These are the cells which are mostly used in this branch of microscopic mounting. The mode of using them, and the different treatment which certain objects require when intended to be preserved in the before-mentioned liquids, may now be inquired into. I may mention, however, that this class of objects is looked upon by many with great mistrust, owing to the danger there is of bubbles arising in the cells after the mounting has been completed, even for years. I know some excellent microscopists who exclude all objects in cells and preservative liquids from their cabinets, because they say that eventually almost all become dry and worthless; and this is no matter of surprise, for many of them do really become so. Perhaps this is owing to the slides being sold before they could possibly be thoroughly dry. As to the air-bubbles, I shall have something to say presently. We will now suppose the cell employed is made by placing a glass ring upon the slide with marine glue or gold-size, and is quite dry. Around the edge of the cleaned thin glass which is to cover it, I trace with a camel-hair pencil a ring of gold-size, and also around the edge of the cell to which it is to adhere. Dr. Carpenter objects to this, as rendering the later applications of the gold-size liable to “run in.” All danger of this, however, is completely done away with by leaving the slide and cover for awhile until the cement becomes partially “fixed,” but still adhesive enough to perform its function (Chapter II.). With many slides this is not accomplished in less than twenty-four hours, even if left two or three days no injury whatever ensues; but with other kinds an hour is too long to leave the exposed cement, so that the operator must use his own discretion. The liquid required may be drawn up by the mouth into the pointed tube mentioned in Chapter I., and then transferred to the cell. In the various books of instruction, the object is now to be placed in the cell; this, however, I think a great mistake, as another process is absolutely necessary before we advance so far. The cell, full of liquid, must be placed under the receiver of an air-pump, and the air withdrawn. Almost immediately it will be perceived that the bottom and sides of the cell are covered with minute bubbles, which are formed by the air that is held in suspension by the liquid. The slide may now be removed, and the bubbles may require the aid of a needle or other point to displace them, so obstinately do they adhere to the surface of the glass. This process may be then repeated, and one cause, at least, of the appearance of bubbles in cells of liquid will be got rid of. The object to be mounted should also be soaked in one or two changes of the preservative liquid employed, and, during the soaking, be placed under the air-pump and exhausted. It may then be transferred to the cell, which will probably cause the liquid to overflow a little. The cover