SECTIONS OF HAKD SUBSTANCES. 191 



with a fixed index ; so that this amount having been once deter- 

 mined, the screw shall be so turned as to always produce the exact 

 elevation required. — Where the substance of which it is desired to 

 obtain sections by this instrument is of too small a size or of too 

 soft a texture to be held firmly in the manner just described, it 

 may be placed between the two vertical halves of a cork of suitable 

 size to be pressed into the cylinder ; and the cork, with the object 

 it grasps, is then to be sliced in the manner already described, the 

 small section of the latter being carefully taken-off the knife, or 

 floated-away from it, on each occasion, to prevent it from being lost 

 among the lamellse of cork which are removed at the same time. — 

 The special methods of preparation which are required in the case of 

 the various substances of which sections may be conveniently cut 

 by this instrument, will be noticed under their several heads. 



138. 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 hard Vegetable Tissues, — can only be reduced 

 to the requisite thinness for Microscopical examination, by grind- 

 ing-down thick sections until they become so thin as to be tran- 

 sparent. General directions for making such preparations will 

 be here given ;* but those special details of management which 

 particular substances may require, will be given when these sub- 

 stances 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 pcrcellanous 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 disk of soft iron (resem- 

 bling that used by the lapidary) charged at its edge with diamond- 

 dust, which disk may be driven in an ordinary lathe. Where waste 

 of material is of no account, a very expeditious method of obtain- 

 ing 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 ' cutting-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 



* 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. 



