m. till noon, the figures were 47 per cent and 
 20 per cent for the same length of exposure. It is certain, therefore, that the 
 presence of oxidizable substances in the air possibly diminishes the consuming 
 power of the sun." 
 
 Mr. Elfving has confirmed this conclusion by the following experiment, which 
 I have, in my turn, repeated and confirmed. It consists in sifting, so to speak, the 
 rays of the sun through a solution of sul^ihate of quinine, which absorbs a part of 
 the chemical radiations before they can react on the oxalic solution. Another sift- 
 ing apparatus, of the same thickness, but consisting of pure water, furnishes a 
 standard of comparison. The latter, rigorously, might be neglected, for the quan- 
 tity of watery vapor or of liquid or solid water which the rays of the sun have 
 traversed before reaching the level of the soil, exceeds by far the thickness of the 
 supplementary screen of liquid ; the absorption due to water, is, moreover, very 
 feeble. In my experiments I suppressed this complication. Mr. Elfving used two 
 glass bell jars with double walls, of which one contained water, the other a solu- 
 tion of sulphate of quinine. He wrote to me on June 17th : 
 
18 ATMOSPHERIC ACTINOMETRY 
 
 " The light which has gone through a layer of water is five times moi-e active 
 than that which has traversed a solution of sulphate of quinine of quite the same 
 depth. I shall continue my observations at the time of the solstice." 
 
 And on July 9th : 
 
 " I have again made an experiment with sulphate of quinine. On June 27th, 
 while there were consumed in the open air during the whole day 87 per cent of 
 the total amount of oxalic acid, and 78 per cent undei- a bell jar filled with water, 
 the decomposition amounted only to 20 per cent under a precisely similar bell 
 filled with a solution of sulphate of quinine." 
 
 Analogous results are obtained by comparing the effect of sifting solar rays 
 through a solution of potasssium bichromate, which by pi-efei'ence allows those 
 radiations to pass which are least i-efrangible, with that of transmission through a 
 solution of sulphate of copper which allows the most refrangible radiations to pass. 
 All this proves that it is mainly the chemical ladiations which are of importance, 
 and that when these radiations ai-e employed in oxidation, or more generally in the 
 transformation of organic or even mineral substances in the air, they reach the sur- 
 face of the soil much weakened. 
 
 Here is, therefore, a local cause of variations in the actinometric degree ; a 
 local and incidental cause, considering that it may be summed up thus : There may 
 exist actinic clouds, clouds scarcely visible to the naked eye and not accessible to 
 our senses, but the effect of which, at least as far as it can be measured by solu- 
 tions of oxalic acid, exceeds by far, in intensity, that of the variations in brilliancy 
 and obscurity produced by ordinaiy clouds. These clouds come and go, are no 
 longer to-day where they wei'e the day before; they dissolve, for they are, like 
 other clouds, no sooner formed than they become subject to ceaseless causes of 
 destruction. This explains very fully why the actinometric degree should vary 
 so greatly from day to day and from one year to another. It may also be that we 
 find here our explanation of the greater actinic power which spring has, in other 
 words, that season during which the atmosphere is certainly pooi'est in organic 
 substances. 
 
 Upon reaching this point we see new vistas open before us. It is well known 
 that the turning green of the foliage and the production of chlorophyll may take 
 place when the intensity of light is very feeble, as for instance in the back of 
 a room lighted by one window, but that, under such circumstances, the chlorophyll 
 does not begin to act and is not decomposed by carbonic acid. It requires a much 
 stronger luminous intensity for the process of assimilation to begin. This phe- 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 19 
 
 noraenoa increases with intensity of light up to a certain raaximxim, differing in 
 different plants ; beyond this it decreases. Starting from this point, it is naturally 
 suggested that those actinic clouds which we have just discovered, cannot be with- 
 out influence on vegetation, since they modify so largely, although often invisibly, 
 the strength of the chemical i-adiations, which is nearly, if not absolutely the only 
 active element in the complex whole, which until now has been studied under the 
 name of Luminous Intensity. And if again the plants themselves disperse into 
 the air the materials which absoib the solar, chemical radiations, how can we 
 avoid thinking that possibly the production of these odoi'iferous and oxidizable 
 effluvia may be for the plant a means of protection ? 
 
 To elucidate this subject fully, would require expei'iments which I have not 
 the time to make.' I have been satisfied with examining it under various aspects. 
 Odorous and essential oils are not alone aide to ai'i'est the passage of chemical radi- 
 ations. The surfaces of plants aie, as is well known, frequently covered with a fatty 
 or waxy layer, which is highly oxidizable. There are, besides, at all times fatty 
 substances in the air, as is pioved by the greasy feel of old dust deposited upon 
 our furnitui'e. What effect can these fatty substances exert upon the combustion 
 of oxalic acid in the sunlight ? If our ideas are correct, a slight layer of fatty 
 matter should protect that acid against solar light. 
 
 INFLUENCE OF FATTY SUBSTANCES. 
 
 The presence of fatty matter on the surface of our test solution brings up a 
 slight experimental difficulty. It is this, that solar oxidation of a fatty substance 
 is always accompanied by a production of acid which raises the titre of the oxalic 
 solution at the same time that the solai- combustion lessens it. We must, therefore, 
 either use a very small quantity of fatty matter so that it may barely form an im- 
 perceptible veil to cover the liquid, or, what is better still, we must spread it out 
 in a transparent layer over a surface of glass interposed in the path of the luminous 
 rays. Here are some experiments made in connection with this subject: 
 
 Map. — On June 27, 1885, I exposed to the sun during six hours seven vessels 
 of the same dimensions, containing each 10 c. c. of oxalic acid in half-deci- 
 normal solution. Two of these vessels, Nos. 1 and 2, had their walls 
 clean. Vessel No. 3 had been rubbed with a weak solution of butter in 
 sulphide of carbon, which left upon the sides, hardly tarnishing them, a 
 greasy layer. Moreover, by virtue of a well-known phenomenon of super- 
 ficial tension, the walls of the vessel have allowed an invisible layer of 
 fatty matter to spread on the surface of the liquid. In order to separate 
 
20 ATMOSPHERIC ACTINOMETRY 
 
 this action as far as possible fi-om that of the roughness of the walls, a 
 fourth vessel is brought up to the same degree of opacity as No. 3, by 
 rubbing it externally with chalk diffused in water. Finally, to increase 
 the quantity of fatty matter pi-esent in the liquid, and also, in order to 
 see the effect which a little oj^acity of the liquid may there produce, new 
 vessels, Nos. 1', 2', and 3', have been prepared like the vessels Nos. 1, 2, 
 and 3, simply adding to each two drops, in other woids 5 milligrammes, 
 of fatty matter. 
 
 These were the results : 
 
 Combustion. 
 
 Vessel No. i, clean sides 33 ^ 
 
 " 2, " " 33^ 
 
 " " 3, dull sides, greasy surface 29 ^ 
 
 " 4, " " (chalk) 32^ 
 
 " " i", like I, plus 2 drops of milk 16 ^ 
 
 " 2', " 2, " " " " 17 $?; 
 
 " " 3'. " 3, " " " '• 17^ 
 
 Other experiments, made the following yeai-s, and the detailed repoi-ts of 
 which have been lost, confirm these first results, and show that the fatty matter 
 contained in a liquid or spread as an invisible layer over the surface, like that 
 which covers the walls of a bell jar placed over the vessel containing oxalic acid, 
 diminishes the actinic effect of the solar rays. 
 
 Finally, it is the same with many substances, more or less easily oxidizable, 
 which also exert a protecting, or at least a retarding, effect upon the influence of 
 the chemical radiations. Such is, for instance, alcohol. 
 
 Mcp. — On the 26th June, 1885, two vessels with 10 c. c. of an oxalic acid solu- 
 tion, containing J^ equivalent per litre, gave me a combustion of 37 per 
 cent, the same for both. Two other vessels, exactly alike, which have 
 received an addition of 2.5 c. c. alcohol of 90 per cent, gave me only a 
 combustion of 21 per cent. 
 Mop. — On September 14, 1888, two vessels with a solution of ^^ equivalent of 
 oxalic acid, gave me identical combustions, rising as high as 10 per cent. 
 They amounted only to 4 per cent in two like vessels, to which a few drops 
 of oil of oranges had been added, so that the essential oil and the alcohol 
 have acted similarly. 
 I have made numerous experiments with divers substances which were oxidi- 
 zing or oxidizable, the details of which have been lost. In a general way I have 
 found that the former increase the combustion of oxalic acid, while the others 
 retard it. 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 21 
 
 But tliis rule is not always confirmed, on account of the intervention of what I 
 have called in another paper phenomena of entanglement. These operate so that 
 one oxidizable body may involve another in the same decomposition which it 
 undergoes itself. But here we begin to stray away from the subject of this paper, 
 which is the actinic study of the atmosphere. I am content, therefore, to draw 
 from the great sum total of the i-esults I have obtained the following conclusions: 
 The nature and the proportion of the oxidizable elements which living nature scat- 
 ters through the air, })etray themselves in the solar combustion of oxalic acid, which 
 is the more feeble on the surface of the solution the more the radiations have met 
 with unstable elements to oxidize dui'ing their passage. The organic substances of 
 the atmosphere are therefore a protection against too intense an action of the chemi- 
 cal rays at the surface of the soil, and the effect which they produce is not only 
 measurable, but sometimes very great. In other woi'ds, we do not know what the 
 chemical power of solar light may be at its entrance into the atmosphere, but on a 
 level with the soil it is so impoverished that a thin layer of turpentine vapor, of 
 sulphate of quinine, or of any oxidizable substance, suffices to destroy it almost 
 completely. 
 
 This conclusion has, however, another side to be considered, which is, that 
 the atmosphere must at every moment be the seat of combustions, such that, on 
 the whole, all luminous radiations are utilized. I shall not insist here on the power 
 and the importance of the phenomena of oxidation which take place in the atmos- 
 phere and at the level of the soil, nor upon the genei'al effect which they have on 
 sanitation over which they pi'eside. 1 have published several memoirs on that sub- 
 ject,' to which I must be content to refer. I have there called attention to the 
 power of the solar rays on microbes, first weakening and then killing them, a 
 power which was first indicated, but incompletely proven, by Messrs. Downes and 
 Blunt.' I have, moreover, studied the influence of the conditions of the medium 
 on the resistance of germs. All that has been done since, has only confirmed the 
 importance which I attached to light and to the chemical portion of the solar spec- 
 trum as principal agents in the hygiene of the globe. 
 
 INFLUENCE OF INCREASE OF LATITUDE. 
 
 This first problem, that of the possible influence exerted by oxidizable sub- 
 stances while in suspension in the atmosphere, having been sufficiently discussed in 
 
 ' Annales de Chimie et de Physique, 6th S., vol. v., May, 1885, and Comptes Rendus, vol. c. and ci. 
 Annales de V Institut Pasteur, vol. I., p. 88. 
 
 ' The conclusions of these scholars had been opposed by Tyndall and by Jamieson, so that 
 when I took up the question anew, it had not yet met with a solution. It has found one to-day. 
 
22 ATMOSPHERIC ACTINOMETRY 
 
 the statements just made, I found myself face to face, as Professor of Meteorology, 
 with the following question : 
 
 It is an acknowledged fact, that the activity of the vegetative process in the 
 northern parts of Eui'ope is very great. The interval between sowing and harvest- 
 ing, for spring wheat, lasts on an average 145 days in Alsace. According to M. 
 Tisserand it amounts to only 133 days at Halsno, in 59° 30' N. latitude ; and it is 
 only 114 days at Skibbotten, in 69° 30' N. latitude. It decreases therefore as we 
 approach the pole ; notwithstanding that the average temperature of the period of 
 vegetation diminishes likewise with the increase of latitude. 
 
 This decrease in the number of days needed for vegetation, as we draw nearer 
 the north pole, seems to be a general law. Accoi'ding to Arnell, barley requires 
 117 days to grow in Southern Sweden, 92 in Middle Sweden, and 89 in Lapland. 
 It is true that these variations depend in part at least on the power which the plant 
 has to adapt itself to external conditions, for if sown in our country the Norwegian 
 grain grows more rapidly than ours, while our own native grain, carried to Norway, 
 lags behind the acclimated variety. But this is not sufficient to explain all, and we 
 must in the end always return, as a final analysis, to the influence of climate. 
 
 We may go even a little farther in our induction. According to Griesbach 
 the increased rapidity in the development of plants cultivated at the extreme north 
 does not affect the whole evolution of the plant, but only the period between ger- 
 mination and blooming. It applies, therefore, only to the green oi'gans of the plant, 
 and thus stai'ts once more the question of light, which actually appears to be of 
 greater importance than that of temperature. In fine, to return to the subject of 
 our Memoir, the actinic influence of the solar rays seems to increase with the 
 latitude. 
 
 To what is this increase due? This question is still open and to it I have 
 tried to find an answer. The first point to determine was whethei' the solution of 
 oxalic acid also showed such an increase of actinic effect ? 
 
 It was on this account that I asked Mr. Elfving to assist me, whose interesting 
 experiment I have mentioned above (page 17). I sent to him at Helsingfors an 
 oxalic solution, and ten vessels exactly alike, such as I had used myself in France, 
 in order to make sure that at least, and as far as possible, those experimental condi- 
 tions which we could control should be as identical as they could be made. 
 
 Unfortunately there were other conditions which wei'e entirely beyond our 
 control. The ideal would have been attained with a series of days equally fine, 
 occurring simultaneously in France and in Finland, and permitting us to make our 
 observations under precisely the same circumstances. But there are obvious reasons 
 why the weather could hardly ever be the same in France and in the Gulf of 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 23 
 
 Bothnia. When that vast mass of air, in I'elative repose, which I have named The 
 Isle of Calms, rests over our part of the world and gives us fine weather, the equa- 
 torial current, which turns it northward, is over Sweden and Norway, to which it 
 brings overcast skies and I'ains ; when, on the other hand, the Isle of Calms rests 
 over the north of Europe, we are in France subject to stormy disturbances which 
 come to us through the Mediteri-anean or the Gulf of Gascony, or we are subject again 
 to the return current, which, after having rounded the " Island," comes back to us in 
 the shape of cold east and northeast winds. To find favorable coincidences in this 
 grand atmospheric dance, we should need months of continued observations, which 
 neither Mr. Elfving nor myself were in a condition to undertake. 
 
 In this difiiculty we availed oui'selves of the meaning of the word " fine day," 
 as I have shown above (page 13), which is so uncertain as to its actinometric defini- 
 tion that every effort to make it absolute as to perfect equality of expei'imental con- 
 ditions becomes rather illusory. We could be content, more modestly, with a first 
 approximation ; it was enough to compare the actinometric combustion of the finest 
 days in the Gulf of Finland and in France, at the same time of the year. 
 
 Nor is this all. The length of the day is greater at the North than at the 
 South during the period of vegetation, and the length of the insolation has, we all 
 know, a direct influence on the relative quantum of combustion. Hence I requested 
 Mr. Elfving to make every day two sets of experiments, one with vessels exposed 
 to the sun from 8 a.m. to 4 p.m., like those which I was using in France, and the 
 other with vessels left out from 8 a.m. till the setting of the sun. 
 
 Mr. Elfving made at Helsingfors between August 27th and September 4th, 1887, 
 five seiies of experiments, which I cannot compare with those which I was making at 
 the same time at the foot of the Puy-de-Dome, and the records of which have been 
 lost. But I am fortunately able to compare them with those which I had begged 
 M. Ch. Mascart to make at the same time near the seashore in the Channel. These 
 may perhaps be better fitted for comparison with those made by Mr. Elfving, as 
 both were made at maritime stations. All that I noted when I received them was 
 that they gave much higher figures than those which I obtained at the same time 
 on the bare table-land which cariies the Puy-de-Dome. 
 
 In the first place, here is a table of the observations made in France ; it is 
 formed in the same way as those which have already been given in this Memoir. 
 
24 
 
 ATJIOSPHERIC ACTINOMETRY 
 
 ST. PIERRE LE PORT, 1 887. 
 
 Date. 
 
 Combustion. 
 
 Remarks. 
 
 August 1 S 
 
 44 ^ 
 
 Clear weather till lo o'clock p.m. ; later cloudy. 
 
 
 i6 
 
 38^ 
 
 Rain till 2 o'clock ; later overcast. 
 
 
 '7 
 
 31 i 
 
 Very clear from 11 till 3 ; afterwards cloudy. 
 
 
 18 
 
 23 ^ 
 
 Three-fourths cloudy till 10 a.m. Clear from 10 till 3 p.m. 
 
 
 '9 
 
 33 ^ 
 
 Half overcast in the morning - then quite clear. 
 
 
 20 
 
 21 ^ 
 
 Half overcast all dav long ; a slight fog. 
 
 
 21 
 
 28 <^ 
 
 Slightly overcast in the morning ; then clear. 
 
 
 22 
 
 3° i 
 
 Slightly overcast in the morning ; then clear. 
 
 
 23 
 
 36^ 
 
 Fine weather. 
 
 
 24 
 
 29 ^ 
 
 tt a 
 
 
 25 
 
 42 i 
 
 Warm. Very close. Clear weather. 
 
 
 26 
 
 32 ^ 
 
 Covered in the morning and evening. Clear from 12 till 3 p.m. 
 
 
 27 
 
 23 ^ 
 
 Overcast. Rain from 10 till 11 a.m. 
 
 
 28 
 
 24^ 
 
 Unceasing rain. 
 
 Here asain the solar coinbustiou increases with the fine weather and diminishes 
 when the sky is overcast or rain falls. Although the weather was on an average 
 less fine than during the corresponding series of exi)erinients cited before, the latter 
 gives, on the average, higher results, a fact which confirms what we have already 
 said concerning the actinometric differences of different years at the same epoch. 
 Here are now the experiments made by Mr. Elfving at the same time in 1887 : 
 Helsingfors, latitude 60° 10'. Length of day, 14 hours. Height of the suu 
 above the horizon at noon, about 38°. ___ 
 
 
 
 Solar Combustion. 
 
 
 Date. 
 
 From 8 A.M. to 4 P.M. 
 
 All day long. 
 
 August 27 
 28 
 29 
 
 September 2 
 4 
 
 42^ 
 
 53^ 
 74!^ 
 77^ 
 
 55^ 
 
 87^ 
 89^ 
 
 " The difference between the first three days and the two others is quite great ; 
 it arises, no doubt from the fact that the atmosphere had been purified by heavy 
 rains on August 30th and September 1st and 3d. In March, I had already observed 
 this effect of rain." (M. Elfving.) 
 
 The figures in the first column are on an average higher than those which cor- 
 respond to them in the preceding report, and this superiority must be all the more 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 
 
 25 
 
 striking, as by a mistake in our agreement the exposure to the sun lasted an hour 
 longer in France than in Finland. The latter ought, therefore, to be somewhat 
 increased, in order to make the comparison raoi-e just. We shall presentl)^ return 
 to the results marked in the last column. Mr. Elf\ ing had woi-ked only during 
 five days ; the comparative experiments were therefore not immerous enough, and 
 by a common understanding another beginning was made in 1888. 
 
 Here is the report of the experiments which I made in France, in the garden 
 of the Agronomic Institute, during the months of May and June, 1888. My official 
 duties prevented me from making tl\eni in an unbroken series, and, moreover, I had 
 to leave out three observations during which a fierce wind suddenly sprang up and 
 covered my vessels with a layer of dust. 
 
 PARIS. EXPOSURE FROM 8 A.M. '1 O 5 P.M. 
 
 Date. 
 
 Combustion. 
 
 Remarks. 
 
 May 
 
 [2 
 
 46^ 
 
 A fine day. Fresh north wind. 
 
 « 
 
 13 
 
 29 ^ 
 
 Very fine day ; rather warmer than the day before. 
 
 (< 
 
 14 
 
 5°!^ 
 
 Cirrus in the morning. Very fine day. 
 
 (( 
 
 '5 
 
 23^ 
 
 Sky overcast. Barometer falls. 
 
 ts 
 
 17 
 
 52;^ 
 
 " South wind. Cirrus. 
 
 ft 
 
 18 
 
 23;^ 
 
 tt tt a 
 
 n 
 
 20 
 
 27^ 
 
 Quite a fine day. N. wind. Cirrus and alto-cumulus. 
 
 (1 
 
 21 
 
 35!^ 
 
 Sky overcast. Lighter in the evening. Fresh east winds. 
 
 <( 
 
 26 
 
 43!^ 
 
 g. g. clouds ; fine at night. Fresh north wind. 
 
 •t 
 
 27 
 
 30^ 
 
 Quite a fine day. g. g. Cirrus. Sudden storm. 
 
 June 
 
 I 
 
 33^ 
 
 Fine day. No wind. 
 
 It 
 
 2 
 
 55;^ 
 
 Warm and stormy day. 
 
 
 3 
 
 39^ 
 
 Very warm day. South wind. 
 
 
 5- 
 
 42^ 
 
 Warm and stormy day. Sky overcast. 
 
 
 12 
 
 64^ 
 
 Fine day. A little air. 
 
 The correspondence between the degree of combustion and the state of the 
 atmosphere is less striking in these observations, which were made in Paris, than in 
 those made in the country, which is less surprising when we bear in mind the 
 incessant heterogeneity and variability of the air in a large city. The influence of 
 the spring season, however, to which reference was made before (page 14) is shown 
 in the relative magnitude of the figures of combustion. The figure 64 %, dated on 
 June 12th, is very exceptional. 
 
 Here follow next the results obtained almost simultaneously by Mr. Elfving 
 at Helsingfors : 
 
26 
 
 ATMOSPHERIC ACTINOMETRY 
 HELSINGFORS. EXPERIMENTS MADE IN 1 888. 
 
 
 
 
 Solar Combustion. 
 
 
 Date. 
 
 
 
 
 
 
 
 From 8 A.M. to 4 P.M. 
 
 
 
 All (lay long. 
 
 May 
 
 •9 
 
 50^ 
 
 
 
 -^ 
 
 
 21 
 
 47 ^ 
 
 
 
 5M 
 
 it 
 
 22 
 
 56^ 
 
 
 
 76^ 
 
 X " 
 
 a.l 
 
 53^ 
 
 
 
 65^ 
 
 X " 
 
 24 
 
 37 ^ 
 
 
 
 55 ^ 
 
 X " 
 
 27 
 
 44^ 
 
 • 
 
 
 -^ 
 
 ik 
 
 30 
 
 46^ 
 
 
 
 72^ 
 
 It 
 
 31 
 
 si;^ 
 
 
 
 72 ^ 
 
 Tune 
 
 4 
 
 48^ 
 
 
 
 63^ 
 
 X " 
 
 7 
 
 48^ 
 
 
 
 70^ 
 
 tt 
 
 8 
 
 -^ 
 
 
 
 74^ 
 
 (1 
 
 9 
 
 56^ 
 
 
 
 79 ^ 
 
 
 to 
 
 57^ 
 
 
 
 77^ 
 
 (( 
 
 II 
 
 54^ 
 
 
 
 80^ 
 
 On the days marked with a cross (x), the sky was more or less overcast at 
 Helsingfors. All the figures iu the second column ought to be raised slightly, in 
 order to make them fit to be compared with those in the preceding table, which 
 correspond to an additional hour of exposure. It will be seen, however, that they 
 are on an average higher, although none of them reach the exceptional figure of the 
 12th of June at Paris. 
 
 The conclusion is the same as that derived from the experiments of 1887. In 
 order to add to its weight we recommenced another series in August and Septem- 
 ber. This time I installed myself on the Mont Dore, at a height of about 1100 
 metres, in a house some distance from that little village. 
 
 MONT DORE, 1 888. 
 
 Date. 
 
 Combustion. 
 
 Remarks. 
 
 August 
 
 9 
 
 26^ 
 
 Cirrus in the morning, which increased towards evening. 
 
 lO 
 
 19^ 
 
 Fine day. 
 
 
 II 
 
 18^ 
 
 " Sky slightly covered. 
 
 
 12 
 
 19^ 
 
 
 
 13 
 
 18^ 
 
 Cumulo-cirrus in the morning. Fine afternoon. 
 
 
 14 
 
 27^ 
 
 Fine day. Sky a little cloudy in the evening. South wind. 
 
 
 16 
 
 ^li 
 
 Very fine day. Atmosphere limpid. Cirrus in the evening. 
 
 
 17 
 
 — 
 
 Rain all day. No exposure. 
 
 
 18 
 
 — 
 
 Sky overcast, and rain. 
 
 
 19 
 
 22 <ji, 
 
 Fine day. Cumulo-cirrus and cirrus. 
 
 
 20 
 
 15^ 
 
 Cirrus all day, especially in the evening. Barometer not falling. 
 
 
 31 
 
 15^ 
 
 Large white cumuli. 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 
 
 27 
 
 Date. 
 
 Combustion. 
 
 Remarks. 
 
 September i 
 
 15^ 
 
 Same weather as day before. 
 
 « 
 
 2 
 
 10 ^ 
 
 Middling day. 
 
 (( 
 
 3 
 
 24^ 
 
 Quite a fine day. Many cirri. 
 
 it 
 
 4 
 
 12^ 
 
 Middling day. 
 
 « 
 
 s- 
 
 11^ 
 
 tt 
 
 (1 
 
 6 
 
 12 ^ 
 
 Quite a fine day. Some cumuli early. 
 
 *( 
 
 7 
 
 11^ 
 
 Middling day. 
 
 (( 
 
 lO 
 
 11^ 
 
 Quite a fine dav with a few clouds. 
 
 tt 
 
 II 
 
 7^ 
 
 If ft' If If 
 
 tt 
 
 12 
 
 18^ 
 
 Fine day, hot sun, a few cumuli. 
 
 11 
 
 '3 
 
 25?^ 
 
 Fine in the morning, middling later. 
 
 11 
 
 14 
 
 >S^ 
 
 Fine day, very warm. 
 
 11 
 
 IS 
 
 9^ 
 
 Very fine day, as yesterday. 
 
 11 
 
 i6 
 
 4^ 
 
 Middling day. Warm and heavy. 
 
 II 
 
 17 
 
 29^ 
 
 Day divided between sun and clouds. 
 
 It 
 
 18 
 
 
 Dark day. 
 
 tt 
 
 19 
 
 30^ 
 
 Superb day. 
 
 'ti 
 
 20 
 
 10 ^ 
 
 Sky fine early ; covered in the evening. 
 
 11 
 
 21 
 
 17^ 
 
 Day rather finer than day before. 
 
 It 
 
 22 
 
 13^ 
 
 Quite fine in the morning. Cloudy at night. 
 
 11 
 
 26 
 
 'S^ 
 
 Cumulus concealing about \ of sky. 
 
 11 
 
 27 
 
 25 ^ 
 
 Rather better than the day before. 
 
 It 
 
 28 
 
 49^ 
 
 Rather a dull day, but no clouds. 
 
 Between the 20th and 30th August there followed a long period of rain and overcast sky. 
 
 What strikes us in reading these figures is their smalluess even on fine days. 
 They are the smallest I have ever had to record in August and September, on an 
 average, and this although the latter month was rather fine at Mont Dore during 
 1888 ; there is also to be noticed a great lack of agreement between the apparent 
 character of the day and its actinometric chai'acter. Thus the very fine day of Sep- 
 tember 15th gave only a combustion of 9 per cent, when the slightly veiled day of 
 September 28th gave a combustion of 49 per cent. This is a new confirmation of 
 what has been stated before. 
 
 I partly attribute the very great want of agreement noticed at Mont Doi'e to 
 the fact that this station is surrounded on all sides by pine woods which diffuse 
 through the air a large quantity of terebinthine exhalations, so that the odoi- be- 
 comes sti'iking. This explanation also agrees with the notions which I have 
 suggested before. Nevertheless I admit that it would require very special com- 
 parative experiments to establish it firmly, and to draw fi'om it the proper signifi- 
 cation. We must be content, for the present, to remark that if our explanation is 
 correct, it will also account, as a whole, for the want of agreement already 
 mentioned. If the exhalations of essential oils are really able to arrest actinic 
 radiations, the effect of what we call a fine day will be very variable according as 
 it will succeed a period of rains which may have washed the atmosphere, as in the 
 
28 
 
 ATMOSPHERIC ACTINOMETRY 
 
 observations made by Mr. Elfving, or as it may come to us after a warm day or a 
 period of great heat, which may have increased the invisible chmd of teiebinthine 
 vapors or other odorous essences. But, I I'epeat, all these points must be investi- 
 gated directly, and this preliminary study, although it has continued for many 
 years, has no other claim than that of suggesting new subjects for the study of the 
 atmosphere. 
 
 Let us now I'eturn to the comparison of the effects which equal periods of 
 exposure have in Fiance and in Finland. The following are the results obtained 
 by Ml'. Elfving at Helsingfoi-s, duiing the same period of the same year: 
 
 HELSINGFORS, l! 
 
 
 Solar Com 
 
 justion. 
 
 
 Viatt^ 
 
 
 
 rv Ol^l 1 T'lj u 
 
 l^dlc. 
 
 From 8 A.M. to4P..\f. 
 
 All day long. 
 
 ixt. Ill (tr Ks, 
 
 August 22 
 
 56^ 
 
 66^ 
 
 Clear sky. 
 
 ■' ■23 
 
 51 ^ 
 
 60;^ 
 
 Almost clear 
 
 26 
 
 35^ 
 
 45^ 
 
 Cloudy. 
 
 " 27 
 
 56^ 
 
 75^ 
 
 Clear. 
 
 28 
 
 50 ^ 
 
 68^ 
 
 Half-overcast. 
 
 • " ^9 
 
 ssi 
 
 71^ 
 
 Very clear sky from 9 a.m. 
 
 " 30 
 
 ii?o 
 
 70^ 
 
 Very clear sky from 9 a.m. to 12. 
 
 " 31 
 
 39!^ 
 
 — 
 
 a tt it (t (( 
 
 September 2 
 
 49^ 
 
 59;^ 
 
 Very clear sky. 
 
 3 
 
 49 ^ 
 
 67^ 
 
 (. i( 
 
 6 
 
 54%; 
 
 — 
 
 Almost clear. 
 
 8 
 
 49^ 
 
 — 
 
 Clear early, overcast afterwards. 
 
 9 
 
 52 i 
 
 62^ 
 
 Very clear sky. 
 
 10 
 
 56^ 
 
 -^ 
 
 « « 
 
 II 
 
 59^ 
 
 — 
 
 tt it 
 
 14 
 
 51^ 
 
 — 
 
 Clear sky. 
 
 " IS 
 
 46^ 
 
 — 
 
 ii 
 
 16 
 
 51 ^ 
 
 — 
 
 u 
 
 " 17 
 
 5'^ 
 
 — 
 
 tt 
 
 18 
 
 42^ 
 
 
 Clouds. 
 
 The regulai-ity is hei-e greater than in France, and what is especially I'cmark- 
 able is the close resemblance in an actiiiometric sense of the days which are marked 
 as "similar," in the column of " Reiuarks " (Aug. 29th and 30th, Sept. 2d and 3d, 
 14th, 15th, 16th, and 17th). But what is perhaps most striking in this table, when 
 compared with that on page 26 is that the figures of solar combustion are notably 
 higher than they were in France at Mont Dore, at the same time of the year. 
 Still, Finland is very rich in resinous woods, and e\'en if the station were less sur- 
 rounded by them than at Mont Dore, the altitude is lower, which to some extent 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 29 
 
 makes partial compensation. Besides, the higher figures obtained at Helsingfors, 
 persist at the three positions of the Frencli station : at Paris, on the coast of the 
 Ciiannel, and on the mountain of Puy-de-Dome. 
 
 There can, therefore, remain no possible doubt on this point : the actinic 
 intensity of light in noithern countries, close to the soil, is greater than in our 
 temperate zone at the same hours of the day. It would no doubt be found still 
 weaker if we approached more nearly to the equator. This conclusion was alto- 
 gether unforeseen. 
 
 The fact once established, explanation becomes necessary. This greater ac- 
 tivity of combustion which the air has in nortliern regions, might be ascribed to 
 ozone, rendered more abundant there by the discharges which constitute the 
 aurora boi-ealis, and more active by the action of light. I have begun to study 
 this subject, but my experiments, interrupted by winter and my retui'n to Paris, 
 are not yet completed, and I shall not be able to take them up again till next 
 spring. I believe, however, that I may alieady say that ozone can have but a 
 vei-y secondai-y influence on the phenomenon, and that if light is more active 
 within the same length of time at the north than in Fiance, the I'eason is that it 
 has lost fewer of its chemical radiations in passing through the atmosphere, because 
 the latter is poorer in oxidizable substances. I know, of course, that there are in the 
 north those pine forests, of which I have spoken before, and that perhaps, if 
 Helsingfors were built in the heart of the woods, instead of being a large city on 
 the sea-coast, tlie [)oints of difference would be somewliat less. But there would 
 always remain the fact that the quantities of vapor diffused through the air 
 increase with the temperature, and that, for one and the same aspect of the fauna 
 and the floi'a of the earth, the equatorial atmosphere will always be more heavily 
 loaded than that of the temperate zones, which in its turn will again be more so 
 than that of boreal regions. 
 
 Whatever finally the explanation of the fact may be, the impoitant point is to 
 show that it exists, and that there is a difference in light, so far as its quality is con- 
 cerned, at the same hours of the da}^, at the north and in the heart of Europe. 
 But this is not all. After having examined this question of quality, we have to 
 look next at the question of quantity. The days which are useful to vegetation at 
 the north are longer than with us — what now is the influence of the duration of light 
 on the chemical phenomenon which serves us as a means of measurement ? Is the 
 effect thus produced proportionate to the length of exposure to the sun ? Does it in- 
 crease more or less slowly ? Such are the fii'st questions which we have to consider. 
 
 I believe they are new, because up to this day, both as I'egards meteoiologic 
 instruments and in theoretical speculations, it has always been held that the effect 
 
30 ATMOSPHERIC ACTINOMETRY 
 
 of illumination was, everything else being equal, proportional to its duration. We 
 shall see that this is not so, and that the effect inci-eases much more rapidly than 
 the increase of time, so that all the notions which we entertain on this subject 
 stand in need of revision. 
 
 INFLUENCE OF THE DURATION OF ILLUMINATION. 
 
 So far we have taken as a measure of the total actinic effect during the period 
 of exposure, the sum total of the oxalic acid consumed. The conclusions which 
 we have thus reached, subsist, whatever may be the law which connects the com- 
 bustion with the actinic effect ; it has been enough for us to expose, duiing the 
 same length of time, solutions equally sensitive, in two different places and to pro- 
 ceed always by comparison. 
 
 But the law of the increase of actinic effect, with the time of insolation, does 
 not the less merit attentive investigation. To begin, let us ask first, if the total 
 effect of combustion, observed at the close of a day, upon a solution of oxalic acid 
 exposed to the sun, represents the sum of the divers actinic effects produced at the 
 different hours. 
 
 One way to answer the question is to expose in the moi-ning, side by side, two 
 vessels holding the same quantity of the same solution. One is to be examined at 
 the end of the day, and this will give us the sum total of the effect. The other is 
 to be examined at the end of an hour, and then to be replaced by another vessel like 
 the first, but containing new liquid, to be likewise studied after another hour's ex- 
 posure. In like manner, we shall renew the study at successive hours. If the 
 actinic effects accumulate, without loss or encroachments in the liquid of the vessel 
 which has been exposed to the sun all day long, the quantity of acid which we 
 shall find has disappeared must equal the sum of the quantities of acid which have 
 vanished in the vessels that were exposed for an hour each. 
 
 The experiment, repeated again and again, shows that it is never so. The sum 
 of the quantities of acid burnt in the vessels which have been exposed each one 
 hour only, is always insignificant in comparison with that consumed in a vessel 
 which has spent the whole day in the sunlight. The difference vaiies from one day 
 to another, and increases with the intensity of combustion. It decreases slightly if 
 we carry the exposure of the successive vessels to two hours, and still more if we 
 extend it to three or four hours, as should be expected. But, even if we divide a 
 day of ten hours into two equal periods, one fi'om 7 a.m. to noon, and the other 
 from noon to 5 p.m., the sura total of acid consumed in the two vessels that correspond 
 to the two periods of exposure sometimes does not exceed half the acid consumed 
 in the vessel which spent ten hours in the sun. The combustion, therefore, does 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 31 
 
 not begin as soon as the vessel is exposed to the light. There is a " lost time " at 
 the beginning; two hours, three hours, are necessary for the solution to put itself 
 in action. During this whole time the work is wholly interior and betrays itself 
 by no diminution of the acidimetric titre. 
 
 This "dead time" at the beginning of the reaction should not surprise us. 
 When we study the different reactions of chemistry, from this point of view, we 
 become aware that there are few which begin immediately upon realization of the 
 exterior conditions of production, even in cases when the energy which comes into 
 play is altogether internal as regards the mixture. The formation of a precipitate 
 of barium sulphate is not instantaneous ; that of calcium sulphate, or of calcium 
 tartrate, is still less so ; a mixture of formate and of permanganate of potassium 
 remains apparently inert for some seconds, after which begins an abundant, and, to 
 some extent, an explosive libei'ation of carbonic acid, proceeding from the combus- 
 tion of formic acid. 
 
 Here the heat produced by the reaction intervenes to increase its activity. We 
 can say the same of the phenomenon which Bunsen and Roscoe discovered and in- 
 vestigated under the name of "photochemical induction," in the combination, in the 
 sunlight, of chlorine and hydrogen. This reaction also requires a cei'tain time to 
 commence, but it accelerates afterwards, because it is exothermic. The same 
 remark applies to the reduction of chloride or bromide of silver in the presence of 
 an organic substance, which also shows a " dead time " at the beginning, and be- 
 comes more energetic afterwards. The same remark applies, moreover, to almost 
 all photogi'aphic operations, whether we wish to obtain luminous impressions, to 
 develop images, or to produce positive prints. 
 
 If we observe a " dead time " when the forces are internal and accelerating, it is 
 not surprising that we should find a like one also in the solar combustion of oxalic 
 acid, where the impulse is to come from without and where the reaction is so feebly 
 exothermic. But this verification presents here an interest which it has not else- 
 where, for we connect it intimately with the phenomena of sensibilization, which 
 we discovered pi-eviously in the solutions of oxalic acid. In both cases a molecular 
 preparation is evidently involved, the mechanism of which is still unknown to us, 
 but which results in placing the molecule upon a kind of inclined plane, down 
 which it may be made to roll by the slightest impulse. As a confirmation of this 
 idea, I have ascertained that in fact the " dead time " at the beginning is less pro- 
 tracted with solutions which have been made sensitive, than with new solutions, so 
 that if the latter do not undergo in the light of the sun, as we have seen before, 
 the same degree of combustion as the othei's, it is partly because the " dead 
 time " at the beginning is shorter. But I say " partly " because there is still another 
 
32 ATMOSPHERIC ACTINOMETRY 
 
 pbenomeuon. We shall see that combustion, once begun, does not go on with I'egu- 
 lar anil equal ste[)s, but is made to proceed faster and faster. In other words, every- 
 thing goes on as if the sensitiv^eness were increasing during the oxidation. To put 
 it still differently, the quantity of burnt oxalic acid, which amounts to little or next 
 to nothing during the first moments of the ex[)Osure to the sun, starts out and in- 
 creases, from that instant, quicker than time, so that there is no pi'oportion between 
 the length of the insolation and its consuming effects. 
 
 In order to take account of effect of insolation upon an oxalic solution, let us 
 slightly modify the conditions of an experiment which I have Just described. Let 
 us expose in the morning a dozen similar vessels to the sun, and let us withdraw 
 every other hour two of them, which will give us the sum total of combustion up to 
 that moment. It will be easy by this means to ascertain the progress of combustion 
 during the whole day. The following experiment I cite, not as being the most 
 complete of those which I have made, but because it was pei-forined with a solution 
 of the same sensitiveness as that used in other experiments which I shall (.pote 
 presently, so that all of them are compai-able. 
 
 Ex]p. — -On September 6, 1888, at 8.30 a.m., on Mont Dore, I exposed to the sun 
 four vessels, which I withdrew at various intervals, and in which I meas- 
 ured the proportion of oxalic acid burnt. 
 
 Solar Combustion % 
 
 After 2 hours o ^ 
 
 4 3 ^ ^- 
 
 " 8 " lo s^ 
 
 " 10 " 12 <^ 
 
 We see at the start the " dead time " of the beginning. We see, moreover, 
 that from the fourth to the eighth hour, that is to say from 12.30 p.m. to 
 4.30 P.M., the combustion was twice as I'ajjid as from 10.30 a.m. to noon, in 
 spite of the gi-adual descent of the sun towards the horizon. During the 
 last two hours, and notwithstanding the obliquity of the solar rays, which 
 is already great at this time of the year, the combustion was still two 
 thirds of what it had been between 10 a.m. and noon. 
 It is always the same, whether the total combustion be feeble, as it was at 
 Mont Dore, or active, as I have at times found it in Paris. Fi'om the sum total of 
 my results I think I may conclude that the progress of solar combustion does not 
 remain constant during the whole of the day, and that instead of increasing towards 
 noon, in oi'der to decrease afterwards in proportion as the sun appi'oaches the hori- 
 zon, it, on the contrary, experiences a progi-essive acceleration which does not cease 
 till the sun is near its setting. 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 33 
 
 Everything, then, goes on as if the sensitiveness which, as we have seen, a solu- 
 tion of oxalic acid attains if left to itself in darkness, were not by any means a 
 maximum sensitiveness, and might be greatly increased in the light. I have in fact 
 ascertained — and we shall presently see an example of it — that a recent solution of 
 oxalic acid may be made very sensitive by a few hours' exposure to the sun, and so 
 be brought up to the level of an old solution, or even beyond it. But then an un- 
 foreseen consequence appears : the sensitiveness acquired in darkness is persistent 
 — might it not be perhaps the same with sensitiveness acquired in the light, so that 
 the accelerating effect of a fine day might spread, as it were, in its totality, or at 
 least in part, over the following day ? 
 
 The following experiment proves in fact that a solution left for a whole day in 
 the sun, and which has not been entirely oxidized, will retain for the next day a 
 greater sensitiveness than another part of the same solution which was not pre- 
 viously insolated. 
 
 Exp. — There were exposed every day to the light four identical vessels, two of 
 which were carefully examined at the close of each day, while the other 
 two were left in leserve foi- the day following; on this day they were 
 again exposed to the sun at the same time with the four new vessels of a 
 second experiment. The total of combustion in the vessels which were 
 exposed for two days was then compared with the sum of combustions in 
 the vessels which wei'e each exposed only one day. Some of the results 
 which I thus obtained are as follows : 
 
 The day of Sei)tember 2d, combustion iQ%) j ■ ,, 
 
 3d, 24 f ) 
 
 Both days together, 68 ^. 
 
 The amount of combustion has therefore doubled. Here are the results 
 of another experiment : 
 
 The day of September 4th, combustion 12 ^ ) , - ,, 
 
 " " '• " sth, " II ^r^^ '"''"• 
 
 Both days together, " 38 i. 
 
 The diffei'ence points in the same dii'ection as the preceding experiment ; only 
 it is not quite as great, because the two days, September 4th and 5th, were both 
 quite indifferent days (page 28), whilst the day of September 3d in the first experi- 
 ment was very fine. 
 
 To sum up, we see that the insolated vessel of the first experiment under- 
 went on the second day a combustion of 68 - 10 =■ 58 per cent, while a new vessel 
 suffered only an oxidization of 24 per cent. As to the second experiment, the cor- 
 
34 ATMOSPHERIC ACTINOMETRY 
 
 responding figures are 26 per cent and 11 per cent; this shows that not only does 
 the sensitiveness of the oxalic solution increase in consequence of insolation, but the 
 gain continues during the night. Some experiments of the same kind, which I will 
 not now explain in detail, prove that this excitation of sensitiveness by means of inso- 
 lation, endures even to the second day after, in a solution which is kept in the dark 
 after having been exposed for a day to the sun. It is only after three days, there- 
 fore, that traces of sensitiveness are no longer discernible. By that time, the 
 insolated solution has nearly i-eturned to the degiee of sensitiveness which the 
 mother liquid possessed, which seems thus to coi-respond to a kind of equilibrium. 
 It is in fact remarkable that the diiferent sensitive licpiids which I have used in my 
 long experiments and which were prepared at very different times, with the single 
 precaution that they were not to be used before the lapse of several months, had all 
 of them, at the moment when I used them, very nearly the same sensitiveness. It 
 was on September 8th and 9th that I had to change my solutions at Mont Dore, 
 and I availed myself of the fact that these two days wei'e but indifferently fair, to 
 interrupt my sei'ies and to compare again and again the old liquid with the new. 
 The titre was always the same for both. Mr. Elfving compared likewise two solu- 
 tions which I had sent him a year apart, and found in four days of experimenting 
 the following corresponding figures for the old and the new: 
 
 
 Old Solution. 
 
 New Solution, 
 
 I St day, combustion, 
 
 58^ 
 
 s^i 
 
 2d " 
 
 52^ 
 
 5ii 
 
 3d " 
 
 635^ 
 
 60^ 
 
 4th " 
 
 42 X 
 
 35 ^ 
 
 The old solution was a little more sensitive, which is the usual rule. But 
 the difference was trifling, and thus our former statement was confirmed (page 9). 
 The oxalic solution, kept in diffused light, reaches a fairly constant sensitiveness in 
 a few weeks, but this maximum, although stable, is not a maximum maximorimi. 
 It may be temporarily exalted in the sun, continue if the illumination continues, 
 and return to its original level after some days of darkness. 
 
 In order to make this conclusion really valuable, we have to overcome one last 
 objection. Might it not happen that the increase of combustion discovered on the 
 second day in a vessel which had been insolated on the day before, might mean 
 simply the suppression of the "dead time" at the beginning ? Starting earlier, the 
 combustion might better utilize the good hours of the day, and thus be enabled to 
 go farther. A priori, the intervention of this cause does not seem to explain suffi- 
 ciently the great difference observed. But it is safer to consult experience. It will be 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 85 
 
 sufficient to cut the two days of observation by an intermediate examination and to 
 discover the intensity of combustion during each one of the intervals. Thus it can 
 easily be seen whether the vessels change at the same rate, after having recovered 
 their " dead time," or whethei- the vessel which had been insolated on the previous 
 day, still progi-esses more rapidly than its neighbor? 
 
 ISayp. — On Septeniber 12, 1888, a fine day with a warm sun, and a few cumuli, 
 there wei'e exposed to the sun 4 vessels, Nos. I, 2, 3, and 4. 
 Vessel 1 was examined 
 
 after 5 houi's ; combustion 10 per cent. 
 Vessel 2 
 
 after 9 hours; combustion 18 percent. 
 Vessels 3 and 4 were put aside and exj)osed anew on the next day with 
 two new vessels, Nos. 3' and 4'. This day, the 13th, was very fine, with a 
 few cirri in the morning. It changed a little for the worse towards evening. 
 Vessel 3' was examined 
 
 after 5 hours; combustion 13 per cent. 
 Vessel 3 
 
 after 5 hours ; combustion 44 per cent. 
 The difference is considerable and is certainly in part at least due to the 
 suppression of the " dead time " in the vessel which was insolated on the 
 day before. But this again is not all, for during the second half of the 
 day, the insolated vessel kept up a much more rapid progress than the 
 other, as the following figures clearly show : 
 Vessel 4' examined after 9 hours ; combustion 25 per cent. 
 Vessel 4 examined after 9 hours ; combustion 62 per cent. 
 The acceleration in the solution which had been insolated on the day 
 before, thus continued throughout the Jay, and while in the second half 
 of the second day, the new liquid only showed an increase of 25 — 13 = 
 12 per cent in its combustion, the liquid insolated the day before rose 
 from 44 to 62, undergoing thus an increase of 18 per cent. 
 It will also be noticed that in the morning this same liquid had increased 
 from 44 — 18 = 26 per cent, while the new liquid experienced a combustion of only 
 13 per cent. Here has come in the double effect of suppression uf "dead time" 
 and that of the accelei'ation. The two solutions V)ecame a little more nearly equal 
 towards evening, but the insolated solution continued its quicker progress. 
 
 Thus there can be no doubt that the insolation during the previous day 
 continued its effects over the next day and the day after that. But this is not all. 
 One fact, no less curious than the preceding, is that the sensitiveness due to the 
 
36 ATMOSPHERIC ACTINOMETRY 
 
 action of light, enables the solution to undergo in diffused light a combustion 
 which is out of question as long as it has only its normal degree of sensibility, that 
 is, the degree obtained by keeping for some weeks in a diffused light. 
 
 Ea^. — In August, 1889, at Noalhac near Aurillac (Cantal), at an altitude of 
 about 700 metres, I prepai'ed a solution of oxalic acid, part of which was 
 left in a flask, exposed to a very feeble light, while another portion was 
 exposed to the sun in a stoppered bottle. This was in order to see if the 
 process of i-endei-ing the solution sensitive was necessai'ily accompanied 
 by a pi'ocess of combustion, or whether it could be accomplished without 
 the latter. The experiment showed that the two effects are independent 
 of each other. The solution contained in the closed flask had at its dis- 
 posal only a very small quantity of dissolved oxygen, which it consumed, 
 moreover, with but a slight diminution of its titre, but in one day of inso- 
 lation it I'eached an intense sensitiveness, which was maintained for 
 seveial days at the same rate by preserving it in a diffused light. 
 
 On August 30th, I exposed to the sun two vessels containing some of 
 this insolated solution, and at the same time, two vessels of the same 
 non-insolated preparation. The figures for solar combustion are : 
 18 and 19 per cent for the non-insolated, and 
 92 and 92 " " " insolated liquid. 
 
 The day was a very fine one; the flask with the insolated solution 
 remained in the sun, but closed. On August 31st, two groups of two 
 vessels each were prepared, one containing non-insolated, the other inso- 
 lated solution. One of these groups was exposed to the sun, the other 
 on a window-ledge, facing the north, wheie it received no light but that 
 coming from the sky and dimmed somewhat by a slight dry fog. The 
 figui'es found after 8 hours' exposui'e, for combustion in the sun and in 
 diffused light, were the following: 
 
 Insolated liquid lost 63 per cent in the sun. 
 
 Non-insolated " " 24 " " " " 
 Insolated " "19 " in diffused light. 
 
 Non-insolated " " 6 
 The next day, September 1st, dui'ing a dark, threatening day, the 
 same arrangement gave the following results : 
 
 Insolated liquid lost 50 per cent in the sun. 
 Non-insolated " " 13 " " " " 
 Insolated " " 6 " in ditt'used light. 
 
 Non-insolated " " 3 " " " " 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 37 
 
 Thus a previous insolation increases the rapidity of combustion, not only in di- 
 rect light but also in diffused light. The experiment of September 2d shows, how- 
 ever, when compared with that of the day before, that this diffused light must 
 have a certain intensity to make its effect measurable after some hours. But, 
 viewed by itself, this experiment shows that even a dark day still has an accelerat- 
 ing effect upon a solution which has not seen the sky since the second day before. 
 The insolated liquid was from that moment kept in diffused light at the back 
 of a room with but one window, facing the north. It was found that after a few 
 days it had not sensibly changed in titre, l>ut variation began to show itself at the 
 end of a month. We tlius see how here also, in spite of good conditions of preser- 
 vation, the phenomena of slow combustion appear which have been observed since 
 Wettstein in solutions of oxalic acid. Not insolated, this solution preserved in the 
 same manner had remained much more stable, which shows that it is necessary to 
 avoid exposure to light, even temporarily, or even in a carefully closed flask, of 
 solutions of oxalic acid which are intended for processes of titration. The lumi- 
 nous impression, once received, persists and makes them much less stable — it con- 
 tinues, as we shall presently see, even after the liquid solution has been placed in 
 darkness. 
 
 Eocfp. — Another experiment was begun identical with those that have just been 
 described except that the flask which contained the insolated solution, 
 sheltered from the air, was kept for three nights and two days in a cupboard 
 of th& laboratoiy before being distributed into vessels on September 6th. 
 Unfortunately the day of the 6th was disturbed by cirri and cloudlets. 
 The insolated liquid Tost 20 per cent in the sun, 
 Non-insolated liquid lost 7 per cent in the sun. 
 • The oxidation in diffused light was msignificant. 
 The proportion of oxalic acid bm-nt in sunlight is, therefore, still, after 60 
 hours of obscurity, tliree times greater in the insolated solution than in the other. 
 But the sensitiveness decreases afterwards and the difference soon ceases to be 
 measurable after a day's exposure. We here meet again with that retrogradatiou 
 which we liave pointed out earlier, and which brings us back to nonnal sensitive- 
 ness. 
 
 I add, in order to close the subject, that this solar impression, which disap- 
 pears slowly, is on the other hand produced veiy rapidly, and that, when investi- 
 gating comparatively, vdth respect to the combustion which they undergo, the 
 solution which I had exposed to the sun, in three flasks, and 1, 2 and 3 days, 
 respectively, I have not been sible to show that there was any essential difference 
 between them ! 
 
38 ATMOSPHERIC ACTINOMETRY 
 
 I have left aside, in all which precedes, the question of the mechanism con- 
 nected Avith both sensitization and combustion. The former goes t)n when the 
 solution is sheltered from the air, and can take place only by a new ari'angement 
 of molecules. Combustion, on the other hand, takes place in contact with the air 
 and possibly with the formation of ozone or of hydrogen peroxide. That is a ques- 
 tion which must be investigated by itself. 1 purpose here only to put in evidence, 
 as regards the constitution of the atmosphere, some properties and a varial>ility of 
 effects, not hitherto observed. 
 
 Metekeologioal, Hygienic, and Agricultural Effects. 
 
 If we now return, with the results which we have obtained, to our investiga- 
 tion of the causes which provoke the rapid development of vegetaticm in the exti'eme 
 northern regions, we see that those regions are superior to ours in a twofold 
 aspect. 
 
 1. That cause which depends on the constitution of their atmosphere consists in 
 this, that the absoi-ption of the chemical radiations of solar light is there less great 
 than with us. The actinic power at the level of the soil exceeds that which we 
 have obsei-ved around ourselves at different hours of the day, and that in spite of 
 a lower sun and a greater thickness of atmosphere, which the rays must traverse. 
 These differences are due mainly to the fact that vegetation in the north sends 
 into the air fewer oxidizable substances to form a screen. 
 
 2. The other point of superiority connected with the geographical situation 
 consists in this, that in the extreme north, during the })eriod of vegetation, the 
 days are longer than in our temperate zones, and that the actinic power, at least 
 so far as it may be measured by a solution of oxalic acid, increases more rapidly 
 than the length of the day, and this out of all proportion. After a pei'iod of prepa- 
 ration, combustion })egins, then accelerates so rapidly as to make up for time lost 
 at the beginning, and finally, towards evening, reaches unusually high figures, such 
 as are unknown to our regions. It is in this way that combustion has risen to 
 
 37 and 89 per cent on September 2 and 4, 1887, 
 79 and 80 per cent on September 9 and 11, 1888, 
 75 per cent on Aug. 27, 1888 at Helsingfors. 
 
 and this at a time when the highest figures, relatively to the same periods and with 
 the same solutions, did not exceed 50 per cent and were even sometimes much 
 lower in our country. In order to reach figures equal to those obtained in the Gulf 
 of Finland, it was necessary for me in France to accumulate upon my vessels the 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 39 
 
 radiation of two consecutive days. It may thus be said that, speaking generally, a 
 day at the north is worth two of ours as regards actinic power. 
 
 Not only does the actinic effect of a fine day increase more rapidly than the 
 length of the day itself, but it may actually spread itself over the next day, and 
 the day after that, and thus make up, in some degree, for the absence of the sun. 
 In like manner, a fine morning may render combustion more rapid even though 
 the evening be dark and stonuy. It is enough that the liquid shall have been 
 made sensitive; and as this sensitization is the more rapid as the actinic intensity is 
 greater, the atmospheric condition of northern countries favors them in this respect 
 beyond us, and a new superiority is thus attained through the superposition and 
 mutual emphasis of the other two causes. 
 
 Finally, the sensitiveness produced by a fine day continues for several days. 
 A number of bad days, following each other, is consequently not a period of 
 inertness and loss ; it draws upon the store which was collected during fine 
 weather. On the other hand, we have seen that the sensitiveness which was 
 acquired in the sun, did not increase without limit, and that it reached quite rap- 
 idly a maximum beyond which it did not go. A succession of fine days, therefore 
 does not develop actinic phencmiena to an extreme. We here meet once more 
 with the system of balancing which weakens gi'eat effects, increases small ones, 
 and which has been i)ointed out with regard to so many other manifestations of 
 the forces of nature. 
 
 Summing up the matter, then, it would seem that we have hitherto missed our 
 way, in considering the chemical action of solar light as independent of locality and 
 proportionate to time of isolation, or as furnished or measured by meteorological 
 instruments. The first of these notions was purely instinctive and was suggested 
 especially by the imiformity which was ascertained to exist at different points of 
 the globe in so many other gi'and meteorological phenomena (such as the composi- 
 tion of the air, the average barometric elevation, the mean distribution of nebulos- 
 ity, etc.) Instead of such a uniformity, we find, on the contrary, actinic climates, 
 limited in point of surface, for they betray the local influence of the surface of the 
 soil — limited also in point of duration, for they are due to two kinds of clouds 
 which are subject, like the others, to the influences of place and season. 
 
 Misjudging thus local influences, only the first cause has been thought of, and 
 all efforts had been directed towards measuring the duration of insolation. On 
 this point, I think I have shown that the wrong way had been taken. The actinic 
 force of a day is not the same for the same day, in different parts of the globe, and 
 its effect increase8"~hiore rjipidly than its length ; such is the principal lesson of this 
 Memoir. 
 
40 ATMOSPHERIC ACTINOMETRY 
 
 One step farther might be taken. We have just ascertained that in the solu- 
 tion of oxalic acid there takes place a kind of storing up of light, which shows 
 itself in an increase of sensitiveness as regards phenomena of oxidation. Might 
 not the oxygen which is present in the solution, or even that which is constantly 
 dissolved there and transformed, might it not itself be rendered sensitive, and so be 
 endowed with an oxidizing power which it could afterwards use in diffused light ? 
 
 I have found nothing, while searching in this direction, with oxalic acid ; this 
 reagent, quite sensitive enough for the study of powerful actions, is not sensitive 
 enough for such weak actions as that which I have just suggested. But I 
 have been more successful with oxidizable substances of sharper reactions, so that 
 the very smallest variations became measurable. This is the case with diastases ; 
 an almost infinitesimal quantity will produce very apparent effects, and it is, there- 
 fore, easy to trace their disappearance by oxidation in the liquids which contained 
 them. With rennet especially the very smallest variations in quantity can be a[)- 
 preciated from corresponding variations in the time of coagulation of equal quan- 
 tities of milk, so that this diastase is very convenient for study. By sitch means 
 I found that it oxidized and disappeared in water which had been previously 
 exposed to the sun, while it remained, if not quite intact, at least nearly so in the 
 same water as freshly drawn from the hydrant. I also found that a glass flask 
 exposed to the sun stored up on its walls enough chemical radiations to accelerate 
 afterwards the oxidation of a solution of rennet, which was allowed to stand in the 
 shade. 
 
 All these facts, upon which I do not insist because they go beyond the limits 
 of this work, enlarge the field of those phenomena to which they apply. If inso- 
 lated liquids and solids, may in certain cases, like our solutions of oxalic acid, 
 acquire properties which they had not before, the phenomena of solar combustion 
 may well extend below the surface, which has been directly illuminated, and assume 
 in the general economy of the world an importance, no doubt as yet inferior to 
 that of microscopic organisms, but certainly no longer to be neglected, as it has 
 been heretofore. 
 
 Since I have entertained the views which I have developed in the preceding 
 pages, I have investigated especially their agricultural and hygienic consequences. 
 
 As far as hygiene is concerned, I have shown more clearly I think than 
 Messrs. Downes and Blunt that solar light kills the germs of microbes suspended 
 in the air. I have proved, moreover, that this destruction was preceded by a 
 veritable attenuation} 
 
 ' Annates de Chimie et de Physique, 6tli ser., vol. v., 1885. 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. , 41 
 
 I have ascertained since that this destruction and this attenuation go on in 
 the supei-ficial layers of the soil and even down to some depth. If, in the many 
 attempts to count the microbes of the soil, it has so often been found that the 
 number is less near the surface than at the depth of a few centimetres, we must 
 attribute this result much rather to solar combustion than to desiccation. The 
 sanitaiy action of oxygen, which is pursued and completed in the atmosphere, 
 begins therefore, thanks to light, at the surface of the soil, and the healthiest coun- 
 tries are those in which the actinic power of the sun is greatest. 
 
 By a curious mechanism, which I have tried to make generally known, the 
 solar action which neutralizes the microbes which it encounters, can act like them, 
 and take their place. I have in fact shown in an extensive work ' that the changes 
 which carbohydrates undergo upon exposure to sunlight are exactly like those 
 which they undergo under the action of ferments. Starting from the same point, 
 these two modes of transformation, apparently so different, resemble each other 
 not only in their variety and marvellous flexibility of conduct, but still more in 
 their intermediary and final products. 
 
 Thus, invert sugar in alkaline solution, oxidized in sunlight, gives intermediary 
 products which are colloidal and identical with humic acids, except that they are 
 not nitrogenous. These black acids are afterwards consumed by light, exactly 
 as we see in the bleaching of the black soil which the spade or the plow has 
 turned up. 
 
 The extreme terms of the transformation of this sugar or of its humic deriva- 
 tives are as numei-ous and as varied in solar combustion as when produced by the 
 action of ferments. Thus, by contact with potash, or with soda, we obtain alcohol 
 through an interior combustion which is identical with that produced during alco- 
 holic fermentation. On the other hand, in the presence of baryta, no alcohol, but 
 lactic acid is produced. In this there is analogy, not with alcoholic fermentation, 
 but with lactic fermentation, and this analogy is all the closer since — as is recog- 
 nized — there may be several lactic acids of different rotaiy power, which may be 
 produced as well by the action of light as by that of fenuentation. 
 
 This solar, lactic fermentation is accompanied by the production of acetic acid, 
 as in the case of microbian fermentation. In other cases, butyiic acid is formed, 
 formic acid, oxalic acid — in short, all the ordinaiy residues of the fennent action. 
 Finally, carbonic acid represents in all cases the extreme tenn of the change of 
 organic matter into gas. 
 
 The~luminous action, varying in quantity according to place and season, as the 
 
 ' Antiales de I'lnstiiut Agronomique, vol. x., 1886. 
 
42 ATMOSPHERIC ACTINOMETRY 
 
 different chapters of this Memoir have shown, may therefore differ in the (quality 
 of the effects which it produces. All these facts lend to the study of chemical 
 radiation, an importance of the highest rank, and I shall consider myself very 
 happy, if the first results contained in this paper shall lead men of science to 
 new researches. 
 
 SUMMARY. 
 
 1. The oxidation of oxalic acid in a weak solution takes place mainly, and 
 almost exclusively, under the influence of the chemical rays of solar light ; it can, 
 therefore, be used as an actinometric measure. 
 
 2. It depends on the concentration of the liquid, which for the best results 
 should not exceed about three grammes per litre. 
 
 3. With an equal volume of solution, combustion decreases as depth increases ; 
 there is an absorption of chemical rays, although the liquid is and remains very 
 transparent. 
 
 4. For equal depths of liquid, combustion is proportional to the surface, and 
 consequently also to the volume. 
 
 5. It depends on the age of the solution, that is to say, of the time which 
 has elapsed since preparation. As it grows older, an oxalic solution becomes more 
 sensitive, and attains a certain niaxiiuuin which is (|uite stal)le and (juite regidar. 
 It is well to wait till this state of sensitiveness has been produced. 
 
 6. The daily combustion, such as is measured with sterilized liquids, varies 
 from one day to another much more than any other meteorological phenomenon, 
 and while subject to the influence of what we call " fine weather " and " overcast 
 weather," it manifests very clearly other influences which are less visible. 
 
 7. It shows also the influence of the seasons, and manifestly exhibits a maxi- 
 mum in spring. 
 
 8. It is but feebly subject to the influence of altitude. 
 
 9. On the other hand, it betrays so strongly the presence of divers oxidizable 
 essences or substances in the air, that we must consider local and daily variations 
 as due to the presence in the atmosphere of actinic clouds, which are discoverable 
 only by the reduction and absorption which they produce in the chemical radiations 
 of sunlight. 
 
 10. The atmosphere of extreme northern regions is less absorbent than that 
 of our temperate zones, and, consequently, at the same hours of the day, actinic 
 radiation is more powerful, at the level of the soil, in the north than at the centre 
 of Europe. 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 43 
 
 11. Northern countries add to this cause of superiority, which they owe to 
 the constitution of their atmosphere, another, which is due to their geographical 
 position, namely : that the actinic effect of the sun increases more rapidly than the 
 duration of its presence above the horizon. The veiy long days of the north, 
 during the period of vegetation, are, therefore, in their actinic effect, more active 
 than an equal number of days in our temperate regions, and we can thus explain 
 the particularly intense rate of the progress which vegetation makes in the vicinity 
 of the polar circle, 
 
 12. This increase of sensitiveness which oxalic acid experiences in the sun, 
 does not cease when the light begins to fade, and may continue several days. 
 Hence follows a conclusion which may also be applied to our temperate regions: 
 this is, that the actinic effect of a number of fine days in succession increases more 
 rapidly than its duration, and also, that the effect of a fine morning is not lost by a 
 dark and cloudy evening. 
 
 13. We must, theiefore, give up the hope of finding, in the duration of a day 
 or of solar action, a measure of its effects, and meteorological instruments, which 
 accept such a proportionality, ai'e to be rejected. 
 
 14. The importance of these actinic phenomena in the general economy of 
 the world is great enough to make it necessary that we should approach the 
 investigation by appropriate means. 
 
SUPPLEMENT. 
 
 OBSERVATIONS MADE IN 1894 IN FRANCE AND ALGIERS. 
 
 Since sending my Memoir on Atmospheric Actinometry, I have been enabled 
 by the courtesy of M. Gessard, Chief Pharmacist of the Military Hospital at Setif 
 (Algeria), to make a number of combined obsei'vations in a temperate region and in 
 a hot climate. It was interesting to discover whether we would meet here with 
 the same diffei'ences as between the obseivations made in France and in Finland, 
 that is to say, if for equal lengths of insolation the chemical activity of the solar 
 rays would continue to diminish in proportion as we approach the equator, and 
 as their calorific power increases. 
 
 For such a comparison the choice of the stations was of some importance. 
 Setif is situated about eleven hundred metres above the level of the sea, on a buttress 
 of the southern slope of the high mountains of the sea-coast, the chain of theBabere 
 or the Bibans. Towards the south it overlooks from a height of two or three hun- 
 dred metres an immense plain, which in its turn isboi'dered at a distance of 35 or 40 
 kilometres (22 to 25 miles) by a chain of not very high mountains, which cuts it 
 off from another more extensive plain, the basin of the Hodna. Beyond this, se[)a- 
 rated again by an insignificant mountainous elevation, lies the Sahara and the desert 
 climate, which makes its influence felt as far as the plain of Setif. This vast heat- 
 ing-centre south of the city frequently procures for the lattei', towards evening, a 
 fresh current of air from the north, and in ordinaiy times it stands on the boundary 
 line where two contrary influences enter into direct conflict, the wind blowing fi'om 
 the coast and the high summits, and the burning wind from the desei't. Thus 
 Setif enjoys a relative freshness on certain days when the plain at its feet is given 
 up to the full ardor of the sirocco, and when it even may happen that the cloud of 
 dust, propelled by this wind, stopping at a distance of 15 or 20 kilos, from the 
 town, screens the neighboring mountains at the very time when the atmosphere 
 remains quite clear about Setif and the immediate surroundings. 
 
 The station which I have chosen in France i'oi- my comparative observations 
 is also situated on the side of a slope, overlooking the plain of Vic-sur-C^re (Cantal), 
 and 750 metres above the level of the sea. I might have gone higher, but I have 
 
 44 
 
ATMOSPHERIC ACTINOMETRY. 
 
 45 
 
 alread}' shown that the difference in altitude is of little importance. At all events 
 it acted in the opposite sense to the phenomenon which I sought to verify. As a 
 compensation, the climate of this station at Olmet is a temperate climate. The 
 place lies on the line where the culture of the vine ceases and is in every respect 
 equal to that of Fau, in the valley of Marmanhac, and of Noalhac, in the valley of 
 Aurillac, where I had made my first observations. 
 
 Furthermore, the procedure of M. Gessard and myself was the same; we 
 exposed, from 8 o'clock a.m. till 5 o'clock p.m., vessels containing the same solution, 
 only, on account of the high temperature of Setif during the summer and of the evap- 
 oiation caused by it, we had to pour into the vessels 20 cubic centimetres of oxalic 
 acid, instead of 10, and place them, not upon wooden or stone suppoi-ts, but upon 
 the water of a great crystallizing pan. I need not say that at Olmet I followed 
 the same practice. It is well known that the degree of solar combustion depends 
 on the depth of the liquid, and this is the reason why the present series of experi- 
 ments is not directly comparable with the preceding series. But it is sufficient for 
 us that the expeiiments made in France and in Algeria should be comparable 
 between themselves. 
 
 This being granted, I subjoin the i-esults obtained by M. Gessard : 
 
 Date. 
 
 Solar 
 Combustion. 
 
 July 
 
 2 
 
 12 
 
 
 
 
 
 3 
 
 14 
 
 
 4 
 
 12 
 
 
 5 
 
 13 
 
 
 6 
 
 i6 
 
 
 
 
 
 7 
 
 '7 
 
 
 8 
 
 62 ' 
 
 
 9 
 
 18 
 
 
 1 1 
 
 40 
 
 
 
 
 
 12 
 
 32 
 
 
 '3 
 
 22 
 
 
 
 
 
 i.S 
 
 25 
 
 
 
 
 
 17 
 
 22 
 
 
 i8 
 
 22 
 
 
 
 
 
 19 
 
 24 
 
 
 
 
 
 20 
 
 22 
 
 
 
 
 
 21 
 
 23 
 
 Remarks. 
 
 Clouds. 
 
 Stormy weather. Sky covered at 3 o'clock. 
 
 Fewer clouds than on preceding days. 
 
 Clear weather. No dust. 
 
 Cloudy at times. (See note below.) 
 
 Tempest. Dust. Sun obscured after noon. 
 
 Cloudy weather. 
 
 Fine weather. 
 
 Overcast weather ; a few drops of rain. 
 
 Clouds, storms, dust repeatedly. 
 
 Fine weather. Hail-storm at 7 p.m. 
 
 The Southern mountain cannot be seen at 5 p.m. 
 
 No clouds. Very clear horizon. 
 
 Overcast from 2 to 4.30 p.m. Dust. 
 
 Overcast after noon. Downpour at 3 p.m. 
 
 In the following experiments, made in August and September, M. Gessard 
 ascei-tained the direction of the wind and the temperature, as read on a thermometer 
 
 ' One of the vessels was found to be submerged on this day, so that the result could not be 
 correctly ascertained. The weather on this day did not essentially differ from that of the preced- 
 ing and of the following days. 
 
46 
 
 ATMOSPHERIC ACTINOMETRY 
 
 hanging in a northern exposure, against the wall of a house with lofty arcades, and 
 consequently under a galleiy formed by them. In the statements concerning the 
 winds, the frequent violent changes of which we spoke at the beginning will be 
 noticed. 
 
 
 
 
 
 Temperature. 
 
 Winds (by 
 
 the vane 
 
 )at 
 
 Date, 
 
 
 Solar 
 
 Kemarks 
 
 
 
 
 
 
 
 
 
 
 Combustion. 
 
 M.^\i*tll<&4 a^^t 
 
 
 
 
 
 
 
 
 
 
 
 
 Max. 
 
 Min. 
 
 Aver. 
 
 8.30 
 
 11 
 
 3 
 
 5 
 
 August 
 
 i8 
 
 6^ 
 
 Clouds at noon. Overcast 
 
 
 
 
 
 
 
 
 
 
 
 sky at 4. Rain at 4.30. 
 
 34- 
 
 15-2 
 
 3'-5 
 
 W 
 
 w 
 
 N 
 
 SW 
 
 ti 
 
 19 
 
 3^ 
 
 Quite a fine day. g. g. clouds 
 
 
 
 
 
 
 
 
 
 
 
 at 2 o'clock. 
 
 30.2 
 
 14- 
 
 27-5 
 
 E 
 
 s 
 
 NNE 
 
 NNE 
 
 a 
 
 20 
 
 3^ 
 
 Sun rather overcast in the 
 morning. Rain after 2 
 
 
 
 
 
 
 
 
 
 
 
 o'clock. 
 
 27.9 
 
 12.5 
 
 25- 
 
 BE 
 
 N 
 
 N 
 
 NE 
 
 <i 
 
 21 
 
 3^ 
 
 Sun all day long and sky 
 
 
 
 
 
 
 
 
 
 
 
 clear. 
 
 28.6 
 
 158 
 
 25-5 
 
 W 
 
 w 
 
 NW 
 
 N 
 
 <t 
 
 22 
 
 8^ 
 
 The same. 
 
 311 
 
 17-5 
 
 28.5 
 
 W 
 
 w 
 
 N 
 
 NE 
 
 ti 
 
 23 
 
 8^ 
 
 4< ti 
 
 32.6 
 
 18.6 
 
 30- 
 
 SW 
 
 w 
 
 S 
 
 SW 
 
 it 
 
 24 
 
 11^ 
 
 (( (( 
 
 33-2 
 
 17.2 
 
 31-5 
 
 SW 
 
 SW 
 
 S 
 
 SW 
 
 (i 
 
 25 
 
 6^ 
 
 (( ti 
 
 31-4 
 
 17- 
 
 19. 
 
 SW 
 
 w 
 
 S 
 
 SW 
 
 « 
 
 26 
 
 8^ 
 
 Clouds in the afternoon. 
 
 32.6 
 
 17.8 
 
 29s 
 
 SW 
 
 SW 
 
 s 
 
 SW 
 
 it 
 
 27 
 
 9^ 
 
 Clouds afternoon. Sun 
 
 
 
 
 
 
 
 
 
 
 
 shines at 3 p.m. 
 
 31.8 
 
 16.2 
 
 295 
 
 W 
 
 w 
 
 NE 
 
 NE 
 
 it 
 
 28 
 
 8^ 
 
 Sun all day long. 
 
 31-3 
 
 16.3 
 
 28.S 
 
 W 
 
 SW 
 
 NNW 
 
 NNE 
 
 a 
 
 29 
 
 Ti 
 
 Same. 
 
 311 
 
 16.3 
 
 29. 
 
 SW 
 
 SW 
 
 SW 
 
 NE 
 
 a 
 
 30 
 
 li 
 
 ti 
 
 31.6 
 
 18.6 
 
 30- 
 
 SW 
 
 SW 
 
 SW 
 
 NE 
 
 a 
 
 31 
 
 Ti 
 
 it 
 
 32.2 
 
 19.0 
 
 29s 
 
 SE 
 
 SE 
 
 SE 
 
 SSE 
 
 September 8 
 
 9^ 
 
 Overcast. Much dust. 
 
 25-4 
 
 17- 
 
 29-5 
 
 E 
 
 s 
 
 SSE 
 
 S 
 
 l4 
 
 19 
 
 10 ^ 
 
 Sun all day long. 
 
 29. 
 
 15-6 
 
 29s 
 
 SE 
 
 s 
 
 SSE 
 
 S 
 
 a 
 
 22 
 
 11^ 
 
 A few clouds all day long. 
 
 29. 
 
 14.8 
 
 27- 
 
 W 
 
 NW 
 
 w 
 
 N 
 
 
 23 
 24 
 
 II i 
 II ^ 
 
 Sun all day. 
 
 Sun rather obscured, after- 
 
 29.4 
 
 17.7 
 
 26.5 
 
 W 
 
 NW 
 
 w 
 
 NW 
 
 
 
 
 noon. Dust. 
 
 30-9 
 
 17.1 
 
 28. 
 
 SW 
 
 SW 
 
 SW 
 
 SW 
 
 i< 
 
 25 
 
 14$^ 
 
 Overcast sun all day. Dust. 
 
 30.8 
 
 17- 
 
 28. 
 
 s 
 
 SW 
 
 SW 
 
 S 
 
 When M. Gessard sent me the reports for August and September, he added 
 what follows : "They seem to me to confirm the former. The combustions have 
 not been more intense than in July. I notice, on the other hand, that during over- 
 cast weather they have not been inferior to those observed on days of full sunshine. 
 The second fact which strikes me is this : that the quality of combustion should 
 be maintained with such constancy in the two vessels under all circumstances. 
 Thus, on the 11th September (I did not report it in the table), with a south wind 
 blowing all day, raising the maximum temperature to 32° 4', the contents of both 
 vessels having evapoi-ated, although they had been allowed to float upon the water, 
 I had the cunosity to re-dissolve the residual acid and to test it : I found 8.4 c. c. 
 
AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 
 
 47 
 
 and 8.6 c. c, which corresponds to combustions of 36 per cent and 35 per cent, 
 respectively. 
 
 It will be seen from all that has been said so far, that the daily actinometric 
 combustions are quite as irregular in Algeria as in France, and have only a very 
 remote connection with the external aspect of the sky and the clearness of the 
 horizon. 1 have not been so fortunate as M. Gessard so far as the weather was 
 concerned. The gi'eater part of the month of August in the Cantal and a part 
 of the month of September we had cloudy or rainy periods, and during this time 
 obsei'vations were impossible. Availing myself of the fact that the method of 
 comparison which we employed consists of a compaiisou of the finest days at the 
 two stations, I shall here I'eport only the figures noted down at Olmet during the 
 rare times of fine weather. 
 
 Date. 
 
 Solar 
 Combustion. 
 
 Remarks. 
 
 August 
 
 27 
 
 29 ^ 
 
 — ■ — — ■ — ■ 
 
 Fine day from beginning to end. 
 
 *i 
 
 28 
 
 42 ^ 
 
 Fine weather, but rather heavy and stormy. 
 
 n 
 
 29 
 
 41 ^ 
 
 A fine day, although rather foggy. 
 
 It 
 
 30 
 
 35 ^ 
 
 A fine day, rather stormy. Some clouds. 
 
 a 
 
 31 
 
 41 !« 
 
 A fine day. Some mist towards 4 o'clock in the west. 
 
 September 2 
 
 27 ;< 
 
 Clear in the morning ; rather foggy in the evening. 
 
 
 17 
 
 40^ 
 
 Quite a fine day in spite of g. g. clouds. N. E. wind. 
 
 
 18 
 
 34 !« 
 
 Cumulus covering } of sky, all day long. 
 
 
 19 
 
 52 ^ 
 
 Less cumulus than the day before. Finer day. 
 
 
 20 
 
 60^ 
 
 A very fine day. 
 
 
 21 
 
 52 ^ 
 
 Rather stormy day. 
 
 
 27 
 
 28^ 
 
 A fine day ; few clouds. 
 
 
 29 
 
 23 ^ 
 
 An indifferent day. 
 
 Comparing this list with M. Gessard's we notice in the first place a coincidence 
 between the days of August 27th, 28th, 29th, 30th, and 31st, which were eithei- 
 fine or very fine at Olmet and at Setif. Now if we compai-e the results of the 
 daily combustion fi'om 8 a.m. till 5 p.m. we find, respectively : 
 
 9, 8, 7, 7, and 7 for Setif ; 
 and 29, 42, 41, 35, and 31 for Olmet. 
 
 If we in like manner compare, leaving out the dates, the combustion on the 
 finest days in France and in Algeria, we find again that it decreases with the lati- 
 tude. This is the same conclusion to M'^hich we were led, when we compared the 
 observations made in Fi'ance and in Norway, and between the limits of the two 
 stations at Helsingfors (lat. 60° 19'), and at Setif (lat. 36° 1 1'). This shows that the 
 
48 ATMOSPHERIC ACTINOMETRY. 
 
 actinic effect of twilight during equal times of insolation goes on diminishing in 
 proportion as we approach the equator and as the mean temperature rises. 
 
 It would be interesting to ascertain whether this law continues into the 
 tropical regions. This is probable, but as yet is only a matter of conjecture. I have 
 taken the proper steps to begin observations on this subject early next spring. 
 
 Whatever this extension may lead to, it is none the less unexpected to find 
 that in our temperate regions, the most densely populated of the globe, the actinic 
 effect of the sun is, so to speak, in the opposite direction to its calorific influence. 
 The operation of this law on the flora cannot be doubted. But the future alone 
 can tell us how this is brought about. 
 
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