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.