OSMIC ACID.                        99
Tissues may be immersed in a solution of one-third glycerin,
or a mixture of glycerin 1 part, alcohol 1 part, water 1 to 3
parts, to which about 1 per cent. of a 1 per cent. solution of
osmic acid has been added and left there for several days
until sufficiently stained.
Osmic-acid staining. Subsequent treatment of the tissues.-
In order to prevent the blackening with time that generally
happens to tissues that have been treated with osmic acid,
several methods are available. The best is probably bleach-
ing by free chlorine (Mayer's method, No. 476), or the speci-
mens may be treated with ammonia carmine or picro-carmine,
or they may be put for some hours into solution of bicbro-
mate of potash or chromic acid. This is a very good plan.
Whitman recommends treatment for twenty-four hours with
Merkel's solution (No. 17), which actually bleaches the tissues
if blackened ('Journ. Roy. Mic. Soc.,' (N.S.) iii, p. 912). I
find the method gives excellent results.
The best carmine stain for osmium objects is, I think, alum-
carmine. Borax-carmine or hematoxylin may also be used.
Picro-carmine is generally recommended, but frequently
results in a very dirty stain.
122. Osmic and Chromic Acid Staining Combination (Max
Flesch's formula)
Osmium       .    .    .     .      0-10 parts.
Chromic acid      .    .     .      0-25
Water .      .    .     .    .    100.00
For twenty-four to thirty-six hours. Is a good fixing and
hardening agent. It gives a general stain of very little pre-
cision, and is certainly notto be recommended as a stain. The
same is the case with Flemming's chromo-aceto-osmic mixture,
No. 14.
123. Bichromate of Potash and Osmium.-By adding a
little osmium and about one-fourth of glycerin to Miller's
solution, a good hardening fluid is obtained, which gives, I
think, a better stain than Max Flesch's formula. A still