FIXING AND HARDENING AGENTS. 75 



others, on the contrary, are not hardened by it, but rendered, 

 011 the contrary, 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; 

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

 GEROTA, in Intern. Monatschr. Anat. u. Phys., xiii, 8, 1896, 

 p. 108; Zeit.f. u-iss. Mik., xiii, 3, p. 311. 



On account of its hardening properties it has been used 

 as a fixing agent. On account of the confusion in termi- 

 nology above referred to, it is not at present possible to give 

 precise instructions as to the strengths that have been 

 employed by the different authors for this purpose. All 

 that can be said is that they will almost certainly be bound 

 to lie between the limits of those indicated by BLDM and 

 HERMANN, that is to say between 0*5 per cent, and 4 per 

 cent, if the formaldehyde be used pure. Only one writer 

 (HoYER, jun., Anat. Anz., ix, 1894, Erganzungsheft, p. 236; 

 Zeit. f. wiss. Mik., xii, 1, 1895, p. 28) appears to have used 

 concentrated solutions. He states that with such solutions 

 tissues are better preserved than with weak ones, even 

 better preserved than with corrosive sublimate. 



There is certainly some mistake here. I find that pre- 

 parations fixed in 13'H per cent, formaldehyde (formol with 

 two volumes of water) have the cells enormously over-fixed, 

 and presenting the homogeneous aspect of osmicated cells. 

 Experimenting further with weak solutions containing from 

 2 per cent, to 4 per cent, of formaldehyde, I have found 

 that like the stronger solution mentioned above, these too 

 give a homogeneous, colloid appearance to protoplasm, and 

 have at the same time a marked swelling and vacuolating 

 action. With the 2 per cent, solution the vacuolation is 

 enormous. I have concluded that used pure formaldehyde 

 is not at all suitable as a fixing agent for cytological work, 

 and should not be employed for that purpose, and I 

 certainly should not think of using it myself, even for 

 general morphological work. 



Some formulae for formaldehyde mixtures for fixing given 

 in the last edition, with which I had had fair results, are 

 suppressed, as I now think that where the results were good, 

 it may have been in spite of the formaldehyde, and not on 

 account of it. See, however, the alcoholic mixtures of 



