and the Absorption of the Tiihonic Rays, 471 



Case of the Bichromate of Potash. — A piece of paper 

 dipped in a solution of this salt turns brown rapidly on expo- 

 sure to the sun's rays ; but if there be placed before it a trough 

 containing a solution of the salt, the change then goes on very 

 slowly. 



Prismatic anali/sis of this case. — On projecting a motionless 

 spectrum on this paper, an impression was obtained in a 

 quarter of an hour, reaching from a line a little beyond the 

 fixed line D, which I will provisionally call a-, to the violet 

 extremity. 



A trough with parallel sides filled with a solution of the 

 salt was next interposed in the beam, and the resulting spec- 

 trum received on a bromoiodized plate. The tithonograph 

 extended from the red extremity to the line x. The use here 

 made of the bromide of silver as a test-surface was originally 

 pointed out by Sir J. Herschel. 



From this we may draw the same conclusions which we met 

 in the case of the chrysotype; they are as follows: — 



1st. When a ray impinges on a sensitive surface, or passes 

 through a changeable medium^ with the chemical effect which 

 takes place the constitution of the ray is correspondingly dis-' 

 turbed. A change in the composition of the medium involves a 

 change in the ray. A specific detithonization of the ray is the 

 necessary attendant on a chemical change i7i the medium. 



2nd. Hays which thus disappear by absorption are occupied 

 in disturbing the constitidion of the ponderable medium. 



3rd. Rays which have become detithoniined or inactive^ with 

 respect to a given medium, and which are therefore not ifivolved 

 in chajiges going on, in it, escape by being transmitted or re- 

 fected. 



Similar laws, I believe, will be found to apply to each in- 

 stance ofaclino-decomposition, holding equally for the thermic, 

 photic, and phosphorogenic rays, as for the tithonic. An il- 

 lustration may render this matter plain. 



Thus in thermic absorption. If the rays of the sun are 

 converged on red oxide of lead by a burning lens, the heat 

 will be absorbed by that substance and oxygen be given off, 

 a lemon-coloured protoxide remaining. As soon as this is 

 accomplished, no further absorption of heat takes place, and 

 no further chemical changes ensue. 



A piece of polished silver exposed to the focus of a burning 

 mirror never melts, not because it is an infusible body, but 

 because its optical constitution is such that it reflects the rays 

 impinging on it. If the polish be taken olf, it melts in an 

 instant, because it can absorb the rays. So two different-co- 

 loured pieces of cloth exposed to the sunshine upon snpw, 



