MOUNTINO OBJECTS IN CANADA BALSAM. 229 



pressure into the place which they occupied. Some objects, 

 however, retain the air with such tenacity, as to require the re- 

 petition of the exhausting process two or three times : and in 

 this case it is preferable to use camphine or oil of turpentine in- 

 stead of balsam, on account of its greater fluidity, and to warm 

 even this to a temperature of about 100°. There are certain 

 cases, on the other hand, in which it is desirable to retain, instead 

 of expelling, the air contained within the cavities of the object. 

 Thus, if minute insects ^such as Fleas) be displayed as transparent 

 objects to show the ramifications of the tracheae, or if it be wished 

 that a section of Tooth or Bone should be so mounted in balsam 

 as to exhibit its canaliculi, the previous maceration in oil of tur- 

 pentine should be never employed, and the balsam employed 

 should be some which has been previously hardened ; this being 

 melted without the use of more heat than is necessary, the object 

 should be surrounded by it, and the cover put on as quickly as 

 possible ; and the slide should then be laid upon a surface ot 

 stone or metal, the good conducting power of which causes the 

 balsam to cool rapidly, and thus diminishes its tendency to pene- 

 trate into the substance of the object. 



129. Wben the object is already attached to the glass slide, 

 the mounting in Canada balsam is usually a matter of very little 

 difiiculty. If it be a soft tissue which has been spread out and 

 allowed to dry upon the glass for the purpose of securing it in its 

 place, all that is necessary in the first instance is to dry it tho- 

 roughly, to shave or scrape it with a sharp knife if it should seem 

 too thick, and. to moisten its surface with oil of turpentine if it 

 should not readily " take" the balsam. The slide is then very 

 gently warmed, a sufficient quantity of balsam is spread over the 

 surface of the specimen, care being taken that it "takes" it in 

 every point, and the glass cover is put on. If the preparation 

 cover a large area, great care should be taken in letting down the 

 cover gradually from one side, so as to drive a wave of balsam 

 before it which shall sweep away air-bubbles ; raising it a little, 

 and introducing a small quantity of fresh balsam, if any vacuity 

 present itself as it descends. The preferable mode, however, ot 

 mounting thin sections of hard bodies, will depend in great de- 

 gree upon the size of the section and the tenacity of its substance. 

 "Where its area is great and its texture brittle, its removal from 

 the glass on which it has been ground down, to another slip, 

 cannot be accomplished, even by the most dexterous manage' 

 ment, without considerable risk of breaking it ; and although, by 

 the friction of the glass upon the stone, the edges of this will 

 probably have been scratched or roughened, yet this is a dis-sight 

 about which the scientific Microscopist will care but little, as it 

 only afiJects the saleable value of such objects. IsTothing more 

 will in this case be necessary, than to lay some liquid Canada 

 balsam on the surface of the section, to warm it gently, and then 

 to place on it^ a thin glass cover of suitable dimensions, gently 



