THe Microscope. 81 
An abundance of the hardening fluid should be used, at 
least twenty or thirty times the bulk of the tissue. If any for- 
eign matters are to be removed, wash the tissue carefully with 
a stream of saline solution. 
It should be borne in mind that tissues may be over-hard- 
ened as well as not sufficiently hardened. When over-hardened 
they will be brittle and crumble; they may be so hardened that 
they are spoiled for cutting and staining. If they are not hard 
enough they feel elastic like rubber, when slightly pressed be- 
tween the thumb and forefinger. When tissue has to be 
transferred from one hardening reagent to another it should be 
removed from the first when it is still in this tough and elastic 
condition. In changing a tissue to complete its hardening in 
alcohol all the first hardening medium should be washed away 
by soaking the tissue in water, frequently changing the water 
until it is no more colored by the hardening reagent. 
Tissues are hardened by coagulating the albumen and ex- 
tracting the water, hence hardening causes shrinking, It is 
desirable that this shrinking take place as little as possible, at 
the same time it should be gradual that it may be uniform. It 
is a general rule, then, in using all hardening reagents that 
cause marked shrinking of the tissue, to commence with weaker 
and advance to stronger solutions. 
It is very important to have each bottle with its contents, 
labelled with a letter or number, with a corresponding mark in 
a note book. The note book should tell the kind of animal, the 
tissue, date of injecting, of changing or substituting fluids, and 
any facts of special interest. 
HARDENING REAGENTS. 
RotassiceBichromate=e2=--——- === = 2 parts. 
Bouieisulphape: 2 - 2s 1 part. +} By weight. 
Mo DUR he See eee oe SC oe 100 parts. 
This fluid has great penetrating power, and is admirably 
adapted for hardening and preserving nearly all the tissues of 
the body. It slowly coagulates the albumen without extracting 
the water and causes but slight shrinking. It is a cheap mix- 
ture and specimens may be preserved in it for an indefinite 
time without losing any of their characteristics. 
