MOUNTING AND PREPARATION OF OBJECTS. 45 
may be applied to the fixation of many tissues, its action 
being so rapid as to forestall the bad effect of the violent 
dehydration which it sets up. At the other extreme, 
dilute alcohol (1 part go% alcohol in 2 of water) pro- 
duces a moderate fixation without harmful osmotic 
changes. . 
After fixing with weak alcohol, or. after washing out 
Flemming’s fluid or corrosive sublimate with water, the 
next step is to dehydrate, which can best be done by 
treatment with alcohol of increasing strength. After 
the specimen has been thoroughly permeated with 30% 
alcohol, it should be transferred to a 50% solution, then 
to 70%, 90%, and 95%, successively. Thus the water 
is removed so gradually that the diffusion currents set 
up are not sufficiently violent to distort the tissue. The 
period of immersion in each grade of alcohol will vary 
with the thickness of the specimen. For sections or 
minute objects three to five minutes will suffice. 
The last step before mounting an object in balsam is 
to treat it with a clearing agent, that is, with some liquid 
of high refractive index which will penetrate its tissues 
and make them clear, just as glycerin makes starch- 
grains more transparent than when mounted in air. 
Since the object is already in alcohol and is to be mounted 
in balsam, it is obvious that the clearing agent should be 
miscible with both substances. Cedar-oil, clove-oil, and 
xylol are perhaps the commonest clearing agents. After 
being treated with any one of them until it is thoroughly 
transparent, the object may be placed in‘ balsam and 
mounted. 
