CHLORIDES ORGANIC ACIDS ETC. 55 



though it enters as a useful ingredient into many mixtures, in 

 which it serves to enhance the power of penetration . 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. 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. 



108. Absolute 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 proportion 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 Ranvier, INIayer (Mitth. Zool. Stot. Neapel, ii, 1880, p. 7) ; 

 Bruel [Zool. Jahrb., Abth. Morph., x, 1897, p. 569) ; and van Rees 

 {ibid., in, 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 hydrates 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. 



Ranvier prepares a sufficiently " absolute " alcohol as follows : — - 

 Strong (95 per cent.) alcohol is treated with calcined cupric sulphate, 

 with which it is shaken up and allowed to remain for a day or two. It 

 is then decanted and treated with fresh cupric sulphate, and the 

 operation is repeated imtil the fresh cupric sulphate no longer becomes 

 conspicuously blue on contact with the alcohol ; or imtil, on a drop of 

 the alcohol being mixed with a drop of turpentine, no particles of water 

 can be seen in it under the microscope. The cupric sulphate is prepared 

 by calcining common blue vitriol in a porcelain capsule over a flame 

 until it becomes white, and then reducing it to powder (see Proc. Acad. 

 Nat. Set. Philad., 1884, p. 27 ; Journ. Roy. Mic. Soc, 1884, pp. 322 and 

 984). 



Test for the presence of water (Yvon, C. R. Acad. Sci., 1897, p. 1181). 

 Add coarsely powdered calcium carbide ; the merest trace of water will 

 cause an evolution of acetylene gas, and on agitation the alcohol will 

 become tiu-bid. 



109. One-third Alcohol. The grade of Aveak alcohol that is 

 generally held to be most useful for fixing is one-third alcohol, or 

 Ranvier's Alcohol. It consists oftivojJarts ofzvater and one part 

 of alcohol of 90 per cent, (and not of absolute alcohol). Sec the 

 Traite Technique of Ranvier, p. 241, et passim. 



