DE-ALCOHOLISATION AND CLEARING AGENTS. 



for balsam, because it is not miscible with either of them. 

 And chloroform is an admirable de-alcoholisation agent and 

 admirable precursor of paraffin or balsam, but can hardly be 

 utilised as a clearer i. e. for the purpose of obtaining the 

 transparency required for examination on account of its 

 volatility, which precludes its use as an examination medium. 

 1 shall, however, still in many cases continue to use the term 

 *' clearing " to signify " de-alcoholising," for the sake of 

 brevity. 



Of course clearing media can serve as Examination Media, 

 if not too volatile. 



106. The Practice of De-alcoholisation or Clearing. The 

 old plan was to take the object out of the alcohol and float 

 it on the surface of the de-alcoholising or clearing medium 

 in a watch-glass. This plan was faulty, because the alcohol 

 escapes from the surface of the object into the air quicker 

 (in most instances) than the de-alcoholising or clearing agent 

 can get into it ; hence the'object must shrink. To avoid or 

 lessen this cause of shrinkage, the operation is now generally 

 done by the method suggested by Mayer and Giesbrecht, 

 which consists in putting the clearing medium under the 

 alcohol containing the object. This is done in the following 

 manner. Take a short glass tube, and put into it enough 

 alcohol to contain the objects (a watch-glass will often do 

 well, but a tube is safer). With a pipette carefully put 

 under the alcohol a sufficient quantity of clearing medium 

 (or carefully pour the alcohol on to the clearing medium). 

 Then put the objects into the alcohol. They will sink down 

 to the level of separation of the two liquids at once ; and 

 after some time they will be found to have sunk to the 

 bottom of the clearing medium. They should, however, not 

 be considered to be perfectly penetrated by the clearing 

 medium until the wavy refraction-lines caused by the mixture 

 of the two liquids at their surface have ceased to form. They 

 may then be removed by means of a pipette, or the super- 

 natant alcohol drawn off and the preparations allowed to 

 remain until wanted. 



The penetration of all clearing media may be hastened by 

 using them warm. 



It frequently happens that the essential oil with which 



