COLLECTION, EXAMINATION, ETC. 323 



in the process of grinding. After the lapse of a httle 

 time, and when the bubbles of air cease to be given 

 off, the flame should be withdrawn, and the specimen 

 allowed to partially cool in the balsam. This is 

 done in order that, during the contraction of the 

 balsam in the rock, the interstitial spaces may be kept 

 completely filled with the medium ; if this is not 

 carefully attended to, the chalk will grind up readily 

 into a mud when we endeavour to produce a surface 

 upon it, and the result will be a hole in that particular 

 part of the section. As soon as the balsam begins to 

 harden, our specimen must be withdrawn and left to 

 cool down. The superficial balsam is then scraped 

 away with a penknife, and a surface produced upon 

 it with a snakestone and water. When one face 

 is sufficiently smooth, presenting an even balsam- 

 filled surface, it should be carefully dried and 

 cemented with balsam to a glass slip. The bulk of 

 the chalk may then be ground away on medium- 

 grain emery, and the finishing proceeded with as in 

 the preparation of the preliminary surface, by rubbing 

 down on a perfectly plane snakestone. As soon as 

 the section is nearing completion little spots of light 

 may appear through the section, showing the pre- 

 sence of organisms. This thinning down should be 

 continued until the matrix is also fairly translucent, 

 and then it may be mounted with fluid balsam, using 

 a very moderate heat to harden the medium. It 

 is often of great convenience to make a section of 

 chalk before proceeding to wash down a quantity for- 

 the extraction of the Foraminifera, for by this means 



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