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 quality of the gelatine, the temperature, and varying conditions, in regard to which experience must be the guide. In very dry weather, for instance, the proportion of sugar may be increased, its chief office being to give pliancy and elasticity to the tissue, and prevent the horniness of the gelatine when perfectly desiccated.

To prepare the tissue compound for use, heat must be applied until it is quite fluid, when one part of a saturated solution of bichromate of ammonia must be added to every ten parts of the gelatinous compound, after which the whole should be strained through flannel. It is desirable, after the chromic salt has been added to the gelatine, to avoid applying a greater heat than is necessary to preserve fluidity, as excess of heat tends to produce spontaneous insolubility. About 100° Fahrenheit will generally answer the purpose. It should be further remembered that frequent and continued application of heat to gelatine destroys its setting powers, which would render the preparation useless.

The thickness of the tissue, and the proportion of the mixture necessary in forming it, depend very much on circumstances. If the tissue be too thin, the finished picture will not possess its proper depth of shade in its darkest parts, unless it has had an unusually large proportion of coloring matter. If too thick, drying is retarded, and it is intractable in mounting and other manipulations; it will also require a longer time in development. As different qualities of gelatine will produce different results, something must be left to experience in determining the amount of the tissue-compound necessary to form a given amount of tissue; as a general rule, however, it may be stated that about two ounces will be required for each superficial foot.

Immediately previous to the preparation of a sheet of tissue, the piece of “patent plate” glass should be placed