INTRODUCTORY                          3
life, but is fixed in death in the attitude it normally had
during life; and second, the hardening of it to such a degree
as may enable it to resist without further change of form the
action of the reagents with which it may subsequently be
treated. Too much stress can hardly be laid on this point,
which is the most distinctive feature of modern histological
practice; without good fixation it is impossible to 'get good
stains, or good sections, or preparations good in any way.
The most convenient fixing agents are picro-sulphuric acid
and corrosive sublimate. The structure having been duly
treated with one of these, or with some other (for details
concerning the employment of these reagents see the chapter
on FixING AGENTS), is to be washed in order to remove from
the tissues as far as possible all traces of the fixing reagent.
If corrosive sublimate, or osmic acid, or a solution into which
chromic acid or a chromate enters, has been used for fixing,
the washing may be done with water. But if picric acid in
any form   has been used, the washing must be done with
alcohol. The reason of this difference is that the first-named
reagents (and indeed all the compounds of the heavy metals
used for fixing) appear to enter into a state of chemical
combination with the elements of tissues, rendering them
insoluble in water; so that the hardening induced by these
agents is not removed by subsequent treatment with water.
Picric acid, on the other hand, produces only a very slight
to   hardening of the tissues, and does not appear to enter into
any combination whatever with their elements, as it is entirely
removable by treating the tissues with water or alcohol. If
t  the removal be effected     by  means of water, the tissue
to   elements are left in a soft state in which they are obnoxious
 to all the hurtful effects of water. Alcohol must therefore
be taken to remove the picric acid, and to effect the necessary
hardening at the same time.
o At the same time that the superfluous fixing agent is being
removed from the tissues, or as soon as that is done, the