GRINDING AND POLISHING SECTIONS 507 



details of management which particular substances may require will 

 be given when these are respectively described. The first thing to 

 be done will usually be to procure a section of the substance, as thin 

 as it can be safely cut. Most substances not siliceous may be divided 

 by the fine saws used by artisans for cutting brass; and these may 

 be best worked either by a mechanical arrangement such as that 

 devised by Dr. Matthews, 1 or. if by hand, between 'guides.' such as 

 are attached for this purpose to Hailes's and some other microtomes. 

 But there are some bodies (such as the enamel of teeth, and porcel- 

 lanous shells) which, though merely calcareous, are so hard as to 

 make it very difficult and tedious to divide them in this mode ; and 

 it is much the quicker operation to slit them with a disc of soft iron 

 (resembling that used by the lapidary) charged at its edge with 

 diamond dust, which disc may be driven in an ordinary lathe. Where 

 waste of material is of no account, a very expeditious method of 

 obtaining pieces fit to grind down is to detach them from the mass 

 with a strong pair of 'cutting pincers,' or, if they be of small 

 dimensions, with 'cutting pliers ; ' and a flat surface must then be 

 given to it, either by holding them to the side of an ordinary grind- 

 stone, or by rubbing on a plate of lead (cast or planed to a perfect 

 level) charged with emery, or by a strong-toothed file, the former 

 being the most suitable for the hardest substances, the latter for the 

 toughest. There are certain substances, especially calcareous fossils 

 of wood, bone, and teeth, in which the greatest care is required in 

 the performance of these preliminary operations, on 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,' or 

 on the ' corundum files ' now sold in the tool shops, which are made 

 by imbedding corundum of various degrees of fineness in a hard, 

 resinous substance. 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 specimen will be rendered 

 much more fit for the processes it has afterwards 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 

 1 Joiirn. Qiiekett Microsc. Club, vol. vi. LSSO, p. 88. 



