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 ma;t 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 



