144 OBJECT OF THE MEMOIR. 



CHAPTER XII. 



ON SOME ANALOGIES BETWEEN THE PHENOMENA OF THE CHEMICAL RAYS AND THOSE 



OF RADIANT HEAT. 



(From the London, Edinburgh, and Dublin Philosophical Magazine for September, 1841.) 



CONTENTS : Object of the Memoir. On the Daguerreotype Process. Chemical Consti- 

 tution of Daguerreotype Pictures. Spectral Images. Film of Iodide torn off Me- 

 chanically. Iodine is not evolved, but corrodes the Plate. The Chemical Rays are 

 absorbed. The Photographic Effects are transient. The Chemical Rays are not 

 conducted. They become Latent.- Optical Qualities control Chemical Action. 

 The Active Rays are absorbed, and the Complementary reflected. Relation of Op- 

 tical Forces and Chemical Affinities. 



567. IT is the object of this memoir to establish some striking analogies which ex- 

 ist between the phenomena of the chemical rays and those of radiant heat. 



568. As most of the experimental illustrations which I shall here give depend upon 

 the use of M. DAGUERRE'S preparation (though I have numerous others which serve to 

 extend these truths to other combinations, and which will be published in due time), I 

 shall also, incidentally, give what appears to be the proper theory of the Daguerreotype. 



569. Without saying anything of the laws of reflexion, refraction, 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, farther 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 AB- 

 SORPTION of those rays by sensitive 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 radia- 

 tion, or conduction, or contact with other bodies, so likewise, by some unknown pro- 

 cess, photographic effects produced on sensitive surfaces are only transient, and gradu- 

 ally disappear. 



3d. That, as when rays of heat fall on a mass of ice, its temperature rises degree 

 by degree, until it reaches 32 Fah., and there stops, until a certain molecular change 

 (liquefaction) is accomplished, and after that proceeds to rise again, so, also, the chem- 

 ical rays impress certain changes proportional to their quantity, up to a certain point, 

 and there a pause ensues ; a very large amount of light being now rendered latent or 

 absorbed, without any indication thereof being given by the sensitive preparation (as 

 the heat of fluidity is latent to the thermometer), a molecular change then setting in, 

 the increments of the quantity of light are again indicated by changes in the sensitive 

 preparation. 



