44 ELEMENTS OF APPLIED MICROSCOPY. 
since this substance sets like a rock and has a high index 
of refraction, which makes opaque objects very distinct. 
It is commonly used dissolved in xylol, the natural fir 
balsam being mixed with an equal volume of the oil; 
the solution after filtering is concentrated to a syrupy 
consistency by evaporation. It is used like glycerin 
jelly, teing placed on the object, covered and set aside, 
when the xylol evaporates and the balsam becomes 
firmly set. 
With an object of firm texture, not containing water, 
the preparation of a balsam mount is very simple. The 
wing or leg of an insect, for example, may easily be 
mounted in this way. With softer specimens, especially 
those made up largely of water, it is necessary to resort 
to some preliminary treatment which shall harden and 
dehydrate the tissues. 
7. Fixing and Dehydration.—The first step in the prep- 
aration of a soft plant or animal tissue for balsam mount- 
ing is to treat it with some agent which shall fix the cell 
structures in exactly the condition in which they occurred 
in life, preventing the disintegrating changes which nor- 
mally follow the death of protoplasm, and which shall 
at the same time so act chemically upon the cell constitu- 
ents as to harden and protect them from the action of 
the chemical agents to be applied later. Flemming’s 
mixture is one of the best of these fixing agents; it con- 
tains .25% chromic acid, .1% osmic acid, .1% glacial 
acetic acid dissolved in water, and should be allowed to 
act for half an hour or more. A saturated solution of 
corrosive sublimate is often useful. Absolute alcohol 
