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 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 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 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