1893,] Microscopy. ` 175 
MICROSCOPY 
On the Restoration of Osmic Acid Solutions.—The 
osmic acid so valuable to the histologist as a fixing reagent, is 
known to the chemist as osmium tetroxide, Os O,, and is one of the 
series of osmium compounds with oxygen. It is readily reduced in 
the presence of an organic substance, giving up two atoms of oxygen 
and forming the deutoxide, Os O,. This is the well-known black pow- 
der so familiar in specimens killed in any of the fluids into which osmice 
acid enters. Osmium tetroxide affects fatty substances first, and these 
are the substances that most readily undergo oxidation. Prolonged 
exposure to the action of the tetroxide blackens the tissue so that it is 
useless for histological purposes. 
In preparing stock solutions of osium tetroxide the greatest care 
must be taken to exclude organic matter, even acetic acid in the quan- 
tity called for in Flemming’s stronger fluid, is liable to cause the 
reduction of the tetroxide. A trace of impurity in the distilled water 
sooner or later produces the fatal blackening, while a solution made 
with pure water will keep indefinitely in the light. Let but a trace 
of dust fall into the bottle and reduction will take place to some 
degree. 
During the summer of 1892, at the Marine Biological Laboratory 
at Woods Holl, Mass., a large number of solutions were treated with 
peroxide of hydrogen, H, O,, and different investigators tested them 
with uniformly good results. The restored solutions acted in the same 
manner as the fresh solutions, and produced the same results ; the only 
noticeable difference was a gradual weakening of the solution, as 
would naturally be expected. Specimens overblackened by the action 
of the peroxide were rendered colorless without perceptibly changing 
the character of the tissue, and a few experiments were made to deter- 
mine whether the bleaching with peroxide could be so controlled as to 
leave certain tissues blackened while others were cleared. This quan- 
titative bleaching with peroxide yielded no very good results in the 
few experiments made, but enough was accomplished to warrant further 
attem 
cacy of the restoration and the bleaching further demon- 
strates that no injurious factor enters into the result as is shown by the 
following equations : 
This department is edited by C. O. Whitman, University of Chicago. 
