xxvm INTEODUCTION 



by impregnating it with some homogeneous substance which 

 is firm enough without being brittle or too hard. The 

 freezing method is the simplest of all, but it is not easily applic- 

 able when a large number of sections are required mounted 

 in consecutive series, nor when very thin sections are re- 

 quired. The tissue to be cut is first soaked in gum-water, 

 then placed on the freezing-plate of the microtome and cooled 

 by an ether-spray or a freezing mixture underneath. The 

 frozen mass is cut when just thawing. 



The imbedding method is more complicated but has many 

 advantages. The specimen, which may be stained previously, 

 is dehydrated by soaking in absolute alcohol ; the alcohol is 

 then removed by soaking in turpentine, xylol, oil of cloves or 

 some similar substance, and the object is then transferred to 

 paraffin kept just melted in a water-oven, the temperature not 

 being allowed to rise above 60° C. Paraffin meltingat 55° or 

 56° C is the best, though a softer paraffin may be used in a 

 cold room, if it be found that the hard paraffin crumbles before 

 the razor. When the object is thoroughly permeated, it is 

 transferred to a small trough of . paper with just enough 

 paraffin to form on cooling a block completely enclosing it. 

 It must be cooled as quickly as possible by placing the trough 

 in a dish containing cold water, as the paraffin is Hable to 

 crystallise if allowed to cool slowly. A couple of L-sbaped 

 blocks of metal, resting on a plate of glass or metal so as to 

 form a bos of adjustable size, may be used in place of the 

 paper trough. The block containing the object is fixed in 

 the microtome ; the most efficient of aU contrivances for this 

 purpose is, perhaps, a grooved metal plate which is simply 

 warmed slightly and pressed against the end of the block, and 

 then cooled quickly as soon as the block has begun to melt. 

 The further procedure depends entirely upon the particular 

 microtome used. The sections are to be mounted as described 

 in section IX. 



IX. ON MOUNTING MICBOSCOPICAL OBJECTS. 



The most convenient slide for microscopical objects is a 

 slip of glass three inches long and one inch wide, or for large 



