DE-ALCOTTOLTSATION AND rLRAltTNO AGENTS. P>9 



ties 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-) food. 



To these should be added the others recommended in the 

 following paragraphs. 



See also the paper of JORDAN (Zeit. w'iss. Mils., xv, 1898, 

 p. 50), as to the behaviour of some essential oils towards 

 celloidin. 



119. 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 

 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, as described 5. The objects should 

 not be considered to be perfectly penetrated by the clearing 

 medium until the wavy refraction-lines caused by the mix- 

 ture of the two liquids at their surface have ceased to form, 

 and they should not 'be mounted or imbedded until they 

 have first been soaked for some time in a fresh quantity of 

 clearing medium, to remove any alcohol that has got into the 

 first bath. 



The penetration of all clearing media may be hastened 

 by using them warm. 



It frequently happens that the essential oil with which 

 objects are being treated in a watch-glass or on a slide 

 becomes cloudy after a short time, and fails to clear the 

 tissues. This is owing to a combination between the essen- 

 tial oil and moisture, derived, I think, rather from the air 

 than from the objects themselves. The cloudiness can 

 usually be removed by warming (as pointed out by 

 HATCHETT JACKSON, Zool. Anzeiy., 1889, p. 630), but in certain 

 moist states of the atmosphere it may persist, notwithstand- 

 ing continued warming. It is for this reason that I advise 



