MOUNTING OF PERMANENT PREPARATIONS. I47 



the former, however, are to be preferred, as they are 

 more easily sealed up, and, besides, they look nicer. 



Canada balsam takes a long time to dry ; in winter 

 and in damp weather it may take months before the 

 outer layer of balsam becomes quite hard like glass. 

 Only then can it be covered over with a layer of 

 varnish which serves both to protect the balsam from 

 the air, and also from the oil, in which the preparation 

 later on may be immersed, and which would otherwise 

 dissolve away the balsam, and thus the preparation 

 .would be endangered. The most suitable varnish for 

 this purpose is the so-called spirit varnish. 



If after having examined some sections in water 

 we decide to preserve them, we place them in ab- 

 solute alcohol, being careful to spread them out 

 immediately on the spatula, to prevent them from 

 rolling up and becoming distorted. After having 

 been placed in fresh absolute alcohol, they are put into 

 xylol, in order to' remove the alcohol ; the superfluous 

 xylol is then dried off with blotting paper, and they 

 are placed in a drop of Canada balsam. Especial care 

 must be taken to observe that all the water was ab- 

 sorbed by the alcohol, and that this in its tarn was 

 completely removed by the xylol, otherwise the pre- 

 paration will become cloudy when it is placed in the 

 Canada balsam, and will be thus quite spoiled. If 

 the sections cannot stand immersion in alcohol 

 without becoming decolourised, it is best to press them 



