5O HOW TO WORK 



94. Canada Ralsam, a thick viscid oleo-resin, which becomes 

 softer on the application of a gentle heat, is much employed by 

 microscopical observers : formerly it was used for cementing cells 

 together, but this is now effected more readily by the aid of marine 

 glue. If it be exposed to too high a temperature, the volatile oil is 

 expelled, and a hard brittle resin remains behind. It is chiefly 

 employed for mounting hard dense textures ; and, in consequence 

 of its great power of penetrating, and its highly refracting properties, 

 the structure of many substances, which cannot be made out by the 

 ordinary mode of examination, is rendered manifest by this medium. 

 Canada balsam should be preserved in a tin box, pi. XX, fig. 127, care 

 being taken to exclude the dust ; or in a bottle having a cap to it. 

 The balsam should be kept very clean, otherwise preparations 

 mounted in it will be spoilt in consequence of the accidental intro- 

 duction of foreign bodies. It has been frequently recommended that 

 the oldest specimens of balsam should alone be employed for micro- 

 scopical examination. By exposure to the air, the balsam becomes 

 very thick, and unfit for use : it may be thinned by the addition 

 of turpentine, ether, or chloroform. Turpentine is apt to render the 

 balsam liable to become streaky some time after the preparation 

 has been mounted, and bubbles are often found in it. 



Vessels for Keeping Canada Balsam in. The tubes, made of 

 thick tin-foil, used for artists' colours, with a small cap that 

 screws on to the top, as suggested by Mr. Suffolk, are very con- 

 venient receptacles for the preservation of Canada balsam. As 

 they contain no space for air, the balsam does not become hard and 

 unmanageable, as is too often the case when it is kept in bottles 

 or tin pots. There is no necessity for using a glass or metal rod, 

 as the quantity of balsam required can always be forced out with- 

 out the slightest difficulty. Other cements and varnishes can be 

 kept in the tin tubes also for any length of time. It is as well, 

 however, to keep them in an upright position, to prevent the cement 

 from running into the thread of the screw, and so fixing the top too 

 tightly. 



Dr. Carpenter recommends a small syringe which is easily filled 

 with the balsam when fluid. A drop can always be obtained by 

 gently warming the end of the syringe, and applying pressure to the 

 piston. By this plan the balsam is kept perfectly clean and free 

 from dust. 



!)5. Solutions of Canada Balsam. Canada balsam is soluble in 

 ether, but its best solvent is chloroform. Many very delicate 

 structures may be mounted in Canada balsam, by immersing them 

 in a chloroform solution. Sufficient chloroform is added to make 



