10 
SIR J. W. F. HERSCHEL ON THE CHEMICAL ACTION OF THE 
time to use (and at the time much to my satisfaction) my own process* in the prepara- 
tions of paper for the copying - of designs, as well as for the camera. By degrees, how- 
ever, an inconvenience, little expected, appeared. The paper with a basis of lead turns 
yellowby keeping in the dark, and the tint goes on gradually deepening to a dark brown. 
But, what is very singular, this change is not equally rapid in all kinds of paper, a dif- 
ference depending no doubt on the size employed, which, it may be observed here once 
for all, is of the utmost influence in all photographic processes. In one sort of paper 
(known by the name of blue wove post ) it is instantaneous, taking place the moment 
the nitrate (if abundant) is applied. And yet I find this very paper to resist disco- 
loration, by keeping, better than any other, when the mordant base is silver in place 
of lead. On the other hand, a paper of that kind called smooth demy , rendered sen- 
sitive by the process described in (28.), was found to acquire, by long keeping, a grey 
or slate colour, which increases to such a degree as might be supposed to render it 
useless. Yet in this state, when it is impressed with a photographic image, the pro- 
cess of fixing with hyposulphite of soda destroys this colour completely, leaving the 
ground as white as when fresh prepared. This fortunate restoration, however, does 
not take place when the paper has been browned as above described. Some of the 
muriatic salts also are more apt to induce this discoloration than others, especially 
those with earthy bases. But the effects in this respect are so capricious, that it is 
vain to attempt giving any connected account of them. 
31. In consequence of this spontaneous discoloration 'f~, I disused, for ordinary 
purposes, this mode of preparation, and adopted the following series of washes, on 
Mr. Talbot’s principle, viz. 1st, nitrate of silver, S. G. = T096 (say 1*1); 2ndly, mu- 
riate of soda, 1 salt -|- 19 water; 3rdiy, nitrate of silver, S. G. T132 (say T15), satu- 
rating the muriatic solution with chloride of silver, and occasionally dividing the last 
application into two consecutive washes of equal strength by dilution. This, as an 
ordinary working paper, is easily prepared, and has sensibility enough for most pur- 
poses. It gives very good camera pictures, and, if that particular sort of paper above 
named be used, it retains its whiteness well in the dark, at least for some weeks. 
32. Paper of this kind may not always be at hand, nor under the same name may 
the same quality be always depended on, and every other sort I have tried discolours 
under this preparation, though much more slowly than when lead is used. Being 
much annoyed with this, I adopted for camera pictures a process which proved both 
convenient and effectual, and which applies equally well to both descriptions of 
* 1st, or mordant wash, saturated solution of acetate of lead. 
2nd, weak solution of common salt, containing of salt. 
3rd, nitrate of silver, S. G. = 1-20, which however is best applied in two consecutive washes each of 
S. G. MO. 
f Some camera pictures which I carried with me to Paris, and which were prepared on the 28th of April, 
were found on arrival much discoloured ; and others prepared at the same time, were turned brown on my re- 
turn from France about a month afterwards. (Neither had been fixed.) 
