68 DE-ALCOHOLISATION AND CLEARING 



134. 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. The objects should not be con- 

 sidered 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 

 still remaining. 



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 essential oil and moisture derived, we 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 

 Anzeig., 1889, p. 630), but in certain moist states of the atmosphere it 

 may persist, notwithstanding continued warming. It is for this 

 reason that we advise that clearing be done, whenever possible, in 

 shallow corked tubes, under which conditions the phenomenon rarely 

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



135. Choice of a De-alcoholisation or Clearing Agent. In 



recent years carbon bisulphide has begun to oust xylol or benzol 

 from many of our laboratories for routine work, and the method 

 of getting the dehydrated pieces between a layer of the oil and the 

 alcohol in a phial (§ 4) is much used, instead of adding benzol 

 or carbon bisulphide little by little to the Stender dish of alcohol. 

 The introduction of dioxan, and the use of alcohols which dissolve 

 paraffin, used in special work, has also to be noted (§§ 124-131). 

 The use of methyl benzoate instead of cedarwood oil, is to be 

 recommended, as the former washes out in benzol much better 

 than the latter in the two-stage clearing used for large pieces and 

 embryos. We advise the beginner to keep on his table the follow- 

 ing : Oil of cedar and methyl benzoate, for general use and for 

 preparing objects for imbedding in paraffin ; clove oil, for inaking 

 minute dissections in (§ 8), 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 ; carbol-xylol and carbolic 

 acid, for rapidly clearing very imperfectly dehydrated objects. 



