498 APPENDIX. 



withdrawing the latter the cover-shp will descend ob- 

 liquely expelling the enclosed air. 



F'inishcd preparations should be examined from time 

 to time, in order that loss of the mounting medium by 

 evaporation may be made good. 



5. To prepare ground sections of shells, bones, or other 

 hard parts. 



A small piece of the structure to be manipulated should 

 be first isolated and then cemented in the desired posi- 

 tion to a piece of plate glass, by means of canada balsam. 

 When firmly set it should be ground down upon a hone 

 or rough surface to the required thinness, and finally dis- 

 lodged for mounting, by immersing the whole in benzole 

 or chloroform. It may then be put up in canada balsam 

 in the manner described above. 



6. Frozen sections. 



Material for this purpose should be preserved in weak 

 glycerine. For the preparation of these sections an ether- 

 spray freezing microtome is desirable ; good preparations 

 may however be made as follows. 



Obtain a metal rod of the calibre of a candle and 2 — 3 

 inches in length : place the preparation (which, unless 

 quite fresh, should be previously soaked in gum-water) 

 on one end of it and add 6 — 8 times its bulk in ordinary 

 fluid gum. Freeze with pounded ice and salt. Cut. 



INIount in weak glycerine. 



