170 HISTORY OF PHOTOGRAPHY. 



Proto-nitrate of mercury simply washed over paper is slowly and feebly 

 blackened by exposure to sunshine. And if paper be impregnated with the 

 ammonio-citrate of iron, already so often mentioned, partially sunned, and then 

 washed with the proto-nitrate, a reduction of the latter salt, and consequently 

 blackening of the paper, takes place very slowly in the dark over the sunned 

 portion, to nearly the same amount as in the direct action of the light on the 

 simply nitrated paper. 



But if the mercurial salt be subjected to the action of light in contact with 

 the ammonio-citrate or tartrate, the effect is far more powerful. Considering 

 at present only the citric double salt, a paper prepared by washing first 

 with that salt and then with the mercurial proto-nitrate (drying between) is 

 endowed with considerable sensibility and darkens to a very deep brown, nay, 

 to complete blackness, on a moderate exposure to good sun. Very sharp and 

 intense photographs of a negative character may be thus taken. They are, 

 however, difficult to fix. ' The only method which I have found at all to suc- 

 ceed has been by washing them with bichromate of potash and soaking them 

 for twenty-four hours in water, which dissolves out the chromate of mercui'y 

 for the most part, leaving, however, a yellow tint on the ground, which resists 

 obstinately. But though pretty effectually fixed in this way against liriht, 

 they are not so against time, as they fade considerably on keeping. 



When the proto-nitrate of mercury is mixed in solution with either of the 

 ammouiacal double salts it forms a precipitate, which, worked up with a brush 

 to the consistence of cream and spread upon paper, produces very fine pictures, 

 the intensity of which it is almost impossible to go beyond. Most unfortu- 

 nately they can not be preserved. Every attempt to fix them has resulted in 

 the destruction of their beauty and force ; and even when kept from light they 

 fade with more or less rapidity, some disappearing almost entirely in three 

 or four days, while others have resisted tolerably well for a fortnight or even a 

 month. It is to an overdose of tartaric acid that their more rapid deterioration 

 seems to be due, and of course it is important to keep down the proportion of 

 this ingredient as low as possible. But without it I have never succeeded in 

 producing that peculiar velvety aspect on which the charm of these pictures 

 chiefly depends, nor anything like the same intensity of color without over- 

 sunning. 



FERKOTARTRATE OF SILVER. 



Extending his inquiries still further into these very remarkable 

 changes, the following process presented itself to Sir J. Herschel, 

 which is in many respects remarkable : 



If nitrate of silver, sjoecific gravity 1 '200, be added to ferro- 

 tartaric acid, specific gravity 1 -023, a precipitate falls which is in 

 great measure redissolved by a gentle heat, leaving a black sediment, 

 which, being cleared by subsidence, a liquid of a pale yellow color is 

 obtained, in which a further addition of the nitrate causes no turbid- 

 ness. AMien the total quantity of the nitrated solution amounts to 

 about half the bulk of the ferrotartaric acid it is enough. The 

 liquid so prepared does not alter by keeping in the dark. 



Spread on paper and exposed wet to the simshine (partly shaded) 

 for a few seconds no impression seems to have beef! made; but by 

 degrees (although withdrawn from the action of the light) it de- 



