44 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
noticed in my first communication to the Royal Society. It was attributed at first 
successively to a variety of causes, such as retention of aqueous vapour, reflection 
backwards and forwards of the incident light between the surfaces of the glass and 
paper, to a development of heat at the point of contact, disengagement of gas, &c. 
None of these causes, however, appear adequate to account for the observed effect. 
Mere moisture has but little influence on argentine papers. The additional light re- 
turned on the paper by continued reflection is much too small, and this explanation is 
opposed by the observed fact, that the effect in question is imperceptible in a gloomy 
day, however long the exposure be continued, and that it increases in degree as the 
sun is stronger. This would certainly favour the idea that the cause is to be sought 
in heat developed and retained by the glass ; and this explanation may appear to be 
supported by the fact, that the effect in question is not produced when the glass, 
instead of being pressed into contact with the paper, is merely placed before it as a 
screen, with a considerable interval between them. When a piece of nitrated paper, 
for instance, was rolled round a cylindrical surface of moderate convexity, covered 
with black velvet, and the glass gently laid in contact with it, the effect of sunshine 
was exalted at the line of contact; but on either side of that line, as the interval in- 
creased, the influence of the glass diminished, and at less than half an inch distance 
no difference could be perceived between the impressions under the glass and in free 
air. Nevertheless, on trying to produce the same effect in gloomy days, by heat alone 
applied to a metal plate in contact with the paper, no increase of action was observed 
unless the heat was fully that of boiling water. But this is far greater than can be 
acquired, under the circumstances of the experiment, by the mere action of the sun. 
Moreover, I find that the same exalting effect is produced when a thin close-textured 
white paper is interposed between the nitrated paper and the glass, and that when the 
sun has far from its full power; under which circumstances only a very moderate 
warmth can be excited at the point of action. 
115. *When first this phenomenon was noticed, the idea was suggested of two op- 
posing influences in the solar light, the difference of which only becomes sensible in 
the immediate chemical effect, and of which the negative or deoxidizing one (using 
the term in a mere conventional sense) is partly stopped by the glass. In pursuance 
of this notion, red light, concentrated by a lens, after reflexion at the surface of a 
looking-glass, was thrown on paper exposed to the free sunshine, but without any 
perceptible effect in retarding its discoloration. 
116. In further prosecution of this subject I fastened (March 24, 1839,) on a piece 
of nitrated paper, colourless, or nearly colourless plates, of the following substances : 
* This paragraph is copied verbatim from my first communication on this subject. The paper used was ni- 
trated, the sun feeble, and all circumstances unfavourable, (February 10, 1839). Had muriated paper been 
used, however, the experiment so conducted could have hardly failed to have verified (at least so far as it goes) 
the peculiar views of solar action which suggested it, and thus to have led to the earlier discovery of those sin- 
gular properties of the less refrangible rays described in the foregoing part of this paper. 
