14 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
42. Papers were washed with the chlorides of gold and of platina, freed from ex- 
cess of acid. In the case of platina, they proved insensible ; in that of gold, a slow 
but regularly increasing darkening takes place, and the paper at length, under the 
influence of light, becomes purple. A wash of nitrate of silver increases, but not to a 
high degree, the sensibility of the aurated paper, but that impregnated with platina 
remains insensible, unless the nitrate be in great abundance. 
43. In the chlorides both of gold and platina, nitrate of silver produces precipitates. 
In that of gold, the precipitate is of a yellow-brown colour, and in that of platina, a 
pale yellow. Both are possibly metallic double salts in which the gold or platina, as 
well as the silver, is in the state of chloride. As relates to the action of light, I 
find the precipitate of gold very little sensitive if spread on glass, that of platina not 
at all. On paper, the aurated precipitate is blackened somewhat more readily. 
44. If paper impregnated with oxalate of ammonia be washed with chloride of gold 
it becomes, if certain proportions be hit, pretty sensitive to light ; passing rather ra- 
pidly to a violet purple in the sun. It is next to impossible to dry paper so prepared, 
however, as a very gentle heat blackens it. It passes also to the same purple hue in the 
dark, though much more slowly ; so that, as a photographic combination, it is useless. 
45. Paper impregnated with acetate of lead, when washed with perfectly neutral 
chloride of gold, acquires a brownish yellow hue and a sensibility to light which, 
though not great, is attended with some peculiarities highly worthy of notice. The 
first impression of the light seems rather to whiten than to darken the paper, by dis- 
charging the original colour, and substituting for it a pale greyish tint, which by slow 
degrees increases to a dark slate colour. But if arrested while yet not more than a 
moderate ash-grey, and held in a current of steam, the colour of the part acted on by 
light (and of that only) darkens immediately to a deep purple. The same effect is 
produced by immersing it in boiling distilled water. If plunged in cold water the 
same change comes on more slowly, and is not complete till the paper is dried by 
heat. A dry heat however does not operate this singular change. The best way of 
making this and similar experiments is to shade one half the paper operated on with 
an opake screen closely applied, and to reserve portions, before and after exposure, 
for comparison. 
46. This is not the only case in which the impression made by light is rendered 
more evident by subsequent applications. A solution of chloride of platina in ether 
being washed over a bibulous paper impregnated with hydriodate of potash, in cer- 
tain degrees of strength and copiousness, browns pretty rapidly in the dark, but much 
more rapidly and to a much deeper tint in sunshine. A paper so washed and partly 
shaded, on exposure produced a well-defined figure of the shading body, which, on 
the addition of a fresh wash of the hydriodate, out of the light, became much more 
strongly contrasted with the surrounding ground. 
4 7- Paper (No. 644) washed with acetate of lead and then with chloride of pla- 
tina, is absolutely insensible, and only becomes very feebly sensible when thoroughly 
