The slide must first be cleaned; then on the centre a quantity of balsam must be placed with a bluntly-pointed glass rod, according to the size of the object about to be mounted. To this a slight heat must be applied, which will cause any bubbles to rise from the surface of the slide, so that they may be readily removed with a needle. The object should be freed from all air by steeping in turpentine, as before described, and then from superfluous liquid by a short drainage, and carefully laid _upon_, or where it is practicable thrust _into_, the balsam, prepared on the slide as above. In the former case, or where the balsam has not totally covered the object, a small quantity must be taken, warmed, and dropped upon it, and any bubbles removed by the needle as before. To cover this, the thin glass must be warmed, and beginning at one side, allowed to fall upon the balsam, driving a small “wave” before it, and thus expelling any bubbles which may remain. This is quite as safely performed (if not more so) by making a solution of balsam in turpentine of the consistency of thick varnish. The thin glass cover may be slightly coated with this, and will then be much less liable to imprison any air, which frequently happens when the cover is dry. Bubbles, however, will sometimes make their appearance in spite of all care; but when the object is comparatively strong, they may be removed by keeping the slide rather warm, and _working_ the cover a little, so as to press them to one side, when they should be immediately removed with a needle point, otherwise they are again drawn under. Where the slide requires keeping warm for any length of time, a _hot-water bath_ is sometimes made use of, which is simply a flat tin, or other metal case, with a mouth at the side, that when the hot water is introduced it may be closed up, and so retain its warmth for a long time. In working, the slide is laid upon it, and so admits of longer operations, when required, without growing cold. Sometimes a spirit-lamp is placed under it to keep up an equal heat through excessively long processes. Where the time required, however, is but short, a thick brass plate is sometimes used (see Chapter I.), which is heated to any degree that is required, and the slide placed upon it. Some objects, which are so thin that they are usually _floated_ upon the slide, as before stated, require no steeping in turpentine or other liquid. These are best mounted by covering with a little _diluted_ balsam, and after this has had time to penetrate the substance, ordinary balsam is laid upon it, and the slide finished in the usual manner.