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HISTORICAL SURVEY 23 explosive properties similar to those of gunpowder. It was conceived that this substance would be an important substitute for gunpowder, but it was soon found that its explosive property was very unequal, being sometimes too strong and at other times too weak. On the other hand, another very useful property of this substance was observed—its solubility in a mixture of alcohol and ether. This solution leaves behind it a transparent membrane forming an excellent sticking-plaster for wounds. Thus the same substance that was destined to be a substitute for gunpowder, as a destructive agent for producing wounds, became actually a remedy for the latter. This solution of gun-cotton was called collodion. In recent years, owing to the work of Count Hilaire de Chardonnet and others, this property has been made use of in the preparation of artificial silk from cotton and wood- pulp, which latter, being mainly composed of cellulose, behaves in a similar manner to cotton Avhen subjected to the above treatment. The thought occurred to several photographic experi- menters to try this substance instead of the white of egg, as a coating for glass plates ; but the attempts did not at first lead to any satisfactory results. At length Archer published in England a full description of a collodion negative process surpassing in the beauty of its results, in simplicity and certainty, Niepce's white of egg process. Archer's Experiments. —Archer coated glass plates with collodion, in which an iodide had been dissolved ; he immersed this in a solution of nitrate of silver, and thus obtained a membrane of collodion impregnated with sensitive iodide of silver, which he then exposed in the camera. The invisible change produced by the light became visible on pouring gallic acid, or the still more powerful chemical agent, p}^rogallic acid, over the plate ; or, instead