PERMANGANATE OF POTASH. 31 



The rationale of this mixture is, that it fixes tissues faith- 

 fully, without causing the production of the delusive reticular 

 precipitates of albuminoids which we have mentioned as being 

 produced by chromic acid that is the part played by the 

 bichromate and sulphate ; and that it also fixes faithfully the 

 chromatin of nuclei that is the part played by the organic 

 acid. 



The Chromates are useful as hardening rather than fixing agents. 

 They have a very mild and even action on tissues, but are not at all pene- 

 trating and act very slowly. They may still be found useful for fixing 

 certain tissues, some of those of Mollusca, for example. For mixtures that 

 may be used for such a purpose, see the chapter on HARDENING AGENTS. 



44. Cupric Sulphate. Cupric sulphate was recommended some few 

 years ago in one of the well-known handbooks. It was recommended (in a 

 place which I cannot now find) to be used for marine organisms in saturated 

 solution in sea-water, the organisms to be preserved in the solution itself 

 till wanted. It has been quite recently (Arch. d. Sci. phys. et nat., Juin, 



1889, t. xxi, p. 556) recommended by BEDOT for the preparation of Siphono- 

 phora and other delicate pelagic animals. See " Siphonophora." 



FBIEDLAENDER (Biol. Centralbl., x, 1890, p. 483 ; Journ. Roy. Mic. Soc., 



1890, p. 804) preserves this class of objects by inundating them with a 

 mixture of 125 parts cupric sulphate, 125 parts zinc sulphate, and 1000 

 parts water. 



45. Alum. Alum has been used for fixing purposes, and may therefore 

 be mentioned here. Although quite superseded for general work by other 

 reagents, it may possibly still be found useful for certain special purposes. 

 For instance, for the preservation of Medusse the following process has been 

 recommended (by Pagenstecher). Take two parts of common salt and one 

 of alum, and make a strong solution. Throw the animals into it alive, and 

 leave them there for twenty-four to forty-eight hours. Preserve in weak 

 alcohol. A saturated solution of alum in sea-water preserves very well the 

 forms of Salpidas, Medusse, Ctenophora, and other pelagic animals. It 

 constitutes a preservative medium in which the objects may remain till 

 wanted. 



According to my experience, it is not to be recommended for any but the 

 very coarsest work. It should be noted, however, that Eanvier (Traite Tech- 

 nique, p. 279) found that it fixed cartilage-cells better than any other reagent. 

 He employed a solution of 0'5 per cent. 



46. Permanganate of Potash (Du PLESSIS, Bull. Soc. Vaud. Sci. Nat., 

 2, ser. xv, pp. 278280, 1878). A strong solution in water. I find this 

 reagent has very slight penetrating power, and, besides, macerates some 

 tissues. It is therefore not adapted for general use, but it preserves very 

 well the forms of cells, and has one great virtue it kills, I fancy, more 

 rapidly than any other agent I have been able to find ; even 2 per cent, osmic 

 acid is not equal to it in this respect. I have found it sometimes very 

 valuable for the study of isolated and very contractile cells, such as some 

 spermatozoa. 



