CHLORIDES ORGANIC ACIDS ETC. 57 



the various methods for the neutralisation of formalin solutions, 

 and advocates the use of borax for this purpose. He adds 

 borax till a good red colour is shown with phenolphthalcin, 

 or a slaty blue with thymol blue, when added to the diluted 

 formalin. 



Atkins states further that formalin neutralised with sodium 

 hydroxide becomes acid on standing {M. B. A. J., 1922), 



It was said above that formaldehyde possesses certain hardening 

 and preserving quahties. It hardens gelatine, for instance, and certain 

 albuminoids ; but others, on the contrary, are not hardened by it, but 

 sometimes even rendered more soluble than they are naturally. For 

 some theoretical considerations concerning its action on tissues, see 

 F. Blum, in Anat. Anz., xi, 1896, p. 718 ; Benedecenti, in Arch. Anat. 

 u. Phys. Abth., 1897, p. 219 ; Gerota, in Intern. Monatsschr. Anat., xiii, 

 1896, p. 108 ; Zeit. wiss. Mik., xiii, p. 311 ; Sjobring in Anat. Anz., 

 xvii, 1900, p. 274 ; and Blum, in Encycl. mik. Technik., p. 393. It 

 seems to be generally admitted that this action consists in the formation 

 of methylene compounds with the substances of the tissues. 



The stock should he diluted ivith isotonic saline, and not in distilled 

 water, according to Carletox {Q. J. M. S., 1922). 



We find that, used pure, it is far from a first-class fixative. For 

 it over-fixes and shrinks some things, and swells and vacuolates 

 others. But notwithstanding this it is frequently very convenient 

 on account of its compatibility with the most various stains. It 

 has a high degree of penetration, and is a valuable ingredient in 

 many mixtures. 



It is a powerful reducing agent, and therefore incompatible with 

 such reagents as chromic acid or osmic acid and the like, which it very 

 rapidly decomposes. 



For fixing Lee finds that a strength of about 4 per cent. (1 vol. 

 formol to 9 of isotonic saline, or to 8 of water if the formol has 

 been long kept) is generally about right ; and this is the strength 

 used by most writers. For cytological purposes a fixation of at 

 least two days seems indicated : this applies especially to gonads 

 which are notoriously difficult to preserve in formol. The strengths 

 used in Cajal's and Da Fano's formol silver nitrate Golgi 

 apparatus methods, generally give fine results for tissues other 

 than genital. For these, injection fixation may be indicated. 

 See also § 698. Mayer takes 1 of formol to 8 of sea- water, for 

 marine animals. Few workers use much stronger solutions. Only 

 one (HoYER, Anat. Anz., ix, 1894, p. 236, Erganzungsheft) seems 

 to have used concentrated solutions. We think this exaggerated, 

 for we have found enormous over-fixation with solutions of 1 to 

 2 vols, of water. Wash out with alcohol (of 50 per cent, or more), 

 not water. 



For hardening, the same strengths may be taken. Hardening 

 is more rapid than with alcohol. For prolonged hardening, con- 



