RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 11 
paper. It consists in simply delaying the last or efficient wash of nitrate of silver, 
on which the sensitive quality depends, till the moment of using it, and in fact using 
the paper actually wet with the nitrate, and applied with its sensitive face against a 
glass plate whose hinder surface is in the focus of the camera. This affords other 
collateral advantages : 1st, that all crumpling or undulation of the paper is avoided ; 
2nd, that, being rendered in some degree transparent, the light is enabled to act deeper 
within its substance. If lead be used as the mordant basis of the paper, it is manifest 
that it can sustain no injury whatever by keeping or exposure, there being no ingre- 
dient on which the light can act, present, until the moment it is used*. 
33. My attempts to generalize the property of lead above described, of acting as a 
mordant, have proved unsuccessful. Those metals only could be practically useful 
which give white combinations ( leucolytes ). Of these I found that paper impregnated 
with hydrate of bismuth, by wetting it with the nitrate of that metal and then soak- 
ing in water (by which it acquires a beautiful whiteness) comports itself as ordinary 
paper. The insoluble salts of mercury, even calomel, are also without effect, as in- 
deed are many of those of silver itself, as the sulphate, borate, &c. I mean without 
effect in giving a high degree of sensibility ; for in other respects they are far from 
inefficacious, as will presently appear. 
34. It frequently happens that however carefully the successive washes are applied, 
so as apparently to drench completely every part of the paper, irregular patches in the 
resulting sheet will be of a comparatively much lower degree of sensibility, which degree 
is nevertheless uniform over their whole area. These patches are always sharply de- 
fined and terminated by rounded outlines, indicating as their proximate cause, the 
spreading of the wash last applied within the pores of the paper. They have been 
noticed and well described by Mr. Talbot, and ascribed by him, I think justly, to 
the assumption of definite and different chemical states of the silver within and with- 
out their area, which it would be highly interesting to follow out. They are very 
troublesome in practice, but maybe materially diminished in frequency, if not avoided 
altogether, by saturating the saline washes used, previous to their application, with 
chloride of silver (see § 16.). By attending to this precaution, and by dividing the 
last wash of the nitrate into two of half the strength applied one after the other 
drying the paper between them, as recommended in Art. 30, Note, their occurrence 
may be almost entirely obviated. 
35. With a view to ascertain how far organic matter is indispensable to the rapid 
discoloration of argentine compounds, a process was tried which it may not be amiss 
to relate, as it issued in a new and very pretty variety of the photographic art. A 
solution of salt of extreme dilution was mixed with nitrate of silver, so dilute as to 
form a liquid only slightly milky. This was poured into a somewhat deep vessel, at 
the bottom of which lay horizontally a very clean glass plate. After many days, the 
* Mr. Talbot, to whom I communicated this process, wrote me word in reply, that he also had been led by 
his own experience to adopt a manipulation of the same kind though somewhat different in detail. 
c 2 
