34 FIXING AGENTS. 



50. Perchloride of Iron. (FoL, Zeit.f. wiss. Zool., Bd. xxxviii, 1883, p. 

 491 ; and Lehrb. d. vergl. mik. Anat., p. 102). Fol recommends vol. 1 of 

 Tinct. Ferri Perchlor. P. B. diluted with 5 to 10 vols. of 70 per cent, 

 alcohol. This gives better results than the weaker (2 per cent.) mixture at 

 first recommended. Aqueous solutions do not give nearly so good results. 



Fix for a short time only and wash with alcohol. The preparations are best 

 stained with pyrogallol (see the chapter on Impregnation Methods, 214). 

 Fol recommends this process chiefly for Infusoria, and other ciliated objects* 

 but also as a general zoological method. I find it fixes cytoplasm well in 

 the cases to which I have applied it (nerve-end organs), but I hear from 

 Naples that it has been tried for the preservation of marine organisms, and 

 found wanting. Its chief value seems to me 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. 



51. 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 (Zellsubstanz, 

 &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 are 

 found in the Yermes and Ccelenterata ; 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. 



