In applying the required heat to the slides, covers, &c., it is necessary in many cases to ensure uniformity, otherwise there is danger of the glass being broken. For this purpose a brass plate at least three inches wide, somewhat longer, and one-eighth of an inch thick, must be procured. It should then be affixed to a stand, so that it may be readily moved higher or lower, in order that the distance from the lamp may be changed at will, and thus the degree of heat more easily regulated. This has also the advantage of enabling the operator to allow his slides, &c., to cool more gradually, which, in some cases, is absolutely necessary,—as in fusing some of the salts, &c.
In order to get rid of air-bubbles, which are frequently disagreeable enemies to the mounter of objects, an air-pump is often very useful. This is made by covering a circular plate of metal with a bell-glass, both of which are ground so finely at the edges that greasing the place of contact renders it air-tight. The pump is then joined to the metal plate underneath, and worked with a small handle like a common syringe. By turning a small milled head the air may be allowed to re-enter when it is required to remove the bell-glass and examine or perform any operation upon the object. The mode of using this instrument will be described hereafter, but it may be here stated that substitutes have been devised for this useful apparatus; but as it is now to be obtained at a low cost, it is hardly worth while to consider them. Much time is, in many instances, certainly saved by its use, as a very long immersion in the liquids would be required to expel the bubbles, where the air-pump would remove them in an hour.
The next thing to be considered is what may be termed Cements, some of which are necessary in every method of mounting objects for the microscope. Of these will be given the composition where it is probable the young student can make use of it; but many of them are so universally kept as to be obtainable almost anywhere; and when small quantities only are required, economy suffers more from home manufacture than from paying the maker’s profit.
Amongst these, Canada Balsam may, perhaps, be termed the most necessary, as it is generally used for the preservation of many transparent objects. It is a thick liquid resin of a light amber colour, which on exposure to the atmosphere becomes dry and hard even to brittleness. For this reason it is seldom used as a cement alone where the surface of contact is small, as it would be apt to be displaced by any sudden shock, especially when old. In the ordinary method of using, however, it serves the double purpose of preserving the object and fixing the thin glass cover; whilst the comparatively large space upon which it lies lessens the risk of displacement. By keeping, this substance becomes thicker; but a very little warmth will render it liquid enough to use even when to some extent this change has taken place. When heated, however, for some time and allowed to cool, it becomes hardened to any degree, which may be readily regulated by the length of time it has been exposed, and the amount of heat to which it has been subjected. On account of this property it is often used with chloroform: the balsam is exposed to heat until, on cooling, it assumes a glassy appearance; it is then dissolved in pure chloroform until it becomes of the consistence of thick varnish. This liquid is very convenient in some cases; as air-bubbles are much more easily got rid of than when undiluted Canada balsam is used. It also dries readily, as the chloroform evaporates very quickly, for which reason it must be preserved in a closely-stoppered bottle. It has been said that this mixture becomes cloudy with long keeping, but I have not found it so in any cases where I have used it. Should it, however, become so, a little heat will readily dispel the opacity. The ordinary balsam, if exposed much to the air whilst being used, becomes thicker, as has been already stated. It may be reduced to the required consistency with common turpentine, but I have often found this in some degree injurious to the transparency of the balsam, and the amalgamation of the two is by no means perfect. (See also [Chapter III.]) The cheapness of the article renders it no extravagance to use it always undiluted; and when preserved in a bottle with a hollow cover fitting tightly around the neck, both surfaces being finely ground, it remains fit for use much longer than in the ordinary jar.
Asphaltum.—This substance is dissolved in linseed oil, turpentine, or naphtha, and is often termed “Brunswick black.” It is easily worked, but is not generally deemed a trustworthy cement, as after a time it is readily loosened from its ground. It is, however, very useful for some purposes (such as “finishing” the slides), as it dries quickly. I shall, however, mention a modification of this cement a little further on.
Marine Glue.—No cement is more useful or trustworthy for certain purposes than this. It is made in various proportions; but one really good mixture is—equal parts of india-rubber and gum shellac; these are dissolved in mineral naphtha with heat. It is, however, much better to get it from the opticians or others who keep it. It requires heat in the application, as will be explained in [Chapter IV.]; but it is soluble in few, if any, liquids used by the microscopist, and for that reason is serviceable in the manufacture of cells, &c. Where two pieces of glass are to be firmly cemented together, it is almost always employed; and in all glass troughs, plates with ledges, &c., the beginner may find examples of its use.
Gold Size.—This substance may always be procured at any colourman’s shop. The process of its preparation is long and tedious. It is, therefore, not necessary to describe it here. Dr. Carpenter says that it is very durable, and may be used with almost any preservative liquids, as it is acted upon by very few of them, turpentine being its only true solvent. If too thin, it may be exposed for awhile to the open air, which by evaporation gradually thickens it. Care must be taken, however, not to render it too thick, as it will then be useless. A small quantity should be kept on hand, as it is much more adhesive when old.
Liquid Glue is another of these cements, which is made by dissolving gum shellac in naphtha in such quantity that it may be of the required consistency. This cement appears to me almost worthless in ordinary work, as its adherence can never be relied upon; but it is so often used and recommended that an enumeration of cements might be deemed incomplete without it. Even when employed simply for varnishing the outside of the glass covers, for appearance’ sake alone, it invariably chips. Where, however, oil is used as a preservative liquid, it serves very well to attach the thin glass; but when this is accomplished, another varnish less liable to “chip” must always be laid upon it. (See [Chapter IV.])
Black Japan.—This is prepared from oil of turpentine, linseed oil, amber, gum anime, and asphalt. It is troublesome to make, and therefore it is much better to procure it at the shops. It is a really good cement, and serves very well to make shallow cells for liquids, as will be described in [Chapter IV.] The finished cell should be exposed for a short time to the heat of what is usually termed a “cool oven.” This renders it very durable, and many very careful manipulators make use of it for their preparations.
Electrical Cement.—This will be found very good for some purposes hereinafter described. To make it, melt together—