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4 THE ADVANCE OF PHOTOGRAPHY
is still more apparent. Silver, placed in nitric acid, is dissolved with effervescence, and if the solution is evaporated, a solid mass of crystals is obtained. This is not silver, but a combination of this body with nitric acid. This nitrate of silver is totally different from ordinary silver ; it is easily soluble in water ; it has a bitter, disagreeable taste ; it fuses readily and destroys organic matter ; and it is therefore used as a corrosive agent, under the name of lunar caustic.
Fingers which have grasped lunar caustic, skin which has been cauterized by it, or in fact any light coloured objects sprinkled with a solution of it, quickly assume a dark colour. This can be at once observed by moistening a small piece of paper with a silver nitrate solution, allowing it to dry, and then placing it in the light.
Indelible Ink. — These properties were soon made use of to produce a so-called indelible ink, which is nothing more than a solution of one part of nitrate of silver in four parts of water, mixed with a thick solution of gum. Written characters traced with this ink upon linen cloth are pale ; but, when dried in the sunlight, quickly become dark brown, and are not injured by washing. A quill, not a steel pen, must be used, as steel decomposes the nitrate of silver.
From the discovery of the blackening of paper saturated with lunar caustic to the invention of photography there was but a step ; yet it was long before anyone thought of producing pictures by the help of light alone, and still longer before these attempts were crowned with success.
Experiments of Wedgwood and Davy. — Wedgwood, the son of the celebrated manufacturer of porcelain who produced the popular Wedgwood ware, and Davy, the celebrated chemist, made the first attempts in the 3-ear 1 802. The}7 placed flat bodies, such as the leaves of plants, upon paper prepared with nitrate of silver. Light was thus kept from the parts of the paper covered by the