Sometimes the object to be mounted is of such a thickness as to require a cell. For this purpose glass rings are used (as described in Chapter IV.), and filled with balsam. The best mode of doing this is thus described by Mr. T. S. Ralph in the _Microscopic Journal_:--“The question was asked me when I was in England, if I knew how to fill a cell with Canada balsam and leave behind no air-bubbles? I replied in the negative; but now I can state how to accomplish this. Fill the cell with clear spirit of turpentine, place the specimen in it, have ready some balsam just fluid enough to flow out of the bottle when warmed by the hand; pour this on the object at one end, and, gradually inclining the slide, allow the spirit of turpentine to flow out on the opposite side of the cell till it is full of balsam; then take up the cover, and carefully place upon it a small streak of Canada balsam from one end to the other. This, if laid on the cell with one edge first, and then gradually lowered until it lies flat, will drive all the air before it, and prevent any bubbles from being included in the cell. It can be easily put on so neatly as to require no cleaning when dry. If the cover is pressed down too rapidly, the balsam will flow over it, and require to be cleaned off when hardened, for it cannot be done safely while fluid at the edges.” Sometimes with every care bubbles are enclosed in the balsam, injuring objects which are perhaps rare and valuable. The whole slide must then be immersed in turpentine until the cover is removed by the solution of the balsam; and the object must be cleansed by a similar steeping. It may then be remounted as if new in the manner before described. The balsam and chloroform described in Chapter I. is thus used; and where the object is thin, the mounting is very easily accomplished. When the object is laid upon the slide with a piece of glass upon it, and the balsam and chloroform placed at the edge of the cover, the mixture will gradually flow into the space betwixt the glasses until the object is surrounded by it, and the unoccupied portion filled. The chloroform will evaporate so quickly that the outer edge will become hard in a very short time, when it may be cleaned in the ordinary way. Sometimes the balsam is dissolved in the chloroform without being first hardened; but this is only to render it more fluid, and so give the operator less chance of leaving bubbles in the finished slide, as the thicker the medium is, the more difficult is it to get rid of these intruders.