CLEARING AGENTS 507 



A variety of reagents with widely different chemical properties are 

 used. Glycerin and acetate of potash are commonly used for frozen 

 sections or teased preparations which, for any reason, cannot be mounted 

 in damar. 



The choice of a clearing agent for damar mounts depends chiefly on 

 two factors, the kind of stain employed and the imbedding medium. 



Xylol. This is the best clearing agent for use after aniline dyes. It 

 clears only from absolute alcohol, through which celloidin sections cannot 

 be passed, since it dissolves celloidin. However, it can be used for cel- 

 loidin or other sections dehydrated in 95 per cent, alcohol by the follow- 

 ing method. Blot the section on the slide with smooth, soft filter paper 

 and pour on a few drops of xylol. Repeat the blotting, followed by xylol 

 two or three times and the section will be perfectly clear. Paraffin sec- 

 tions attached to the slides are cleared by immersion in a vial of xylol; 

 this has already been mentioned as the last step in the staining processes. 



Oleum Origani Cretici. This clears readily from 95 per cent, alcohol 

 without dissolving celloidin and affects aniline colors slowly. Although 

 particularly recommended for clearing after celloidin imbedding, it is 

 useful for all kinds of sections. 



Carbol-xylol. 



Carbolic acid crystals i 



Xylol 3 



Used for clearing thick sections of the central nervous system after 

 carmine and haematoxylin stains. Clears from 95 per cent, alcohol with- 

 out affecting celloidin, but extracts the basic aniline dyes. 



Chloroform. Since osmic acid reduced by fat is soluble in xylol, 

 chloroform is used in cases where permanent mounts of such preparations 

 are desired (as after fixation in Marchi's fluid). 



MOUNTING. 



Frozen sections which cannot be mounted in damar are mounted 

 in glycerin, potassium acetate or glycerin jelly. 



Glycerin Jelly. Soak 7 gm. of gelatin for about 2 hours in 42 c.c. 

 of distilled water. Add 50 gm. of glycerin. Warm, stirring constantly 

 for icr-i5 minutes. Filter hot through moistened cotton. 



Bring sections on a slide and blot off excess water. Put on a small 

 piece of glycerin jelly and warm gently until it melts. Cover and cool. 



Preparations mounted in any of these three substances may be 

 rendered more or less permanent by coating the edges of the cover and 

 adjacent surfaces of the slide with paraffin or wax. 



Gum Damar. Of the two substances most commonly employed for 

 permanent mounts, namely, Canada balsam and damar, the latter is 

 preferable, since balsam turns yellow with age. 



