366 PHOTOCHEMISTRY. 



"What we must first remark is tlie following' observation of jM. Nicpce de 

 Saint Victor : tlie function of the two active snV)stance3 is reciprocal ; it is a 

 matter of indifference whether the substance styled sensitive, iodide of silver 

 for example, be exposed to the light and afterwards washed with the revealing 

 substance, pyrogallic acid, or, inverting the process, the gallic acid be impressed 

 and the sheet washed in the l)ath of silver. As regards intensity, the same 

 effect is always obtained by exposing one or the other to the sun. 



M. Niepce de Saint Victor studied this phenomenon with care, and the fol- 

 lowing ai'e some of tlie experiments which he made to this effect. He exposed 

 to the stui, for some time, a sheet of white paper which had received no prepara- 

 tion, and he perceived that this paper, placed in tlie camera ohscura on a slieet 

 of senfitive paper, l)lackened it as the light itself would have done. Upon this, 

 lie conceived the idea of insolating or exposing to the sun his sheet of paper 

 after having first covered it with a photographic stereotype ; he then placi^d it 

 on a sensitive paper. Not only had the white paper thus insolated acquired the 

 propertA^ of reducing the salt of silver of the sensitive paper, but it reproduced 

 on its surface the figure of the stereotype. 



This curious proi)erty is exerted at a certain distance, but it ceases if a plate 

 of glass or of mica be interposed. The insolated paper retains this property 

 for some time in darkness, but when once this paper lias produced a certain effect, 

 the property is completely annulled ; a new exposure to the sun is necessary to 

 restore it. 



jM. Niepce made this discovery under rather singular circumstances. He 

 exposed a sheet of paper to the action of the sun, enclosed it in a tube of tin, 

 and, at the end of some months, having opened the tube, he placed the paper on 

 a sheet which had been rendered sensitive. The latter received the impression 

 in the whole extent of the section of the tube. 



How is this phenomenon to be explained ? There are two modes of doing it. 

 M. Niepce supjtoses that there is light imi)risoned in the insolated substance. 

 This light sometimes remains stored therein for a considerable time, till the op[)or- 

 tunity occurs for it to produce an eflicient chemical action. It is scarcely neces- 

 sary to say that this explanation, simple as it is, has not met with universal acqui- 

 escence ; difficulty has been felt in comprehending how light could thus remain 

 in a latent state on the surface of bodies for a time which may be tenned iiideliiiite. 



Another explanation has therefore lieen advanced : it has been supposed that 

 the sohar radiation occasions the fonnation of liighly oxydable chemical pro- 

 ducts when it takes impression on the organic material. It would be easy then 

 to ex[dain the indefinite preservation of a volatile substance capable of acting at 

 a distance, and disappearing after having produced a determinate chemical ettect. 



To which of tliese two explanations should we give the preference "? For my 

 own part, I strongly incline to that of M. Niepce, chiefly on account of the fol- 

 lowing experiment : JI. Niepce takes a piece of porcelain presenting a fresh 

 fracture, exposes it to the sun, and then plunges it in the bath of silver, avoiding 

 with care the contact of organic substances ; having afterwards washed it with 

 pyrogallic acid, he sees the silver immediately reduced. It is impossible, how- 

 ever, to admit the production of an organic matter under these conditions. The 

 action of the sun, whether on the salts of silver, or on organic matter, or on 

 porcelain, always produces the same effect. 



I have said that I incline to the explanation of M. Niepce : I do S(i, however, 

 with some degree of hesitation ; and yet, when I see the phenomenon of phosjiho- 

 rescence, when I see an alkaline sulphur absorb liglit and give it out, in the end, 

 under the form of light, how can we persist in doubting that a substance like a 

 fracture of porcelain may so far retain light as to cause it to produce, after a time, 

 not [)hospliorescence, indeed, but a chemical action ? It is, in a word, a force 

 which remains there, for quite a long interval, until it finds the occasion for exert- 



