MKMOIU XI11.] EXPERIMENTS MADE IN VIRGINIA, 197 



MEMOIR XIII. 



AN ACCOUNT OF SOME EXPERIMENTS ON THE LIGHT OF 

 THE SUN, MADE IN THE SOUTH OF VIRGINIA. 



Abstract from the Journal of the Franklin Institute of Philadelphia for June, July, 

 August, and September, 1837 ; Philosophical Magazine, Feb., 1840. 



CONTENTS: Absorption of luminous radiations. The reference spec- 

 trum. Absorption of heat radiations ; the apparatus employed. Ab- 

 sorption of chemical radiations. Screen of bromide of silver. Color- 

 -ation of chloride and bromide of silver by radiations that have passed 

 through correspondingly colored solutions. Early application of pho- 

 tography to the investigation of physical problems. Crystallization of 

 camphor towards the light. The side of vessels towards the sky is the 

 colder. 



IF a beam of the sun's light be passed through a so- 

 lution of chromate of potassa, it can no longer blacken a 

 piece of sensitive paper paper covered over with chlo- 

 ride or bromide of silver. If the light which has thus 

 passed through a stratum of this liquid be converged by 

 means of a lens, the chloride of silver will remain for a 

 long time without change in the focus. 



I made many such experiments with a view of deter- 

 mining the effect of absorbent media on the luminous, 

 calorific, and chemical rays. Such, at that time, was the 

 accepted subdivision of the solar radiations. A ray of 

 the sun was caused to pass through a trough with paral- 

 lel sides containing the absorbent solutions, and for the 

 sake of exactness a reference spectrum was employed, 

 such as is now used in spectroscopic experiments. In 

 this manner the particular luminous rays absorbed by 

 any given solution could be determined, the absorbed 



