INTRODUCTORY. 3 



special precautions in order to avoid injury to them through the violent 

 diffusion-currents that are set up in the passage from water to alcohol, or 

 from one bath of alcohol to another of considerably different density. Some 

 kind of diffusion-apparatus should be used in these cases. The objects may 

 be placed in a tube plugged at one end and closed at the other by a 

 diaphragm of chamois skin or other suitable membrane, the tube being 

 then immersed in a vessel containing the grade of alcohol that it is desired 

 to add to the liquid in the tube, and the whole allowed to remain until by 

 diffusion through the diaphragm the two liquids have become of equal 

 density. Or COBB'S differentiator (Proc. Linn. Soc., N.S.W., v, 1890, 

 p. 157 ; Journ. Roy. Mic. Soc., 1890, p. 821) may be employed. Or, more 

 conveniently in most cases, the apparatus described and figured by HASWELL 

 (Proc. Linn. Soc., N.S.W., vi, 1891, p. 433; Journ. Roy. Mic. Soc., 

 1892, p. 696). This consists of two wash-bottles connected in the usual way 

 by tubing, and furnished, the one with an overflow-tube, and the other with a 

 feeding-tube leading from a reservoir connected with it by means of a 

 regulating tap or drop arrangement. The objects are placed in the first 

 bottle ; some of the same liquid as that containing the objects is placed in 

 the second bottle ; and alcohol of the grade that it is desired to add is led 

 into it from the reservoir. The mixture of liquids therefore takes place in 

 the bottle that does not contain the objects, and the mixture itself is 

 gradually led over to the objects through the siphon-tube connecting the 

 two bottles. Another apparatus for rapid dehydration, devised by CHEATLE, 

 will be found described in Journ. Pathol. and Bacterial., i, 1892, p. 253, 

 or Journ. Roy. Mic. Soc., 1892, p. 892. 



It is sometimes stated that it is necessary that the last alcohol bath should 

 consist of absolute alcohol. This however is incorrect, a strength of 90 

 per cent., or at all events 95 per cent., being sufficient in almost all cases. 

 For the small amount of water that remains in the tissues after treatment 

 with these grades of alcohol is efficiently removed in the bath of clearing 

 agent, if a good clearing agent be employed. Oil of cedar will remove the 

 remaining water from tissues saturated with 95 per cent, alcohol ; oil of 

 bergamot will "clear" from 90 per cent, alcohol, and anilin oil will clear 

 from 70 per cent, alcohol. 



It is not generally necessary that pure ethylic alcohol be employed for 

 dehydration ; methylated spirit will suffice in most cases. 



I am not aware of any substance that can entirely take the place of 

 alcohol for dehydration and preservation. Acetone, and methylal, have been 

 lately (PARKER, Zool. Anz., 403, 1892, p. 376) substituted for alcohol in the 

 dehydration of methylen-blue preparations ; but the great boon of an efficient 

 substitute for alcohol in general work remains yet to be discovered. 



Considered as a mere dehydrating agent, alcohol fulfils its functions fairly 

 well. But considered as a histological preservative agent, it is far less 

 satisfactory. If tissues be left in alcohol for only a few days before sub- 

 jecting them to the further stages of preparation, the injurious effects of a 

 sojourn in alcohol will perhaps not be very disagreeably evident. But it is 

 otherwise if, as must often be the case, they are put away for many weeks 

 or months before the final preparation can be carried out. The dehydrating 



