Analogies between the Chemical Bays and Radiant Heat. 195 



Svanberg. Zwenger. Hagen. 



Catechin, dried in vacuo, C 15 H 12 O 6 C 14 H 18 O 9 



Dried at 100° C C 20 H 20 O 9 C 14 H 14 O 7 



Fused catechin C 20 H 18 O 8 



Combined with oxide of lead C 14 H 12 O 6 



The pyrocatechin has been discovered and examined both 

 by Wackenroder and Zwenger. It is obtained by heating 

 catechin above its point of fusion; white acid vapours are 

 evolved, and a more or less brown coloured acid fluid distils 

 over ; by evaporation the pyrocatechin may be obtained cry- 

 stallized, and may then be purified by sublimation. Melts at 

 20° C; easily soluble in water, alcohol and tether. The 

 aqueous solution is easily decomposed by exposure to the air. 

 Decomposed by alkalies and their carbonates. Formula 

 C 6 " W O 2 . In combination with oxide of lead, C 6 H 4 O.— 

 (Annalen der Chemie und Pharmacie, xxxvii. pp. 306, 320, 

 336.) 



XXX* On some Analogies between the Phenomena of the 



Chemical Bays, and those of Radiant Heat. By John 



William Draper, M.D., Professor of Chemistry in the 



University of New York*, [Illustrated by Plate I.) 



(1.) TT is the object of this memoir, to establish some stri- 



A king analogies which exist between the phenomena 



of the chemical rays and those of radiant heat. 



(2.) As most of the experimental illustrations which I shall 

 here give depend upon the use of M. Daguerre's prepara- 

 tion, (though I have numerous others which serve to extend 

 these truths to other combinations, and which will be pub- 

 lished in due timo,) I shall also incidentally give what appears 

 to be the proper theory of the Daguerreotype. 



(3.) Without saying anything of the laws of reflexion, re- 

 fraction, polarization, and interference, to which these rays 

 are subject, the study of which I commenced more than five 

 years ago on paper rendered sensitive by the 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 follow- 

 ing propositions. 



(4.) 1st. That the chemical action produced by the rays of 

 light, depends upon the absorption of those rays by sensitive 

 bodies ; just as an increase of temperature is produced by the 

 absorption of those of heat. 



* Communicated by the Author. 

 02 



