ON STAINING FLUIDS, AND STAINING. 119 



an indefinite time in the mixture, indeed, perma- 

 nently. The specimens must have been carefully 

 hardened and thoroughly freed, by prolonged soak- 

 ing in water, from all traces of the hardening 

 reagents, before placing them in the gum and syrup. 

 The ether freezing process is conducted thus. 

 The gum and syrup is wiped off the specimens with 

 a cloth, and they are placed in the strongest possible 

 solution of gum acacias for a few hours before being 

 frozen and cut. The microtome and freezing ap- 

 paratus having been prepared for use, a specimen 

 is placed upon the freezing plate and painted round 

 the edges and over the surface by means of a brush 

 with the strong, pure gum-water from which it was 

 taken. It is then frozen by means of the ether 

 spray until it can be easily and smoothly cut. 

 Practice and experience will soon enable the 

 operator to stop the freezing at the right moment. 

 Should the ice, however, become too hard, and 

 therefore brittle, a few seconds' delay before com- 

 mencing to cut the sections will ensure its being 

 in the right condition for cutting. 



On Section Cutting and Microtomes. 



Section cutting, like staining, is an art requiring 

 not only practice and experience, but knowledge 

 and judgment. Thorough knowledge of the struc- 

 ture and functions of the organisms, organs and 

 specimens to be sectionised is equally necessary with 

 the judgment enabling one to decide as to the 

 thickness or thinness of the sections of, say, an 

 organ which shall most satisfactorily display its 

 structure. It is quite possible to cut sections too 



