CHAPTER VI 

 DEHYDRATION 



124. Dehydration is still almost universally carried out with 

 ethyl alcohol, and the other substitutes have made little headway 

 since their introduction into biological laboratories. Ethyl 

 alcohol is supplied as absolute alcohol, or rectified commercial 

 spirit which is about 96 or 97 per cent. The tissues must be 

 dehydrated fairly thoroughly before adding the oil which is the 

 solvent for paraffin wax, or the ether celloidin alcohol mixture for 

 celloidin-imbedding. In recent years various substitutes have 

 been suggested, which dehydrate, and will at the same time dis- 

 solve paraffin wax. Thus the so-called de-alcoholisation-stage 

 (xylol, carbon bisulphide) is dispensed with, and the imbedding 

 process hastened. 



There is a number of substances which have been developed in 

 connection with the aeroplane " dope " and lacquer industry which 

 have remarkable solvent powers, as, for instance, dioxan (di- 

 ethylene oxide), and cellosolve (ethylene glycol mono-ethyl ether), 

 and which have been recently proposed for microtomy. Some of 

 these, especially the amyl derivatives, have a physiological action, 

 and should not be used in small or ill-ventilated rooms unless it is 

 certain that they are harmless. 



It is generally believed that the dehydration of tissues should 

 be carried out in graded strengths. Some workers always begin 

 at 30 per cent., bringing the material through 50, 70, 90, to absolute 

 alcohol, often a day and night in each strength. Others simply 

 drop the fixed material, after washing, into 90 to 95 per cent, 

 alcohol, and then into absolute alcohol. This may be quite satis- 

 factory for rough histological materials, but is not recommended 

 for delicate objects, such as embryos or insect larvae. See J. A. 

 Murray's method, § 91. 



A table is given on p. 54, which gives figures for diluting 96 per 

 cent, alcohol in order to get the lower strengths. 



The dehydration with alcohols is carried out in corked, or glass - 

 stoppered phials, or in Stender dishes with ground or ordinary 

 glass tops. In damj) climates the corked bottles are necessary 

 with the higher strengths, but in dry climates the Stender dish 

 method is quite good enough. The periods of immersion of pieces 

 in the various grades of alcohol obviously vary with the size and 

 penetrability of the materials being used. But this is not all — it 



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