10 ON SECTION CUTTING. 
harden it: this may be effected by freezing, but the more usual 
plan is by means of reagents. The general action of these 
hardening reagents is to coagulate the protoplasm of the tissues ; 
and the objects to be attained are to effect this coagulation 
quickly, before the tissues can undergo any alteration ; and 
thoroughly, ¢.e., throughout the whole thickness of the object to 
be hardened. ‘To ensure the latter result it is always advisable 
to use small pieces of the substance to be cut. 
The hardening reagents in most common use are : 
1. Osmic Acid. For this purpose a 1 per cent. solution in 
water is used: it acts almost instantaneously, and so allows no 
change to occur in the tissues ; it has also the merit of staining 
the tissues as well as hardening them. It can, however, only 
be employed when the specimens are very small, as it hardens 
the surface layers so rapidly that it is unable to penetrate 
beyond a very slight depth. A few minutes immersion is 
usually sufficient. 
2. Corrosive Sublimate. A saturated solution in water is 
employed, in which the object is placed for half an hour or 
more. After removal it is thoroughly washed with water or 
weak alcohol, and then transferred to 70 per cent. alcohol 
before staining. 
3. Chromic Acid, A 0°25 to 0°5 per cent. solution of chromic 
acid in water is a useful hardening reagent ; it acts much more 
slowly than osmic acid, but is thereby enabled to penetrate to 
greater depths. Specimens should usually be left in the solution 
for 24 hours. 
4, A Mixture of chromic acid with a few drops of osmic 
acid is often very useful, as it combines the advantages of both 
reagents. 
‘5, Picric Acid. A very valuable hardening reagent, of which 
the best preparation is Kleinenberg’s. Specimens should be 
left in it from 12 to 24 hours. It is prepared thus: with 100 
ec. of water make a cold saturated solution of picric acid: add 
2 cc. of concentrated sulphuric acid: filter, and add to the 
filtrate three times its volume of water. 
6. Absolute Alcohol. Often a useful hardening reagent. 
B. Dehydration. 
Specimens that have been hardened in any of the preceding 
reagents, with the exception of the last, should, on removal, 
