CHAPTER VII 

 DE-ALCOHOLISATION AND CLEARING AGENTS 



133. Introduction. De-alcoholisation agents are liquids em- 

 ployed for the purpose of getting rid of the alcohol which has been 

 employed for dehydrating tissues (§ 12-t), and facilitating the pene- 

 tration of the paraffin used for imbedding, 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 functions it is to make micro- 

 scopic preparations transparent by penetrating amongst the highly 

 refracting elements of which the tissues are composed, 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 of 

 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 of the resins, and not all de- 

 alcoholising agents can serve as clearers. We shall, however 

 still in many cases continue to use the term " clearing " to signify 

 " de-alcoholising," for the sake of brevity. 



Neelsex and Schiefferdecker {Arch. Anal. Phys., 1882, 

 p. 206) examined a large series of ethereal oils (prepared by 

 Schimmel & Co., Leipzig), with the object of finding a not too 

 expensive substance that should combine the properties of clearing 

 quickly alcohol preparations, not dissolving out anilin colours, 

 clearing celloidin without dissolving it, and not evaporating too 

 quickly. 



Of these, the following three fulfil the conditions : Cedar-wood, 

 Origanum, Sandal-ivood. 



To these should be added the others recommended in the 

 following paragraphs. 



See also the paper of Jordan {Zeit. zviss. Mik., xv, 1898, p. 50) 

 as to the behaviour of some essential oils towards celloidin. 



67 3—2 



