44. PREPARATION AND MOUNTING 
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. This will be most readily done by 
baking it in what we should call a “cool oven.” The time 
required will most likely be 20 or 30 hours. Care must be 
taken that the heat is not too great, else the balsam will be 
discoloured. It must then be dissolved in pure chloroform 
or benzole (the latter is preferable) until it becomes of the 
consistence of thick varnish. This liquid is very convenient 
in some cases, as air-bubbles are much more easily dispelled 
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. Cloudiness is most frequently, if not always, 
caused by dampness in the object, as mentioned in Chapter 
IV. Should it, however, become so, a little heat will gene- 
rally 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 by no means 
perfect. (See also Chapter IV.) Its cheapness renders it 
no extravagance to use it always undiluted; and when pre- 
served 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. Canada bal- 
sam may now be procured in collapsible tin tubes, like those 
used by artists; and its manipulation is thus rendered much 
more easy, cleanly, and convenient, as well as economical. 
Chloroform is, however, frequently used for dilution, and is 
perhaps the safest solvent we can employ. 
Dammar VarnisH.—Some complain that this varnish is 
not easily procurable in a pure transparent state. It is 
often used by our American friends in mounting diatoms 
