CHAPTER V. 61 



with magnesium or sodium carbonate ; some workers use lithium 

 carbonate, but this should not be used for BOUIN mixtures : it will 

 generally suffice to make them up with spring water. 



It was said above that formaldehyde possesses certain hardening 

 and preserving qualities. 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. Monatschr. 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. 



I 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 I find that a strength of about 4 per cent. (1 vol. formol 

 to 9 of water, 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 indi- 

 cated : this applies especially to gonads which are notoriously 

 difficult to preserve in formol. The strengths used in CAJAL'S and 

 DA FANG'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 141. MAYER takes 1 of formol 

 to 9 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, I 

 think this exaggerated, for I 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, consider- 

 able volumes of liquid should be taken, and the liquid should be 



