EFFECTS OF YELLOW SOLUTIONS ON THE CHEMICAL RAYS. 105 



direct experiment discountenance this, but the analogy of their action on the chemical 

 ray renders it extremely improbable, an action which I proceed to develop,^. 50, 

 being still consulted. 



409. Having removed the differential thermometer and its case, and produced a cone 

 of light converging from the lens, and passed it through a solution of sulphate of copper 

 and ammonia contained in the trough, if now we hold in the focus a piece of bibu- 

 lous paper imbued with chloride of silver, although little or no heat is transmitted 

 through the solution, yet an extremely dark spot is produced, characteristic of the black- 

 ening of that substance by the solar rays. Though, therefore, the double salt transmits 

 the ray of heat with difficulty, the ray of chemical action passes with great facility. 

 If a trough containing a strong solution of bichromate of potassa be now substituted, a 

 far larger quantity of light will pass, and vastly more beat ; but a paper imbued with 

 chloride of silver being laid in the focus, no chemical change whatever goes on, the 

 chloride retaining its usual whiteness. 



410. I placed a piece of paper imbued with chloride of silver in a cubical box, one 

 of the sides of which was formed of a pair of glass plates, with a solution of bichromate 

 of potassa between them ; it was exposed for many days to the sun's light, and only 

 assumed a faint bluish stain, while a similar piece exposed to the direct rays was fully 

 blackened in fifteen minutes. So powerful is the action of this salt, that when a stratum 

 of it not more than the hundredth part of an inch thick was included between two 

 plate glasses, it stopped the decomposition of chloride of silver. It was after a long 

 examination of a variety of substances that I first became acquainted with the great 

 absorptive power of the chromates of potassa. In my earlier experiments, I had made 

 use of the chloride of platinum and the chloride of gold, both of which have an anal- 

 ogous action. The solutions which I have recognised as possessing this power in the 

 most eminent degree are, 



Bichromate of potassa. Muriate of iron. 



Chromate of potassa. Chloride of gold. 



Yellow hydro-sulphuret of ammonia. Chloride of platinum. 



Hydro- sulphate of lime. Coloured vegetable solutions. 



It is remarkable that all the mineral solutions on this list are yellow ; the absorptive 

 power, however, is by no means connected with that colour, for the yellow tint is a 

 compound one ; all the rays of homogeneous light are absorbed by one or other of the 

 bodies on the foregoing list. 



411. It is interesting to know whether these absorptions be really the abstraction of 

 something from the solar ray, or merely some change impressed upon it. If light 

 consists of tremblings, pulses, or undulations, or any other kind of motion of a homo- 

 geneous elastic medium, in virtue of which it is competent to excite sensations of heat 

 and effect chemical changes, we might explain the action of these media as the result 

 of some change occurring to that motion, either in direction or degree. We might 

 suppose, too, that when a ray had been deprived of its power by passage through one 

 medium, it might have it restored, in a greater or less degree, by being transmitted 

 through another. I have not found, in thus comparing together nearly three hundred 



O 



