A Manual Of Photography

PHOTOGRAPHIC PROPERTIES OF FERRO-TARTRATE OF SILVER. 61 either of the ammoniacal 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 unfortunately, they cannot 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 over-dose of tartaric acid that their more rapid deterio- ration 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 colour without over-sunning." Section IV.—Ferro-Tartrate 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, specific 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 ac sediment, which, being cleared by subsidence, a liquid of a pale yellow colour is obtained, in which a further addition of the nitrate causes no turbidness. When the total quanti y o e nitrated solution amounts to about half the bulk o e err tartaric acid, it is enough. The liquid so prepared does not alter by keeping in the dark., Spread on paper, and exposed wet to the sunshine (partly shaded) for a few seconds, no impression seems to Kave heen made ; but by degrees (although withdrawn from the action of the light) it envelopes itself spontaneously, and at length be- comes very intense But if the paper be thoroughly dried in the dark (in which state it is of a very pale greenish-yellow colour), it possesses the singular property of receiving a dormant or invisible picture, to produce which (it it be for instance an engraving that is to be copied), from thirty seconds to a minute s exposure in the sunshine is requisite. It should not be continued too long, as not only is the ultimate effect less striking, but a picture^begins to be visibly produced, which darkens sponta-