OF ARTS AND SCIENCES. 217, 



rays pass through the gaseous vapor of the same substances placed 

 between our eyes and the point of their combustion; and thus the 

 original monochromatic rays are absorbed by this gaseous vapor. 



The theory of absorption of colored solutions may also be explained, 

 by assuming that certain of the rays excited in the solution are the 

 result of the motion of waves of unequal length. Now, if we call to 

 mind that each ray gives its own illumination or image of the slit, 

 and consequently that there are side by side a series of these images, 

 whose illumination is the result of the ether molecules vibrating in 

 wave-lengths gradually increasing in length, — for instance, at the 

 solar line H being 3,928 according to iingstrom's map, orjo^yo^^ij^ 

 in. to 7605 or T(y,oo^o?(y(To ^"- ^*' ^ YvaQ, — we may easily conceive 

 that certain of these ether molecules, passing through a solution 

 whose molecules are of a size capable of receiving their motion, 

 transfer their energy ; whilst certain others of different length, both 

 of longer and shorter size, only partially transfer their energy ; 

 and again certain others, not losing any of their energy, because 

 they do not excite any motion of their own kind, pass through at 

 the same initial rate. The consequence would follow, that the first 

 set of waves become extinguished, and hence their illumination ceases, 

 after their projection into the solution ; the second rays are partially 

 quenched ; and the third set appear as bright as when first projected. 

 Thus, instead of seeing the absorption-bands of the spectrum trans- 

 mitted through a colored solution, always with definite limits, in some 

 instances we observe that the greatest amount of light absorbed is at 

 the middle, and shades lighter towards the outside limits of these 

 bands. The energy consumed in extinguishing certain rays of light 

 must, then, by the law of conservation of energy, become stored up 

 in a latent form in all those colored substances which give absorption- 

 bands. 



Herrmann Vogel (Jahresbericht, 1861, and in Watt's Diet, of Chem. 

 vol. V. p. 295) maintained that silver iodide or bromide or chloride 

 are reduced by certain rays of solar light to the subiodide, subbromide, 

 and subchloride ; or, in other words, — 



2 (Ag. Br.), 



by exposure to those refrangible rays of light which to our eye appear 

 violet or blue, becomes 



Ag.^ Br., 



and that one portion of bromine is set free. 



