and of Carbonates by the Light of the Sun. 1 65 



red, the blue, the indigo, and the violet, exerting no percep- 

 tible effect. This being the case, we should expect that by 

 passing a beam through absorbent media of such a nature 

 that the extreme red, the blue, the indigo, and violet are ab- 

 sorbed, this decomposition should nevertheless go on. A so- 

 lution 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 corre- 

 spond to the more refrangible yellow and less refrangible green. 

 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, ad- 

 justed 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 bichromate of potash, the sun's 

 beams came through it, and in the interior of the box an 

 arrangement of leaves and carbonated water could be ex- 

 posed to the rays that had escaped absorption. The thick- 

 ness of the liquid stratum was about half an inch. I had se- 

 veral such boxes made, so that 1 might compare the simul- 

 taneous 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 exposed to party-coloured 

 light — blue, yellow, red, &c. 



Whenever an experiment was commenced in these closets, 

 simultaneously a similar one was commenced in the unob- 

 structed sunshine. It is needless to repeat, that in all these 

 care was taken to have the different arrangements for decom- 

 position as nearly alike as possible. 



On comparing together the amount of gas evolved in un- 

 absorbed 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. 



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. 



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 



