(34 CHAPTKK V. 



for instance, and certain albuminoids; but others, on the 

 contrary, are not hardened by it, but sometimes even 

 ivndered 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; BENEDECEKTJ, in 

 Arch. Anat. u. Phys., Abth., 1897, p. 219 ; GEROTA, in Intern. 

 Monafwlir. Anat., xiii, 1896, p. 108; Zeit. uiss. Mik., xiii, p. 

 311 ; SJOBRING in Anat. Anz., xvii ; 1900, p. 274; and BLUM, 

 in Encycl. Mik. TecJinik., p. 3$3. 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. 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, considerable volumes of liquid should be taken, 

 and the liquid should be renewed from time to time ; for. 

 the formaldehyde fixes itself on the tissues with which it 

 comes in contact, deserting the solution, which thus becomes 

 progressively weaker. The specimens should be suspended 

 in the liquid or otherwise isolated from contact with the 

 containing vessel. The hardening obtained is gentle and 



