17 g DECOMPOSITION UNDER ABSORBENT MEDIA. 



feet. This being the case, we should expect that, by passing a beam through absorb- 

 ent media of such a nature that the extreme red, the blue, the indigo, and violet are 

 absorbed, this decomposition should nevertheless go on. A solution of bichromate of 

 potash nearly fulfils these conditions, and not only does it absorb the luminous rays in 

 question, but also all the tithonic rays, except a trace of those which correspond to the 

 more refrangible yellow and less refrangible green. 



784. A remarkable proof of the correctness of the foregoing prismatic analysis 

 comes out when leaves are made to act on carbonated water in light which has passed 

 through a solution of bichromate of potash. I took a wooden box of about a cubic 

 foot in dimensions, and having removed its bottom, adjusted to it a trough made of 

 pieces of plate glass. The box being set on end, its lid served as a door, and the 

 trough being filled with a solution of the bichromate of potash, the sun's beams came 

 through it, and in the interior of the box an arrangement of leaves and carbonated wa- 

 ter could be exposed to the rays that had escaped absorption. The thickness of the 

 liquid stratum was about half an inch. I had several such boxes made, so that I might 

 compare the simultaneous effect of light which had undergone absorption by different 

 media. They formed, as it were, a series of little closets in which bodies could be ex- 

 posed to party-coloured light blue, yellow, red, &c. 



785. Whenever an experiment was commenced in these closets, simultaneously a 

 similar one was commenced in the unobstructed sunshine. It is needless to repeat, 

 that in all these care was taken to have the different arrangements for decomposition 

 as nearly alike as possible. 



786. On comparing together the amount of gas evolved in unabsorbed light and in 

 light that had undergone absorption by the bichromate of potash, in three out of five 

 trials the gas collected under the latter circumstances exceeded in volume that collected 

 under the former ; this was probably due to a slightly higher temperature which obtained 

 in the box. 



787. On comparing together the volumes of gas collected under the bichromate of 

 potash and under litmus water, the latter was not equal to one half the former. 



788. I compared together the gas evolved in unobstructed light, under bichromate of 

 potash, and under ammonio-sulphate of copper ; the results were as follows : 



Unobstructed light .... 4'75 

 Bichromate of potash . . . 4-25 

 Ammonio-sulphate of copper . -75 



789. Comparing these experiments, made by the aid of absorptive media, with those 

 made by the prism, we are enabled to come to a definite conclusion as to the character 

 of the rays which cause this decomposition. 



790. The true office of prismatic analysis is to determine the refrangibility of the 

 rays which produce given actions; but inasmuch as rays of heat, rays of light, and 

 tithonic rays are found throughout the spectrum, in many cases the prism fails to indi- 

 cate to which of these imponderable agents phenomena are to be ascribed. The case 

 before us furnishes a striking example. Although the decomposition of carbonic acid 

 is most energetically brought about by rays whose index of refraction corresponds to 



