124 NATURALISTS 7 ASSISTANT. 



(say 60 per cent.) spirit and after a short time is placed in 

 strong alcohol (90 to 95 per cent.). The object after a day 

 or two in this will be found to be much harder, and to pos- 

 sess a consistence fitting it for the use of the razor. The 

 object of the successive uses of spirit of increasing strength 

 is to prevent that contraction and distortion of the object 

 which would occur were it placed at once in the strongest 

 alcohol. 



Muller's fluid is also extensively used for hardening objects, 

 as are also chromic acid and Kleinenberg's picric acid. The 

 modus operandi is essentially the same with either. The 

 specimen is placed in a large quantity of the solution (Mul- 

 ler's fluid as directed on p. 138, or chromic acid 1 and per 

 cent.) and after a day or two, is transferred to alcohol. These 

 solutions must not be too strong nor must the specimens be 

 kept too long in them, else they will become so brittle as to 

 crumble under the section knife, rendering it impossible to ob- 

 tain thin sections. A little experience will enable one to esti- 

 mate the proper time for various tissues. 



Osmic acid (one to one-tenth per cent, solution) is also 

 very useful for hardening and at the same time it stains the 

 section more or less darkly from a gray to a black. As noted 

 on another page, it is selective in its staining, affecting nerves 

 and fatty tissues more strongly than other tissues. The object 

 is placed in the solution a varying number of hours according 

 to the tissue, and then is washed thoroughly with distilled water 

 and transferred to alcohol. 



Other methods advocated by some students consist of the 

 use of bichromate of potash, and among the older workers 



