4 "76 Prof. Draper 07i the Interference Spectrum, 



which the iodide of silver acts as the absorbent, and the me- 

 tallic silver behind it assumes the relation of the hydrogen of 

 the foregoing experiment. 



There is no reason to believe that oxygen, hydrogen, or ni- 

 trogen gases, in masses of ordinarj' magnitude, exert any per- 

 ceptible absorptive effect on light, heat, tithonic or phospho- 

 rogenic rays. These bodies, therefore, and all others having 

 the same relation, can exert no action on each other, even 

 though they are under the influence of the most intense radia- 

 tion. 



A mixture of oxygen and hydrogen gases exposed to a bril- 

 liant light, can never produce water, because neither of its 

 constituents has the power of absorbing the incident I'ays. 



But a mixture of chlorine and hydrogen gas explodes in an 

 instant, under the influence of light, because the chlorine can 

 exert a powerful absorbent action. 



If the reason that oxygen and hydrogen cannot form water, 

 under the influence of the sunlight, be due to the circumstance 

 that neither of those gases can absorb tithonic rays, but are 

 perfectly transparent and colourless; and the reason that 

 chlorine and hydrogen at once form muriatic acid, be due to 

 the absorbent capacity of the chlorine; it results, that when a 

 mixture of these latter gases intercepts a ray, the absorbent 

 action upon that ray should not be greater than that of the 

 chlorine alone, and even not more than one-half, because of 

 the diluted state in which the chlorine is presented. But this 

 is the same conclusion to which we have just arrived by expe- 

 riment. 



Although chemical action is the uniform result of absorp- 

 tion, the converse of the proposition does not hold good, — 

 absorption is not necessarily attended by chemical action. 

 Nevertheless it is attended with a certain effect. Even in the 

 case of an elementary, and therefore unchangeable substance, 

 like chlorine, a disposition or capacity for union is communi- 

 cated. Chlorine, which has been exposed to the sun, unites 

 with hydrogen more readily than chlorine which has been 

 made and kept in the dark. 



In some preceding memoirs I have shown that the decom- 

 position of carbonic acid by the leaves of plants is brought 

 about by the yellow photic ray. There seems to be a general 

 relation, though the details of it have not yet been traced, be- 

 tween rays of a particular refrangibility and ponderable sub- 

 stances of a particular kind. Thus, in the case of most of the 

 salts of silver, the point of maximum action falls among the 

 blue rays. In the same way the question naturally arises, 

 does the point for the maximum action on carbon compounds 



