on Photographie Processes. 507 
stances (after carbon) known. I find that this high degree 
of blackness and opacity may be induced on a mercurial pho- 
tograph prepared as in Art. 228, by a process which is in 
itself not a little curious and instructive, as affording a kind 
of parallel to the stimulating action of Mr. Talbot’s second 
application of nitrate of silver, in his beautiful kalotype process. 
The nature of the process in question will be best illustrated 
by describing the experiment which led to it. 
234. It frequently happens that papers prepared with nitrate 
of mercury and the ammonio-citrates or tartrates, with or 
without addition of tartaric or citric acid, fail to exhibit the 
peculiar properties described in Arts. 228, 229 at all satisfac- 
torily. Indeed, to bring on the peculiar velvety effect there 
described, a high degree of intensity of sunshine seems to be 
an essential requisite, as, in a feeble sun, I have never ob- 
tained even an approach to it. A paper prepared (Oct. 28, 
1842) according to the instructions of Art. 229 in every re- 
spect, except in the proportion of tartaric acid (which was 
somewhat Jess than that recommended), proved very little 
sensitive. A strip of this paper, half shaded, acquired after 
a few minutes’ exposure to sunshine only a feeble brown co- 
lour over the sunned portion. Being then withdrawn, it was 
washed over with nitrate of mercury. Immediately the sunned 
portion began to darken very rapidly while the shaded part 
was unaffected, and ultimately assumed a deep brown hue. 
Exposed while yet wet to the sunshine, this passed rapidly to 
intense blackness, while the portion originally shaded, which 
had undergone the same subsequent application, and which 
was now equally exposed to the sun, sustained in the short 
timerequired for bringing on this effect, no appreciable change. 
Indeed it seemed rather to have become more insensible than 
before. 
235. Not alone nitrate of mercury is capable of thus ex- 
citing or stimulating the dormant photographic impression on 
such paper. ‘To my very great surprise, I found the same 
effect to be produced by water sparingly applied, so as only 
to moisten the paper. Across the sunned and shaded por- 
tions of a strip of the mercurialized paper, exposed till a pale 
brown was developed in the former portion, were drawn two 
streaks, one of weak nitrate of mercury and one of spring 
water. Both, after a very short interval, passed to an intense 
brown on the sunned half, the shaded remaining unchanged. 
Edging the streak produced by the nitrate was a black border, 
that produced by the water was uniform, The whole paper 
was now exposed for a short time to the sun, which rapidly 
converted to intense blackness both the streaks on the pre- 
2M2 
