192 SECTIONS OF HARD SUBSTANCES. 



account of their extreme friability ; the vibration produced by the 

 working of the saw or the file, or by grinding on a rough surface, 

 being sufficient to disintegrate even a thick mass, so that it falls 

 to pieces under the hand ; such specimens, therefore, it is requisite 

 to treat with great caution, dividing them by the smooth action of 

 the wheel, and then rubbing them down upon nothing rougher 

 than a very fine ' grit.' Where (as often happens) such specimens 

 are sufficiently porous to admit of the penetration of Canada 

 Balsam, it will be desirable, after soaking them in turpentine for 

 a while, to lay some liquid balsam upon the parts through which 

 the section is to pass, and then to place the specimen before the 

 fire or in an oven for some little time, so as first to cause the 

 balsam to run-in, and then to harden it ; by this means the speci- 

 men will be rendered much more fit for the processes it has after- 

 wards to undergo. — It not unfrequently happens that the small 

 size, awkward shape, or extreme hardness of the body, occasions a 

 difficulty in holding it either for cutting or grinding ; in such a 

 case, it is much better to attach it to the glass in the first instance 

 by any side that happens to be flattest, and then to rub it down 

 by means of the ' hold ' of the glass upon it, until the projecting 

 portion has been brought to a plane, and has been prepared for 

 permanent attachment to the glass. This is the method which it 

 is generally most convenient to pursue with regard to small bodies ; 

 and there are many which can scarcely be treated in any other way 

 than by attaching a number of them to the glass at once, in such a 

 manner as to make them mutually support one another. * 



139. The mode in which the operation is then to be proceeded 

 with, depends upon whether the section is to be ultimately set up 

 in Canada balsam (§ 157), or is to be mounted dry (§ 154), or in 

 fluid (§ 164). In the former case, the following is the plan to be 

 pursued : — The flattened surface is to be polished by rubbing it 



* Thus, in making horizontal and vertical sections of Foraminifera, as 

 it would be impossible to slice them through, they must be laid close 

 together in a bed of hardened Canada balsam on a slip of glass, in such 

 positions, that, when rubbed down, the plane of section shall traverse 

 them in the desired directions ; and one flat surface having been thus 

 obtained for each, this must be turned downwards, and the other side 

 ground away. The following ingenious plan has been suggested by Dr. 

 Wallich ("Ann. of Nat. Hist." July, 1861, p. 58), for turning a number 

 of minute objects together, and thus avoiding the tediousness and diffi- 

 culty of turning each one separately : — The specimens are cemented 

 with Canada Balsam, in the first instance, to a thin film of mica, which 

 is then attached to a glass slide by the same means ; when they have 

 been ground down as far as may be desired, the slide is gradually heated 

 just sufficiently to allow of the detachment of the mica-film and the 

 specimens it carries ; and a clean slide with a thin layer of hardened 

 balsam having been prepared, the mica-film is transferred to it with 

 the ground surface downwards. When its adhesion is complete, the 

 grinding may be proceeded with; and as the mica-film will be found to 

 yield to the stone without the least difficulty, the specimens, now 

 reversed in position, may be reduced to any degree of thinness that may 

 be found desirable. 



