CHAPTER VI. 



I'K-ALCOIIOLISATION AND CLEARING AGENTS. 



118. Introduction. De-alcoholisation agents are liquids 

 employed for the purpose of getting rid of the alcohol 

 \\liich has been employed for dehydrating tissues ( 3), and 

 facilitating the penetration of the paraffin used for imbed- 

 ding, or the balsam or other resinous medium in which 

 preparations are, in most cases, finally mounted. Hence 

 all of them must be capable of expelling alcohol from 

 tissues, and must be at the same time solvents of Canada 

 balsam and the other resinous mounting media. The 

 majority of them are essential oils. 



Clearing agents are liquids whose function it is to make 

 microscopic preparations transparent by penetrating amongst 

 the highly refracting elements of which the tissues are com- 

 posed, the clearing liquids themselves having an index of 

 refraction superior, or equal, or, at all events, not greatly 

 inferior to that of the tissues to be cleared. Hence all 

 clearing agents are liquids of high index of refraction. 



The majority of de-alcoholisation agents being also liquids 

 <>f high refraction, it follows that they serve at the same time 

 for de-alcoholisation and for clearing; and in consequence 

 it has come about that de-alcoholisation agents are generally 

 spoken of as clearing agents. But that practice is not 

 strictly correct, for not all clearing agents are solvents 

 ol t)u- resins, and not all de-alcoholising agents can serve 

 as clearers. I shall, however, still in many cases continue 

 to use the term " clearing ; to signify " de-alcoholising," for 

 the sake of brevity. 



NEKLSKN find SCHIEFFERDECKEE (ArcJi. Anat. Phys., 1882, 

 ]>. 206) examined a large series of ethereal oils (prepared 

 by Schimmel and Co., Leipzig), with the object of finding a 

 not too expensive substance that should combine the proper- 



