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 keeping in a damp place. The addition of substances to give elasticity, such as glycerine, which retard the drying of the excited gelatine film, also tend to produce spontaneous insolubility. Heat, in conjunction with the moisture, increases the tendency. The use of too much bichromate of potash, or too prolonged immersion in the solution of bichromate, will produce spontaneous insolubility. Immersion in very hot water, prior to development, is at times conducive to insolubility, also drying the tissue in an impure atmosphere, and especially one vitiated by the burning of gas. Some samples of gelatine are said to become readily insoluble; but this requires confirmation.

—The same causes which will produce spontaneous insolubility, when present in less degree, cause tardy solution of the unaltered gelatine, and slow development. The more rapidly the tissue has dried, and the more horny and perfectly desiccated it appears, the more readily, as a general rule, the superfluous gelatine and pigment are removed by warm water, and complete development is effected. When the development is slow, hotter water may be employed; but care should be taken that the free soluble bichromate has first been removed by tepid water.

—If the tissue be suffered to remain too long in a saturated solution of bichromate of potash, the salt will crystallize on the surface during drying, and the tissue will be useless. The remedy, of course, is the employment of a weaker solution, or a shorter immersion in the full-strength solution.

—If the print be suffered to float to the surface of the warm water, allowing portions