ON SPECTRAL IMAGES AND LATENT LIGHT. 



forces can control, and even check the play of chemical affinities. While thus it ap- 

 pears that there are points of analogy between this chemical agent and radiant heat, 

 we must not too hastily infer that the laws which regulate the one obtain exclusively 

 also with the other. As is well known, there are striking analogies between radiant 

 heat and light, but there are also points of difference ; the convertibility of heat of one 

 degree of refrangibility to another does not occur with light ; there are also dissimili- 

 tudes in the phenomena of radiation and its consequences. I do not doubt that what 

 has been communicated in this memoir will, by the researches of others, be greatly 

 extended ; but it is not to be expected that a complete parallel can be run between 

 radiant heat and the chemical rays, any more than between radiant heat and light. 



640. From the phenomena of the interference of these rays, of the sensitiveness or 

 non-sensitiveness of the same chemical compound being determined merely by the fact 

 of its thickness or thinness, these, and many other similar results, obviously depending 

 upon mechanical principles, it seems to me that very powerful evidence may be drawn 

 against the materiality of light, and its entering into chemical union with ponderable 

 atoms. Those philosophers who have endeavoured to prove the undulatory theory, 

 will probably find, in studying these subjects, cogent evidence in favour of their doctrines. 



Note added to the preceding Chapter. 



ON CERTAIN SPECTRAL APPEARANCES, AND ON THE DISCOVERY OF LATENT LIGHT. 

 (Being a Letter to the Editors of the London and Edinburgh Philosophical Magazine, inserted in that Journal, November, 1842.) 



641. GENTLEMEN If there be a thing in which I have a disinclination to engage, 

 it is controversy of a personal kind with scientific fellow-labourers. But as, you well 

 know, it ordinarily happens that there is no other gain to philosophers beyond the 

 mere credit of their discoveries, they may be forgiven for reluctantly endeavouring to 

 secure this, their only reward. 



642. I have recently returned from a long journey, undertaken for the purpose of 

 making trials on the sunlight in lower latitudes, and am surprised to see in the reports 

 that have reached this country of the Proceedings of the British Association, certain 

 announcements received from Professor BESSEL, of phantoms which can be produced 

 on surfaces by mercury vapour, by the breath, and other means, as though the thing 

 were new. Years ago, if you look in your own Journal (February, 1840, p. 84 ; Sept., 

 1840, p. 218 ; Sept., 1841, p. 198, 199), you will find that I had published facts of 

 the kind ; spectral appearances, that could be revived on metals, glass, and other bodies, 

 by the breath, by vapour of camphor, by mercury vapour, &c. The very purpose for 

 which I described them was the striking resemblance of some of them to Daguerre- 

 otype images. I have repeatedly shown that, by placing a coin or any other object 

 on iodized silver, in the dark, the vapour of mercury will bring out a representation of 

 it. And in one of the papers just quoted, the condition under which camera images 



