in a levelling stand, in a perfectly horizontal position, a spirit-level being used in the adjustment. The tissue-compound, warmed to 100°, should be strained through a piece of moist flannel or muslin, and when the compound is ready the plate should be warmed until it is of the same temperature as the compound. The proper amount is then poured on the collodionized plate, and caused to flow over its surface, a glass rod being used to spread the solution. A little care is necessary to prevent the formation of bubbles of air, which, when once formed, are not easily disengaged from the viscous solution, and, unless eliminated, result in defects in the tissue, producing white spots in the picture. The coated plate is then left on the levelling stand until it is quite set. As will easily be seen, a very little inclination will cause the coating to run into uneven waves, or to accumulate and form a greater thickness in parts, the disadvantage of which is manifest.
When once thoroughly set, the plates may be placed away in an upright position to dry. The more quickly the drying is effected, provided heat be not applied, the better. The temperature should not exceed 60° or 70° Fahrenheit, as a higher temperature may cause the gelatine to run, and form uneven waves. A very low temperature, which would retard drying, is, of course, undesirable. A damp place is especially to be avoided, as the protracted drying, caused by a damp atmosphere, materially tends to the production of a spontaneous decomposition and general insolubility of the tissue. In a dry, well-ventilated dark-room, kept at a temperature of about 60° Fahrenheit, drying will generally take place within twelve hours, and without any danger to the solubility of the tissue. It may be found desirable in damp weather to use a drying box, containing chloride
