CHAPTER VI. 



DE-ALCOHOLISATION AND CLEARING AGENTS. 



118. Introduction. De-alcpholisation agents are liquids employed 

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

 for dehydrating tissues ( 3), and facilitating the penetration 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 function 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-alcoholi- 

 sation 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. I shall, however, still in many cases continue to 

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

 of brevity. 



NEELSEN and SCHIEFFERDECKER (Arch. Anat. 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-wood. 



