124 NATURALISTS’ ASSISTANT. 
(say 60 per cent.) spirit and after a short time is placed in 
strong alcohol (go 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. 
Miuller’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 } 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 
