CUTTING AND GRINDING THIN SECTIONS. 207 



lost among the lamellae of cork which are removed at the 

 same time. The special methods of preparation which are re- 

 quired in the case of the various substances, of which sections 

 may be conveniently cut by this instrument, will be described 

 under their several heads. 



108. Grinding and Polishing of Sections. Substances which 

 are too hard to be sliced with a cutting instrument in the manner 

 last described, such as bones, teeth, shells, corals, fossils of all 

 kinds, and even some recent vegetable tissues, can only be re- 

 duced to the requisite thinness for Microscopical examination, 

 by grinding down thick sections, until they become so thin as 

 to be transparent. The general method of making such pre- 

 parations will be here described; 1 but those special details of 

 management which particular substances may require, will be 

 given when these substances 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 ; but there are some bodies (such as the enamel of 

 teeth, and porcellanous shells), which, though merely calcareous, 

 have their mineral particles arranged in such a peculiar state of 

 aggregation, 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 disk of soft iron (resembling that used by the lapi- 

 dary) charged at its edge with diamond-dust, which 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 it be of small dimensions, with " cut- 

 ting-pliers;" and a flat surface must then be given to it, either 

 by holding it to the side of an ordinary grindstone, or by rubbing 

 it 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." Where (as often happens) such speci- 

 mens are sufficiently porous to admit of the penetration of 

 Canada balsam, it will be desirable, after soaking them in tur- 



1 The following directions do not apply to Siliceous substances; as sections of these 

 can only be prepared by those who possess a regular Lapidary's apparatus, and who 

 have been specially instructed in the use of it. 



