18

NEGATIVE PHOTOGRAPHS.

it is instantaneous, taking place the moment the nitrate, if abundant,is appliod. And yet I find this very paper to resist discolouration, bykeoping, better than any other, when the mordant base is silver in placeof lead. On the other hand, a paper of that kind called smooth demy,rendered sensitive by the process first described, was found to acquire,by long keeping, a grey or slate colour, which increases to such a degreeas might be supposed to render it useless. Yet in this state, when it isimpressed with a photographic image, the process of fixing it with thehyposulphite of soda dostroys this colour completely, leaving the groundas white as when fresh prepared. This fortunate restoration, however,does not take place when the paper has been browned as above des-cribed. Some of tho muriatic salts are moro apt to induce this discol-ouration than others, especially those with earthy bases. But thoeffects in this respect are so capricious, that it is in vain to attemptgiving any connected account of them.

“ Iu consequence of this spontaneous discolouration, I disused forordinary purposes this mode of preparation, and adopted the followingseries of washes, on Mr. Talbot’s principle,—viz., 1st, nitrate of silver,S. G. = 1.096, (say 1.1;) 2ndly, muriate of soda, 1 salt + 19 water;3rdly, nitrate of silver, S. G. 1.132, (say 1.15,) saturating the muriaticsolution with chloride of silver, and occasionally dividing the last appli-cation into two consecutive washes of equal strength by dilution. This,as an ordinary working paper, is easily prepared, and has sensibilityenough for most purposes. It gives very good camera pictures, and, ifthat particular sort of paper above named is used, it retains its white-ness well iu the dark, at least for some weeks.”

From the description which Sir John Ilerscliel has given of thospontaneous browning of tho paper, it may appear that little more needbo said on the subject, or that if any thing more is required, that itshould follow in this place; it has, however, been thought best to occupya future chapter with it, as some new matter somewhat distinct fromthe subject of the present chapter will be introduced.

It would bo tedious and useless to mention all the combinations ofalkaline and earthy muriates which have been devised to vary the effect,or incroase tho sensitiveness of the silver preparations; the very con-siderable differences produced through the influence of those salts, willafford peculiarly interesting results to any inquirer, and furnish himwith a curious collection of photographic specimens. As a general rule,the solutions of the muriate, and indeed all other salts, and of tho silverwashes, should be made in tho combining proportions of the materialused; with a scale of chemical equivalents at hand, the photographicexperimentalist need not err far, taking care that a slight excess of puronitrate of silver prevails.

c. Iodidated Papers.

I distinguish by this name those papers which are impregnated withthe iodide of silver, cither by applying tho already formed iodide sus-