MAKING PREPARATIONS 5 



them as nearly as possible in the condition in which 

 they are during life. For this purpose one or other of 

 the fluids mentioned below may be used. Care must be 

 taken that the objects shall be of small size, that the 

 quantity of hardening fluid is large relatively to the 

 bulk of the object, and that the fluid has ready access 

 to all parts of it. Large objects should be cut up into 

 pieces of moderate size, so that the reagent may readily 

 gain access to all parts of the tissue. 



The following are the best fluids for this purpose : 



1. Absolute alcohol or methylated spirit. 



2. Picric acid (saturated solution in water). 



3. Chromic acid (O'l 0*5 per cent, solution in 



water). 



4. Osmic acid (*1 1* per cent, solution in water). 



These reagents are only to be applied to fresh material. 



When absolute alcohol is used, the object may be 

 kept in it for an indefinite period. Such treatment 

 generally makes the object brittle ; this may be remedied 

 when the object is to be mounted in glycerine by 

 placing it, for at least twenty-four hours before it is to 

 be cut, in a mixture of glycerine and absolute alcohol 

 in equal parts, leaving it exposed to the air so that the 

 alcohol may gradually evaporate. The glycerine slowly 

 saturates the object and restores its toughness. This 

 can only be done when the sections are to be mounted 

 in glycerine. 



When picric or chromic acid is used, the object should 

 be immersed in it until each part of it is thoroughly 

 permeated by the reagent ; the length of time required 

 for this varies with different material, and in the case 



