FIXING AGENTS              19

10. Absolute Alcohol (as a fixing agentl).-Mayer finds
that boiling absolute alcohol is often the only means of killing
certain Arthropoda rapidly, enough to avoid maceration
brought about by the slowness of penetration of common cold
alcohol (especially in the case of Tracheata).
11. Acidulated Alcohol (Paul Mayer's formulal).-To 97
vols. of 90 per cent. alcohol, in which is dissolved a small
quantity of picric acid, add 3 vols. pure hydrochloric acid.
Leave the specimens in the mixture only just long enough to
ensure that they are thoroughly penetrated by it. Wash out
with 90 per cent. alcohol, the disappearance of the yellow
stain of the picric acid being a sign that all the acid is
removed.
The use of this mixture is for the preparation of coarse
objects it is intended to preserve in alcohol. The object of
the acid is to prevent both that glueing together of organs by
the perivisceral liquid, which is often brought about by the
coagulating action of pure alcohol, and the precipitation on
the surface of organs of the salts contained in sea-water,
which is a hindrance not only to the penetration of the
alcohol, but also to subsequent staining.
Whitman 2 states that " acid alcohol as above prepared
loses, its original qualities after standing some time, as ether
compounds are gradually formed at the expense of the acid."
He also states that 70 per cent. alcohol may be taken instead
of 90 per cent., for washing out.
12. Osmic Acid.-Osimic acid is best employed in the form
of vapour, and its employment in this form is indicated in all
cases in which it is possible to expose the tissues directly to
the action of the vapour. The tissues are pinned out on a
cork which must fit well into a wide-mouthed bottle in which
is contained a little solid osmic acid (or a small quantity of
I per cent. solution will do). They remain there until they
I ' Mitth. Zool. Stat. Neapel,' ii (1881), p. 7.
2 ' Journ. Roy. Mic. Soc.' (N.S.), ii (1882), p. 870.