498 Major-Crenpral J. Wnterliouse. On the Sensiiiceness 



Prohohlf ('iia</' of the E fiefs ilesciihf'<l. — As regai'ds the cause of the 

 peculiar effects described, and the nature of the visil>le and invisible but 

 developable images produced by the action of light iipon plain silver 

 surfaces, it is very difficult to give any definite opinion. It would 

 seem that in this, as in most photographic processes, the first action of 

 li^ht is principally molecular, but if the exposure be prolonged and 

 takes place in the air under ordinary conditions, there is a certain 

 chemical decomposition of the surface of the plate, and the impressed 

 image becomes distinctly visible. 



— Moser was of opinion that the action of light does not neces- 

 sarily consist in the separation of two chemically combined bodies, 

 even in the Daguerreotype, in which process he maintained there "is no 

 separation of iodine from silver under the action of light. (We now 

 know that the iodine evolved is absorbed by the underlying silver.) 

 He brought forward his experiments on pure surfaces of silver, where, 

 he says, there can be no possibility of a chemical action, to show that 

 the effects of the Daguerreotype could be produced in quite a different 

 way. 



Waidele. — Waidele,"^ although he does not refer to Moser's observa- 

 tion of the direct action of light upon silver, and concerns himself 

 more with the contact actions attributed to invisible light, concludes 

 that IMoser's effects are not produced by any action of invisible light, 

 but must be rather explained by the action of atmospheres on bodies 

 and the absorption of gases. He points out that polishing powders, 

 sach as tripoli, charcoal, &c., ordinarily contain moisture and absorbed 

 gases, for which they have a strong attraction, and if used in this 

 state they give up these vapours to the surface polished. If, how^ever, 

 they are heated to a red heat to drive off all moisture and gases, they 

 then act as absorbents, drp«wing out the moisture and gaseous impuri- 

 ties from the surface of the metal, and producing a far purer and more 

 perfect polish. He also notes the effect of carbonic acid, hydrogen, 

 and other vapours on iodised silver Daguerreotype plates. 



So far as my experiments go, I do not think that the nature of the 

 polishing materials has had much influence on the results obtained, 

 though it is an interesting point which should be looked into. 



Boscoe. — Roscoet also attributes the Moser effects to absorbed gases, 

 and says : — "Almost all of the singular phenomena first investigated 

 by Moser, and ascribed by him to the action of latent light, may be 

 more rationally explained by the authenticated facts of the absorption 

 of gases by solid bodies." He also refers more particularly to the con- 

 tact or " breath " images produced by laying coins, &c., upon polished 

 plates of metal. 



Action of Gases. — It is evident that if there be any chemical action 



* ' Fogg. Ann.; vol. 59, 18i3. 



t Watts' Diet, of Chem.. IT, 1872, p. 805. 



