ON STAINING FLUIDS, AND STAINING. 99 



tissues or specimens for temporary observation and 

 examination only. 



Canada Balsam. The most universally used 

 and useful of all mounting media is the oleo-resin 

 obtained from Abies Balsamea and Pimis Canadensis 

 and is mostly imported from Quebec. The balsam 

 is obtained from the tree by puncturing the blisters, 

 or vesicles, which form under the bark of the trunk 

 or its branches, and collecting their fluid-contents 

 in a bottle. It is at first opaque, but gradually 

 clarifies and becomes transparent. It consists of 

 24 per cent, of volatile oil, 60 per cent, of resins 

 soluble in boiling alcohol, and of 16 per cent, of 

 resins soluble in ether, but insoluble in alcohol. 



As it hardens very slowly, it is, for histological 

 purposes and for use as a mounting medium, 

 generally evaporated to dryness and re-dissolved 

 in either benzol or xylol, preferably in the latter. 

 These solutions harden rapidly, are perfectly 

 transparent, practically permanent and preservative. 

 For some exceptional purposes, e.g., for specimens 

 which require to be mounted in deep cells, pure, 

 undiluted balsam will be found useful. 



In order to prepare Canada Balsam for histo- 

 logical purposes it should be gently heated over a 

 sand bath and thus slowly evaporated until it be- 

 comes quite hard. Great care must be taken that 

 the heat does not become too great so that the 

 balsam is burned, in which case it becomes of a 

 dark brown colour and is consequently spoiled. 

 When quite dry and brittle it is to be reduced to a 

 coarse powder and re-dissolved in an equal quantity 

 of one or other of the solvents above named. For 

 delicate entomological and some vegetable prepara- 



