RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 
9 
nitrated paper, in less than a fifth of the time, and moreover acquiring a beautiful 
ruddy-brown tint almost amounting to crimson, with a peculiarly rich and velvety 
effect. 
29. Alumina, similarly precipitated from the same liquid, gave no such result. 
Struck by this difference, which manifestly referred itself to the precipitant, it now 
occurred to me to omit the organic matter, (whose necessity I had never before thought 
of questioning,) and to operate with an alkaline precipitant on a mere aqueous solution 
of nitrate of lead, so as to produce simply a hydrate of that metal. The result was 
instructive. A cream of this hydrate, being applied and dried, acquired, when washed 
with nitrate of silver, a considerable increase of sensitiveness over what the nitrate 
alone would have given, though less than in the experiment where organized matter 
was present. The rich crimson hue also, acquired in that case under the action of 
the light, was not now produced. Two peculiarities of action were thus brought into 
view ; the one, that of the oxide of lead as a mordant, (if we may use a term borrowed 
from the art of dyeing,) the other, that of the organic matter as a colorific agent. 
The former action, as more immediately interesting, claimed the first attention, and 
the more so, on account of the analogy which it seemed to offer with a similar mor- 
dant action of the chloride of silver itself, in Mr. Talbot’s curious process of succes- 
sive alternate washes with salt and nitrate of silver, which by this time had been di- 
vulged by him. Of this action, the observed effect seemed to promise an explanation, 
should it be found to occur when other metallic oxides were substituted for lead, or 
other salts of this metal for the hydrate. 
30. Paper washed with acetate of lead was therefore impregnated with various in- 
soluble salts of that metal, such as the sulphate, phosphate, muriate, hydriodate, bo- 
rate, oxalate, and others, by washing with their appropriate neutral salts, and when 
dry, applying the nitrate of silver, as usual. The results, however, were in no way 
striking as respects sensitiveness, in any case but in that of the muriatic applications. 
In all cases where such applications were used, a paper was produced infinitely more 
sensitive than any I had at that time made. And I may here observe that, in this 
respect, the muriate of strontia appeared to have decided advantage. I should observe 
that up to this time I had purposely abstained from repeating Mr. Talbot’s experiment 
above alluded to, being desirous of seeing what progress I could make in the inquiry 
without external aid. Comparative trials now made with some specimens of his pre- 
paring, satisfied me that in point of sensitiveness the two papers (with mordant bases 
of lead and silver) were nearly on a par*. In consequence of this I continued for some 
* This does not refer to the later improvements made by Mr. Talbot in the preparation of his paper, 
whether by the use of the bromurets or other means yet undescribed by him. I have already alluded to the 
exquisite sensitiveness of some specimens he has been obliging enough to send to me. It may be right to 
mention the date of the experiments described in (29.) and (30.) of the text, which are respectively April 19 
and 27, 1839. 
MDCCCXL. 
C 
