WITH THE MICROSCOPE. 185 



which silica is present the section must be made as described in 

 p. 179. If very brittle, one surface of a thick section may be 

 ground and polished. This is then to be cemented to the glass slide 

 with Canada balsam, and the opposite side ground until sufficiently 

 thin, when it may be polished in the usual way, wetted with balsam, 

 and covered with thin glass. 



Thin chips of flint and other siliceous structures often answer as 

 well as thin sections ground and polished with a great expenditure 

 of labour. 



The structure of many fossils, the mineral matter of which con- 

 sists of carbonates or phosphates, may be investigated after the salts 

 have been dissolved out, or only softened by being soaked for some 

 time in hydrochloric acid diluted with water or mixed with glycerine. 

 Dr. Carpenter gives the following directions for demonstrating the 

 structure of Eozoon Canadense : - - " The minute structure of 

 Eozoon may be determined by the microscopic examination either 

 of thin transparent sections, or of portions which have been 

 subjected to the action of dilute acids, so as to remove the cal- 

 careous shell, leaving only the internal casts, or models, in silex, of 

 the chambers and other cavities, originally occupied by the sub- 

 stance of one animal." 



272. Of Preparing Specimens of Coal for Microscopical Exami- 

 nation. Coal is one of the most difficult substances to- cut into 

 thin sections. It is so opaque that no structure can be discerned 

 unless it is ground exceedingly thin, and so brittle that it often 

 breaks up in the operation of grinding. It is said that coal may be 

 softened by maceration in a solution of carbonate of potash,, when 

 sections may be cut with a razor. The sections are partially 

 decolorised by being gently heated for a short time in strong nitric 

 acid. When of a brown colour they are to be washed in cold water 

 and preserved in glycerine (Micrographic Dictionary). Cannel eoal 

 being less brittle than ordinary coal is more easily prepared. 



THE WORK TABLE OF MAKING AND RECORDING OBSERVATIONS- 

 FALLACIES TO BE GUARDED AGAINST, 



213. The Work Table. Although beautiful work tables furnished 

 with every possible requirement have been designed for microscopists, 

 I think the student will find that an ordinary table which is firm 

 and steady is all that he really requires. It should, however, be 

 well made and provided with a few drawers in which the student 

 can place partitions for himself and arrange according to his 



