in reply to his Animadversions. 421 



cing definite changes in the condition of the surface of solid 

 bodies, and the remarkable property of two substances in 

 juxtaposition, inducing upon each other changes which may be 

 rendered evident to the senses ; and these changes, it must be 

 remembered, are not confined to the surface merely, but they 

 penetrate to a considerable depth into the solid structure of 

 the mass, as I have already proved in several of my published 

 experiments. 



These results, in addition to many which I have previously 

 given, also show the impropriety of considering these spectral 

 images as the effects of " invisible light" " light radiated in 

 absolute darkness" as they have been by M. Moser. I must 

 in this place express my conviction, that any term involving 

 an idea contrary to our received ideas, is calculated to produce 

 much confusion. Light is that element which affects the organ 

 of sight, enabling us to distinguish objects; that which does 

 not do this is no longer light. The prismatic spectrum has 

 been proved to consist of one element giving light and colour, 

 of another element affording heat, and of a third element, which 

 is active in producing chemical change. Now M. Moser in- 

 forms us that it is neither of these, but a class of rays which 

 are still more refrangible than those which have been called 

 the "invisible chemical rays." "According to my experi- 

 ments," he says, "the invisible rays of light pass very readily 

 through aqueous solutions of various kinds, and through dif- 

 ferent oils, but they decidedly do not pass through the thinnest 

 plates of glass, mica, or rock-salt, &c." Now, as M. Moser 

 has not given us his experiments, it is very difficult to deal 

 with them. In the first place, he has not, as it appears to me, 

 as yet afforded any evidence of the existence of such a class 

 of rays as those he speaks of; and if the " thinnest plates of 

 glass, &c." are not permeated by these rays, upon what prin- 

 ciple does he, by the use of coloured glasses, prove "that light 

 and mercurial vapour are identical in their effects?" The 

 prism must be useless in this inquiry on M. Moser's own 

 showing; I am therefore quite at a loss to know by what 

 means he has succeeded in establishing, with so much accu- 

 racy, the refrangibility of these "invisible rays of light." I 

 am very far from denying that the phaenomena in question 

 have been produced by invisible solar emanations. I am not 

 at all prepared to deny the absorption of such emanations by 

 solid bodies, or their existence in a latent state, or their ra- 

 diation in darkness. I contend only that light has nothing 

 whatever to do with the phaenomena. In the present state of 

 the inquiry it does appear to me that heat is a very active^ 

 agent in producing the effects in question ; indeed M. Moser 

 himself says " if the temperature be raised, they (the invisible 



