FIXING AND HARDENING AGHNTS. Gl 



for instance, easily oxidises it, first into aldehyde, and then 

 into acetic acid. It follows that alcohol should not be com- 

 bined in mixtures with oxidising agents of notable energy. 

 Further, alcohol is a reducing agent, and therefore should 

 not be combined with easily reducible substances. These 

 remarks particularly apply to chromic acid, see §§ 37, 38, 48. 

 For fixing, alcohol is a very third-class reagent, only to 

 be used alone where better ones cannot be conveniently 

 employed, though it enters as a useful ingredient into many 

 mixtures, in which it serves to enhance the power of pene- 

 tration. For hardening it is an important one. 90 to 95 

 per cent, is the most generally useful strength. Weaker 

 alcohol, down to 70 per cent., is often indicated. Absolute 

 alcohol is seldom advisable. You ought to begin with 

 weak, and proceed gradually to stronger, alcohol. Large 

 quantities of alcohol should be taken. The alcohol should 

 be frequently changed, or the tissue should be suspended 

 near the top of it (§ 31). Many weeks may be necessary 

 for hardening large specimens. Small pieces of permeable 

 tissue, such as mucous membrane, may be sufficiently 

 hardened in twenty-four hours. 



103. Absdlute Alcohol. — This is sometimes valuable on 

 account of its great penetrating power. Mayer finds that 

 boiling absolute alcohol is often the only means of killing- 

 certain Arthropoda rapidly enough to avoid maceration. 



It is important to employ for fixing a very large pro- 

 portion of alcohol. Alum-carmine is a good stain for small 

 specimens so fixed. For preservation, the object should be 

 put into a weaker alcohol, 90 per cent, or less. 



As to the supposed superiority of absolute alcohol over 

 ordinary strong alcohol, see last § ; and amongst authors 

 upholding its superiority, see besides Eanviek, Mayels 

 [Mitth. Zool. Stat. Neapel, ii, 1880, p. 7) ; Bkuel {Zuol. 

 Jahrb., Ahth. Morph., x, 1897, p. 569) ; and van Kees 

 {ibid., iii, 1888, p. 10). 



Absolute alcohol is a product that it is almost impossible to preserve 

 in use, on account of the rapidity with which it hydi-ates on exposure to 

 air. Fol recommends that a little quicklime be kept in it. This absorbs 

 part at least of the moisture drawn by it from the air. 



