another action of this light still more prompt and energetic. When a mixture of oxymuriatic acid gas and hydrogen gas are exposed to the action of solar light, a detonation takes place, and water and muriatic acid are formed. These different phenomena enabled M. Berard to examine the chemical powers of the different rays of the spectrum. By exposing to the different coloured rays, pieces of card impregnated with muriate of silver, or small phials filled with the detonating mixture, he was enabled to judge of the energy of each by the intensity or rapidity of the chemical change which it produced. He found that the chemical intensity was greatest at the violet end of the spectrum, and that it extended, as Ritter and Wollaston had observed, a little 'beyond that extremity. When he left substances exposed for a certain time to the action of each ray, he observed sensible effects, though with an intensity continually decreasing in the indigo and blue rays. Hence we must consider it as extremely probable, that if he had been able to employ reactives still more sensible, he would have observed analogous effects, but still more feeble, even in the other rays. To show clearly the great disproportion which exists in this respect' between the energies of the different rays, M. Berard concentrated, by means of a lens, all that part of the spectrum which extends froin the green to the extreme violet; and he concentrated, by means of another lens, all that portion which extends from the green to the extremity of the red. This last pencil formed a white point so brilliant that the eyes were scarcely able to endure it; yet the muriate of silver remained more than two hours exposed to this brilliant light without undergoing any sensible alteration. On the other hand, when exposed to the other pencil, which was much less bright, and less hot, it was blackened in less than six minutes. M. Berard concluded, from this experiment, that the chemical effects produced by light are not solely owing to the heat developed in the body by its combining with the substance of the body; because, on such a supposition, the faculty of producing chemical combinations ought to be greatest in those rays which possess the faculty of heating in the greatest perfection: but perhaps we should find less opposition between these two opinions, if we attended to the different results which may be produced by the same agent placed in different circumstances, and if we considered that agents of a nature quite dissimilar may determine the same combinations when they are employed. Such is an epitome of the principal facts which M. Berard has established in his memoir. To great accuracy he has united an excellent arrangement in his account of his experiments, He has presented the physical properties of the different rays merely as the results of experiments, the hypothetical causes of which he has abstained from inquiring into: and he has always employed terms so general as to be applicable, whether the properties treated of belong to a substance really distinct and combined with light, or result simply from original differences which exist among the different molecules of the same principle, which, according to differences in the size or the velocity, or in both united, become capable of producing chemical combinations, vision, and heat. Without attempting to decide between two opinions, which go both beyond the facts observed, we may at least weigh their relative probabilities, and compare the number of hypotheses necessary in each to represent the same number of facts. If we wish to consider solar light as composed of three distinct substances, one of which occasions light, another heat, and the third chemical combinations; it will follow that each of these substances is separable by the prism into an infinity of different modifications, like light itself; since we find, by experiment, that each of the three properties, chemical, colorific, and calorific, is spread, though unequally, over a certain extent of the spectrum. Hence we must suppose, on that hypothesis, that there exist three spectrums one above another; namely, a calorific, a colorific, and a chemical spectrum. We must, likewise, admit that each of the substances which compose the three spectrums, and even each molecule of unequal refrangibility which constitutes these substances, is endowed, like the molecules of visible light, with the property of being polarized by reflection, and of escaping from reflection in the same positions as the luminous molecules, &c. Instead of this complication of ideas, let us conceive simply, according to the phenomena, that light is composed of a collection of rays unequally refrangible, and of course unequally attracted by bodies. This supposes original differences in their size and velocity, or in their affinities. Why should those rays, which differ already in so many things, produce upon thermometers, or upon our organs, the same sensations of heat or light? Why should they have the same energy to form or separate combinations? Would it not be quite natural that vision should not operate on our eyes, except within certain limits of refrangibility; and that too little or too much refrangibility should render it equally incapable of producing that effect. Perhaps these rays may be visible to other eyes than ours, perhaps they are so to certain animals, which would account for certain actions that appear to us marvellous. In a word, we may conceive the calorific and chemical faculty to vary through the whole length of the spectrum, at the same time with the refrangibility, but according to different functions; so that the calorific faculty is at its minimum at the violet end of the spectrum, and at its maximum at the red end; while, on the other hand, the che mical faculty expressed by another function is at its minimum at the red end, and at its maximum at the violet end, or a little beyond it. This simple supposition, which is only the simple statement of the phenomena, equally agrees with all the facts hitherto observed, and accounts for those established by M., Berard, and even enables us to predict them. In fact, if all the rays, which produce these three orders of phenomena, are rays of light, they must of course be polarized in passing through Iceland crystal, or in being reflected from a polished glass with a determined incidence: and when they have received these modifications, thy must be reflected by another glass, if it is properly placed, to exert its reflecting energy on the luminous molecules. On the other hand, if that force is null on the visible luminous. molecules, the invisible light will not be any longer reflected: for the cause which occasions or prevents reflection appears to act equally upon all the molecules, whatever their refrangibility may be. It ought, therefore, to act upon the molecules of invisible light, the condition of visibility or invisibility relating merely to our eyes, and not to the nature of the molecules which produce these sensations in us. But though this mode of viewing the facts appears to us the most natural and simple, we cannot but approve the sage reserve of M. Berard in not attempting to decide questions upon which experiment has not yet accurately, pronounced. The Class heard with pleasure the detail of these interesting experiments, which were presented by the author on the same day that he and M. Delaroche obtained the prize offered for the determination of the specific heat of the gases. We propose to the Class to confirm, by its approbation, this new and valuable set of experiments; and we regard it as very worthy to be printed in the Recueil des Savans Etrangers. (Signed) BERTHOLLET, CHAPTAL, and BIOT, 2 I ARTICLE II. On the Daltonian Theory of Definite Proportions in Chemical Compounds. By Thomas Thomson, M.D. F.R.S. (Continued from p. 115.) I SHALL continue the table of chemical compounds in this Number a little farther, observing the same method as in the preceding part of this paper. a By hydrate of potash, I mean caustic potash which has been exposed to a red heat. If we suppose it composed of an integrant particle of potash, and an integrant particle of water, it should consist of 100 potash + 18.867 water. Now Davy, by heating potash and boracic acid together, actually separated between 17 and 18 of water. This I consider as a full confirmation. Berzelius obtained 16 per cent. (Lärbok i Kemien, ii. 594;) which is very nearly the exact quantity. It ought to have been 15.872. This is caustic soda exposed to a red heat. Supposing it a compound of an integrant particle of soda and of water, it ought to consist of 100 soda + 14.362 water, or 84 128 soda + 12.558 water. I do not know that any accurate experiments have been made to determine the proportions of this hydrate. c According to this statement, slacked lime (which is the hydrate) is composed of 100 lime + 31.27 water. Now Lavoisier found it composed of 100 lime + 28.7 water, and Dalton of 100 lime + 33 333 water. The mean of these results gives us 31.016, which very nearly coincides with the number in the table. d By comparing the experiments of Berthollet (Mem. d'Arcueil, ii. 42,) with those of Berzelius, (Ann. de Chim. lxxviii. 50,) it appears that crystallized barytes exposed to a red heat (hydrate of barytes) is composed of 100 barytes + 12∙121 water. If we suppose it a compound of an atom of barytes and an atom of water, its composition will be 100 barytes + 11-632, which almost coincides with experience. Hence the number in the table. The e That a hydrate of strontian exists is certain, but no direct experiments have been made upon its composition; but from analogy, it is probable the number in the table is correct. crystallized hydrate, according to Dr. Hope's experiments, is composed of 1 atom strontian and 13 water. Hydrate of magnesia is obtained by precipitating magnesia from an acid, by means of potash, and drying it in a gentle heat, It is composed, according to Davy, of 100 magnesia + about ...... 141. Hydrate of alumina..1 a + 1 w acid, or acid of 1.85 147. 2d hydrate of sul 4.632 $ 4.732 ..11.796 i 6.788* .1 si + 1 w...... 5.1981 1 s + 1 w ...... 6.132 m n phuric acid, or acid 1 s + 2 w...... 7.264 " 148. 3d hydrate of sul phuric acid, or acid of 1 s + 3 w...... 8.396 149. Hydro-nitric acid, or acid of 1.620 2 n + 1 w...... 8.888 25 water, which agrees nearly with the number in the table. It would not be surprising if a hydrate of magnesia existed consisting of an atom of magnesia united with an atom of water, but incapable of being dried without losing one-half of its water, 8 Wavellite may be considered as a native hydrate of alumina. If it be composed of 74 alumina and 26 water, it must consist of an atom of alumina + an atom of water. Alumina precipitated from a solution, and dried at 60°, would appear, from Saussure's experiments, to be composed of 1 atom of alumina and 4 atoms of water. h Stated merely from analogy, without any direct experiment. From the experiments of Klaproth, it appears that yttria precipitated from acids, and dried, is composed 69 yttria + 31 water. Hence the number in the table. k According to Davy, zirconia, when in the state of a hydrate, contains more than th of its weight of water. Hence the number in the table must represent its composition. We have no direct analysis of the hydrate of silica; but the earth is known to absorb about 4th of its weight of water. Hence it must be a compound of 1 atom silica and 1 atom water. m This is the strongest possible sulphuric acid. It is composed of 100 real acid + 22.64 water. n This is the acid which freezes at the highest temperature of all, about 42° Fahrenheit. It is that on which Mr. Keir made his experiments. It is composed, as Dalton has shown, of 100 real acid 45-28 water. Hence its composition is as stated in the table. • This and the three following hydrates have been ascertained |