This section is from the book "A Manual Of Photography", by Robert Hunt. Also available from Amazon: A Manual of Photography.
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 black 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 quantity of the nitrated solution amounts to about half the bulk of the ferro-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wet to the sunshine (partly shaded) for a few seconds, no impression seems to have been made ; but by degrees (although withdrawn from the action of the light) it developes itself spontaneously, and at length becomes very intense.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 (if 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the ultimate effect less striking, but a picture begins to be visibly produced, which darkens spont-neously after it is withdrawn. But if the exposure be discontinued before this effect comes on, an invisible impression is the result, to develope which all that is necessary is to breathe upon it, when it immediately appears, and very speedily acquires an extraordinary intensity and sharpness, as if by magic. Instead of the breath, it may be subjected to the regulated action of aqueous vapour by laying it in a blotting-paper book, of which some of the outer leaves on both sides have been damp, or by holding it over warm water.
Many preparations, both of silver and gold, possess a similar property, in an inferior degree ; but none that I have yet met with, to anything like the extent of that above described.
These pictures do not admit of being permanently fixed; they are so against the action of light, but not against the operations of time. They slowly fade out even in the dark; and in some examples which I have prepared, the remarkable phenomenon of a restoration after fading, but with reversed lights and shades, has taken place.