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.
I have stated that the balsam is usually applied to the slide and objects with a “bluntly-pointed glass rod;” but for the purpose of drawing the balsam from the bottle, and conveying it to the desired place, Dr. Carpenter uses a glass syringe with a free opening. These are his instructions:—“This (the syringe) is most readily filled with balsam, in the first instance, by drawing out the piston, and pouring in balsam previously rendered more liquid by gentle warmth; and nothing else is required to enable the operator at any time to expel precisely the amount of balsam he may require, than to warm the point of the syringe, if the balsam should have hardened in it, and to apply a very gentle heat to the syringe generally, if the piston should not then be readily pressed down. When a number of balsam objects are being mounted at one time, the advantage of this plan in regard to facility and cleanliness (no superfluous balsam being deposited on the slide) will make itself sensibly felt.”
When the “mounting” is thus far accomplished, the outer “wall” of balsam may be roughly removed after a few hours have elapsed; but great care is necessary lest the cover be moved or interfered with in any way. In this state it may be left for the final cleansing until the balsam becomes hard, which takes place sooner or later, according to the degree of warmth it has been subjected to. A mantel-piece, or some place about equal to it in temperature, is the best suited to this purpose; and when the requisite hardness is attained, it may be proceeded with as follows:—With a pointed knife the balsam must be scraped away, taking care that the thin glass be not cracked by the point getting under it. If used carefully, the knife will render the slide almost clean; but any minute portions which still adhere to the glass must be rubbed with linen dipped in turpentine or spirit. If the balsam is not very hard, these small fragments are readily removed by folding a piece of paper tightly in a triangular form with many folds, and damping the point with which the glass is rubbed. As the paper becomes worn with the friction, the balsam will be carried off with it. In some cases I have found this simple expedient very useful.
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.
It has been before mentioned that some have objected to chloroform and balsam, believing that it became clouded after a certain time. Perhaps this may be accounted for in part by the fact that almost all objects have a certain amount of dampness in them. Others are kept in some preservative liquid until the time of mounting, and these liquids generally contain certain salts ([Chapter IV.]). If this dampness, as well as all traces of these salts, however small, are not totally removed—the former by drying, the latter by repeated washings—the addition of chloroform will render the balsam much more liable to the cloudiness than when balsam alone was used, as before mentioned.
This mode of employing the balsam, however, will not be always applicable, as chloroform acts upon some substances which balsam alone does not. Some salts are even soluble in it, the crystals disappearing after a few days or weeks, whereas in the balsam alone they are quite permanent. Experience is the only guide in some cases, whilst in others a little forethought will be all that is required.