All dry objects, however, which are to be preserved should be mounted on glass slides in one of the cells (described in Chapter I.) best suited to them. Where the object is to be free from pressure, care must be taken that the cell is deep enough to ensure this. When the depth required is but small, it is often sufficient to omit the card, leather, or other circles, and with the “turn-table” before described, by means of a thick varnish and camel-hair pencil, to form a ring of the desired depth; but should the varnish not be of sufficient substance to give such “walls” at once, the first application may be allowed to dry, and a second made upon it. A number of these may be prepared at the same time, and laid by for use. When liquids are used (see Chapter IV.), Dr. Carpenter recommends gold-size as a good varnish for the purpose, and this may be used in “dry” mountings also. I have used the asphaltum and india-rubber (mentioned in Chapter I.), and found it to be everything I could wish. The cells, however, must be _thoroughly dry_, and when they will bear the heat they should be baked for an hour at least in a tolerably cool oven, by which treatment the latter becomes a first-rate medium. All dry objects which will not bear pressure must be firmly fastened to the slide, otherwise the necessary movements very often injure them, by destroying the fine hairs, &c. For this purpose thin varnishes are often used, and will serve well enough for large objects, but many smaller ones are lost by adopting this plan, as for a time, which may be deemed long enough to harden the varnish, they exhibit no defect, but in a while a “wall” of the plastic gum gathers around them, which refracts the light, and thus leads the student to false conclusions. In all _finer_ work, where it is necessary to use any method of fixing them to the slide, a solution of common gum, with the addition of a few drops of glycerine (Chapter I.), will be found to serve the purpose perfectly. It must, however, be carefully filtered through blotting paper, otherwise the minute particles in the solution interfere with the object, giving the slide a dusty appearance when under the microscope.