MEMOIR XIII.] EXPERIMENTS MADE IN VIRGINIA. 201 



For the investigation of chemical radiations, there was 

 placed upon the screen,/, Fig. 25, a paper covered with 

 bromide of silver. It darkened in those parts on which 

 the more refrangible rays fell. 



Or, having removed the differential thermometer and 

 its shade, Fig. 31, the cone of light converging from the 

 lens passed through a solution of sulphate of copper and 

 ammonia in the trough, and a piece of paper painted 

 with chloride of silver was placed so as to receive the 

 focus. Though but little heat was transmitted through 

 the solution, a dark spot was at once produced charac- 

 teristic of the blackening of the silver salts by the sun- 

 rays. Though, therefore, this double salt transmits the 

 rays of heat with difficulty, the rays of chemical action 

 pass with facility. If in the trough there be placed a 

 strong solution of bichromate of potassa, a far greater 

 quantity of light will pass, and much more heat ; but a 

 paper painted with chloride of silver being placed in the 

 focus, no chemical change whatever goes on, the chloride 

 retaining its usual whiteness. 



NOTE. The foregoing experiments were made be- 

 tween 1834 and 1837. The reader of this volume must 

 not regard them from the present elevated view of Radi- 

 ations. At that time very little respecting these radi- 

 ations was known. They present to me personally this 

 point of interest that they were the beginning of a 

 series of researches to which I devoted many subsequent 

 years. 



The following is a list of solutions possessing an ab- 

 sorptive action on the more refrangible or chemical radi- 

 ations : 



