8 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
that position to be cemented together at the edges, or rather at four points, one in 
the middle of each side. The operation is easier than it would appear to be by the 
description, and the resulting effect offers an infinitely nearer approach to that of an 
ordinary copper-plate engraving or mezzotinto, than anything that can be got from 
a single model. 
III. Of the Preparation of Photographic Paper. 
26. It would be tedious and serve little purpose, were I to recite individually the 
numberless combinations I have tried with a view to the increase of sensitiveness 
and facility of preparation of this useful material ; especially as, after all my trials, I 
am obliged to admit (which I most readily do) that the specimens recently placed in 
my hands by Mr. Talbot far surpass in respect of sensitiveness any that I have yet 
produced of a manageable kind, and that for all ordinary purposes I have ended in 
adopting his process of preparation, as far as I understand it. Nevertheless, as all 
my earlier inquiries were conducted in ignorance of these processes, and as they have 
led to some results worth notice even in this line, I shall set down a few particulars. 
2 7- My first attention was directed to the discovery of a liquid, or emulsion, which 
by a single application, whether by dipping or brushing over, should communicate 
the desired quality. The presence of organic matter having been considered by 
some late chemists an essential condition for the blackening of the nitrate of silver, 
I was induced to try in the first instance a variety of mixtures of such organic soluble 
compounds as would not precipitate that salt. Failing of any marked success in this 
line, (with the somewhat problematic exception of the gallic acid and its compounds,) 
the next idea which occurred was that of introducing organized salts of silver into 
the pores of paper, by first washing it over with an organic, soluble, precipitable salt, 
with alkaline base, and then with nitrate of silver. Here also, no distinct result was 
obtained, except that when the uric acid and alkaline urates * were employed, a 
paper was obtained decidedly more sensitive than that resulting from the nitrate alone, 
but which blackened spontaneously in the dark. 
28. A great many experiments were made by precipitating organic liquids, 
both vegetable and animal, with solutions of lead, as also, after adding alum, with 
alkaline solutions. Both alumina and oxide of lead are well known to have an affi- 
nity to many of those fugitive organic compounds which cannot be concentrated by 
evaporation without injury; an affinity sufficient to carry them down in combination, 
when precipitated either as hydrates or as insoluble salts. Such precipitates when 
collected were applied in the state of cream on paper, and when dry were washed 
with the nitrate. It was here that the first prominently successful result was ob- 
tained. The precipitate thrown down from a liquid of this description by lead was 
found to give a far higher degree of sensitiveness than any I had before obtained ; 
receiving an equal depth of impression, when exposed, in comparison with mere 
* From the Boa Constrictor. My specimen had been long kept, at least fifteen years. 
