CHAPTER I. 3 



necessary for two reasons : first, in the interest of preservation, as 

 above explained ; and secondly, because all water must be removed 

 in order to allow the tissues to be impregnated with the imbedding 

 material necessary for section-cutting, or with the balsam with 

 which they are to be finally preserved. This dehydration is per- 

 formed as follows : The objects are brought into weak alcohol, and 

 are then passed through successive alcohols of gradually increased 

 strength, remaining in each the time necessary for complete satura- 

 tion, and the last bath consisting of absolute or at least very strong 

 alcohol. 



In dealing with delicate objects, it may be necessary to take 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 kinds of diffusion-apparatus may conveniently be used in these 

 cases. The objects may be placed with some of their liquid in a tube 

 corked at one end and closed at the other by a diaphragm of muslin or 

 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. Boy. Mic. Soc., 1890, p. 821) may be employed. Or, the 

 apparatus of HASWELL (Proc. Linn. Soc., N.S.W., vi, 1891, p. 433 ; 

 Journ. Eoy. Mic. Soc., 1892, p. 696). Or that of CHEATLE, described in 

 Journ. Patliol. and Bacteriol., i, 1892, p. 253, or Journ. Eoy. Mic. Soc., 

 1892, p. 892. See also SCHULTZE (Zeit. wiss. Mik., ii, 1885, p. 537) ; and 

 SUSUKI, ibid., 1909, p. 211 ; KOLSTER (ibid., xvii, 1900, p. 294). 



The " Siebdosen," or sieve-dishes of STEINACH, ZIMMERMANN, and 

 SUCHANNEK (vide Zeit. wiss. Mik., iv, 1887, p. 433, and vii, 1890, p. 158), 

 are useful for many purposes. See also TISCHATKIN, ibid., xxiii, p. 45. 

 FAIRCHILD'S perforated porcelain cylinders for washing (ibid., xii, 1896, 

 p. 301) seem to be a very neat idea. See also the similar device of 

 SCHAFFER (ibid., xvi, 1900, p. 422 ; Journ. Eoy. Mic. Soc., 1900, p. 394). 

 For EWALD'S section -washing apparatus, see Zeit. Biol., xxxiv, 1897, 

 p. 264. 



That of SCHOEBEL (ibid., xx, 1903, p. 168) is simple and efficient ; as 

 also that of KREIGBAUM (ibid., xxvii, 1910, p. 504). 



A capillary siphon for the aspiration of liquids in the fixing, staining, 

 and washing of suspended blood-corpuscles, sperm -cells, protozoa, and 

 the like, is described by EWALD, ibid., p. 253. 



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 95 per cent, being sufficient in most 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 



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