230 CHEMICAL RAYS AND RADIANT HEAT. [MEMOIR XVII. 



MEMOIR XVII. 



ON SOME ANALOGIES BETWEEN THE PHENOMENA OF THE 

 CHEMICAL RAYS AND THOSE OF RADIANT HEAT. 



From the Philosophical Magazine, September, 1841. 



CONTENTS: The chemical rays are absorbed. Photographic effects are 

 transient. The chemical rays are not conducted ; they become latent. 

 Optical qualities control chemical action. The active rays are ab- 

 sorbed and the complementary reflected. Relation of optical conditions 

 and chemical affinities. 



IT is the object of this Memoir to establish some strik- 

 ing analogies existing between the phenomena of the 

 chemical rays and those of radiant heat. 



Without saying anything respecting the laws of re- 

 flection, refraction, polarization, and interference, to which 

 the chemical rays are subject, the study of which I com- 

 menced more than five years ago on paper rendered sen- 

 sitive by bromide of silver, further than that a general 

 similitude holds in all these cases between the rays of 

 heat and the chemical rays, I shall at present confine my 

 observations to establishing the following propositions: 



1st. That the chemical action produced by the rays of 

 light depends upon the absorption of those rays by sen- 

 sitive bodies; just as an increase of temperature is pro- 

 duced by the absorption of those of heat. 



2d. That as a body warmed by the rays of the sun 

 gradually loses its heat by radiation, or conduction, or 

 contact with other bodies, so likewise, by some unknown 

 process, photographic effects produced on sensitive sur- 

 faces are only transient, and gradually disappear. 



