196 THE MICROSCOPE AJSfD ITS REVELATIONS. 



191. "When the freezing process is employed, the substance to be cut 

 (which may either be fresh, or have been hardened by some of the 

 processes to be hereafter described, 199) must be thoroughly penetrated 

 by a thick solution of gum; for this, when frozen, does not become 

 crystalline, and may be cut like cheese. If the substance to be cut has 

 been immersed in alcohol, this must be completely removed in the first 

 instance by immersion in water for from six to twenty-four hours, 

 according to the size of the mass; for the gum will not penetrate any part 

 which is still alcoholized. And the substance should be then immersed 

 in the gum-solution for from twelve to twenty-four hours before it is 

 frozen; in order that every part may be permeated by the gum, and no 

 water be left to form crystals of ice. If the freezing Microtome of 

 Prof. Kutherford 1 be employed, the freezing-box should be filled with 

 alternate spoonsful of salt and either snow or finely powdered ice, which 

 are to be stirred round the well previously filled with the gum solution. 

 With the Ether-spray Microtome, the freezing is produced by the rapid 

 evaporation of the liquid injected into the freezing chamber. In either 

 case, the substance to be cut is to be introduced into the well, as soon as 

 the gum begins to harden at its periphery; and should be held in place 

 until fixed by the advancing congelation. In cutting the sections, no 

 wetting of the knife is necessary; as it is kept sufficiently wetted by the 

 thawing gum. The sections should be placed in methylated spirit diluted 

 with twice its volume of water; and this soon not only dissolves out the 

 gum, but removes any air-bubbles the section may contain. If the section 

 is to be at once mounted (which should always be done if it is very 

 delicate and liable to be spoiled by manipulation) it should be placed on 

 a slide before it has thawed, and washed by forming around ifc a little 

 pool of dilute spirit, which may be readily changed two or three times 

 by the glass syringe ( 127). Sections cut by the freezing process may 

 for the most part be mounted in glycerine jelly, for which no other 

 preparation will be needed than the use (if desired) of the Staining 

 process hereafter to be described ( 202). But if, for the sake of 

 rendering the sections more transparent, mounting them in Canada 

 balsam or Dammar is preferred, they must be treated first with strong 

 spirit, then with absolute alcohol, and then with either oil of cloves or 

 oil of turpentine. It is claimed by Dr. Rutherford as the special advan- 

 tage of the freezing process, that " delicate organs, such as the retina, 

 the embryo, villi of the intestines, lung, trachea with its ciliated epithe- 

 lium, may all be readily cut without fear of their being destroyed by the 

 imbedding agent." When imbedded in paraffine, very delicate structures 

 are more liable to damage; the villi of the intestine, for instance, being 

 often denuded of their epithelium, and sometimes themselves torn. 



192. Grinding and Polishing Sections of Hard Substances. Sub- 

 stances which are too hard to be sliced in a Microtome such as Bones, 

 Teeth, Shells, Corals, Fossils of all kinds, and even some dense Vegetable 

 Tissues can only be reduced to the requisite thinness for Microscopical 

 examination, by grinding-down thick sections until they become so thin 

 as to be transparent. General directions for making such preparations 



1 This instrument has received various improvements since it was first devised, 

 and should be obtained from Mr. Gardner, South Bridge, Edinburgh, the maker 

 recommended by its inventor. It may be employed also as an ordinary ' imbed- 

 ding' Microtome, when the 'imbedding' is thought preferable to the freezing 

 process. 



