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 (i part 90% 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 subhmate 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. 



