4>24 Mr. Hunt on the Spectral Images qfM. Moser, 



faint image on the plate under the yellow fluid, but no trace 

 of an impression could be detected, by the influence of the same 

 exposure, under the blue fluid. Surely these results prove, that 

 the calorific rays are the most active in these phaenomena ; and 

 instead of inventing the purely conjectural notion of " invisible 

 light," is it not much more rational, when we have distinct 

 evidence of the powerful action of heat, to look for an expla- 

 nation of these phaenomena in the calorific radiations, which 

 are equally active in light or darkness ? Instead of assuming 

 that bodies are all self-luminous for the purpose of explaining 

 these curious facts, which " self-luminosity " we are not in a 

 condition to prove, is it not more consistent with the spirit of 

 inductive philosophy, to seek for the cause in the invisible ra- 

 diations of heat, which we know take place under all cir- 

 cumstances? And it is admitted, even by Moser himself, that 

 heat is a powerful accelerating agent. 



M. Moser has not told us how he has determined that his 

 dark light passes readily through aqueous solutions and oils. 

 I have tried some experiments which are instructive, and I 

 therefore record them. Having put an edge of wax around 

 a polished copper plate, to the depth of one-eighth of an inch, 

 I covered the plate with water, and upon two small pieces of 

 glass I supported, by the edges, a silver medal, so that it just 

 touched the upper surface of the water; in twelve hours the 

 plate was much tarnished over every part, except that directly 

 under the medal, which remained as bright as at first. Under 

 one of the pieces of glass a very decided change of colour was 

 produced, and from its whiteness I was at first inclined to 

 think, that silver had been removed from the medal, and depo- 

 sited on the plate; I have, however, since proved that it was 

 an oxidation of the copper plate merely. The same arrange- 

 ment was made with very fine olive oil, and the result was si- 

 milar ; but upon leaving the medal and plate undisturbed for 

 some days, the whole surface of the copper was oxidized, the 

 oil became a very fine green colour, and the under surface of 

 the silver was covered with a film of copper. Here we have 

 analogous phaenomena to those we have been considering, but 

 in these cases it is very evident the cause was neither light 

 nor heat, but voltaic electricity. . The plates, after the water 

 and oil were removed, were thoroughly cleaned and exposed 

 to the vapour of mercury, which gave images of the medal in 

 outline, and of the glasses. 



It may not be out of place here to say a few words relative 

 to the explanation given by M. Fizeau of the production of 

 these images. There is no doubt " if different parts of a po- 

 lished surface are unequally soiled by extraneous bodies, even 



