2 
SIR J. F. W. IIERSCHEL ON CERTAIN IMPROVEMENTS 
monio-citrate of iron, one part by weight of the salt to eleven parts water. No 
immediate precipitation takes place, and before any has time to do so, the mixture 
must be washed over paper (which should have rather a yellowish than a bluish cast), 
and dried. It is now ready for use, and I do not find that it is impaired by keeping. 
To use it, it must be exposed to the light till a faint, but yet perfectly visible picture 
is impressed, and till the border (if it be an engraving which is copied) has assumed 
a pale brown colour. Being withdrawn it is to be brushed over as rapidly as possible 
with a broad flat brush, dipped in a saturated solution of prussiate (ferrocyanate) of 
potash diluted with three times its bulk of gum-water, so strong as just to flow freely 
without adhesion to the lip of the vessel. All the care that is required is, that the 
film of liquid be very thinly, evenly, and above all, quickly spread. Being then 
allowed to dry in the dark, it rarely fails to produce a good picture. And what is 
very remarkable, it is ipso facto fixed as soon as dry, so at least as not to be injured 
by exposure to common day-light, immediately; and after a few days’ keeping it 
becomes entirely so, and will bear strong lights uninjured. By long keeping, details 
at first barely seen come out, and the whole picture acquires a continually-increasing 
intensity, without however sacrificing distinctness ; and by the same gradations its 
colour passes from purple to greenish-blue. Some experience, to be acquired only 
by practice, is necessary to determine the proper moment for withdrawing the pho- 
tograph from the action of the light. If it be over-sunned, only the darker shades 
appear ; if too little, the whole, though beautifully perfect in the first moments of its 
appearance, speedily runs into an indistinguishable blot. 
233. The principal obstacle in the way of the employment of gold and silver as 
photographic ingredients for the production of negative models, to be used for re- 
transfers, so as to multiply positive copies, arises from the want of absolute opacity 
in these metals or their oxides when in a state of minute division. The same objec- 
tion does not apply, or applies with much less force, to mercury, which (probably 
owing to its fluid state, which prevents its particles from acquiring that excessive 
tenuity which a laminated form would admit, by reason of their capillary forces con- 
tracting each separately deposited particle into a sphere) is one of the most opake 
substances (after carbon) known. I find that this high degree of blackness and 
opacity may be induced on a mercurial photograph prepared as in Art. 228, by a pro- 
cess 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 satisfactorily. 
Indeed, to bring on the peculiar velvety effect there described, a high degree of in- 
tensity of sunshine seems to be an essential requisite, as, in a feeble sun, I have never 
