60 DE-ALCOHOL1SATION AND CLEARING AGENTS. 



To these should be added the others recommended in the following 

 paragraphs. 



See also the paper of JORDAN (Zeit. wiss. Mik., 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-alcoho- 

 lising 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 mixture 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 com- 

 bination between the essential oil and moisture, derived, I think, 

 rather from the air than from the objects themselves. The cloudi- 

 ness can usually be removed by warming (as pointed out by HATCHETT 

 JACKSON, Zool. Anzeig., 1889, p. 630), but in certain moist states of 

 the atmosphere it may persist, notwithstanding continued warming. 

 It is for this reason that I advise that clearing be done, whenever 

 possible, in shallow corked tubes, under which conditions the pheno- 

 menon rarely occurs. In any case, be careful not to breathe on the 

 liquid. 



120. Choice of a De-alcoholisation or Clearing Agent. I advise 

 the beginner to keep on his table the following : Oil of cedar, for 

 general use and for preparing objects for imbedding in paraffin ; 

 clove oil, for making minute dissections in ( 122), and for much work 

 with safranin, etc. ; oil of bergamot, which will clear from 90 per 

 cent, alcohol, and which does not extract coal-tar colours ; carbolic 

 acid, for rapidly clearing very imperfectly dehydrated objects. 



For special clearers for celloidin sections see Chapter IX. 



