36 FIXING AGENTS. 



also as a general zoological method. I hear from Naples that it has been 

 tried for the preservation of marine organisms, and found wanting. Its 

 chief value seem to lie in the pyrogallol staining method, which may be 

 found very useful for nerve-end organs, which are impregnated somewhat 

 selectively by it. 



Perchloride of iron (the tincture diluted with 3 to 4 vols. of either alcohol 

 or water) has lately been recommended for fixing medullated nerve by Platner 

 (Zeit.f. wiss. Mik., vi, 2, 1889, p. 187). 



Organic Acids. 



53. Acetic Acid. The place of honour amongst organic 

 acids considered as fixing agents appears rightfully to belong 

 to this old-fashioned reagent. In the first edition of this 

 work it was merely stated that acetic and formic acid " are 

 useful and well-known fixatives of nuclei. Flemming,, who 

 has made a special investigation of their action, finds (Zell- 

 substanz, &c.,p. 380) that the best strength is from 0'2 to 1 per 

 cent. Strengths of 5 per cent, and more bring out the nuclein 

 structures clearly at first, but after a time cause them to swell 

 and become pale, which is not the case with the weaker 

 strengths" (ibid., p. 103). It must now be stated that, thanks 

 to v. BENEDEN, the strong acid has become established as a 

 most precious fixative of the most varied zoological objects. 

 It is particularly applicable to very contractile objects, such 

 as many Vermes, Ccelenterata, and Nudibranchs ; it kills with 

 the utmost rapidity, and has a tendency to leave them fixed 

 in the state of extension. The modus operandi is in general 

 as follows : Pour glacial acetic acid in liberal quantity over 

 the organisms, leave them until they are penetrated by it 

 which should be in five or six minutes, as the strong acid is a 

 highly penetrating reagent and wash out in frequent changes 

 of alcohol of gradually increasing strength. Some persons 

 begin with 30 per cent, alcohol, but this appears to me rather 

 weak, and I think 70 per cent, or at least 50 per cent, alcohol 

 should be preferred. 



In the Traite des Neth. Techn., 1887, 1 stated that the reason 

 why glacial acetic acid was not more used was that it did not 

 faithfully preserve delicate histological and cytological detail. 

 I now believe that if the instructions above given be followed, 

 in particular as regards the employment of the glacial acid, 

 and the washing out with somewhat strong alcohol, the most 



