376 OIIAPTIilR XXXT. 



temperature of fi-om 30° to 40° C. preparations may be 

 sufficiently hardened in solution of Miiller in eight or ten 

 days, and in solution of Erlicki in four days, whilst at the 

 normal temperature two or three times as long would be 

 required. 



But it seems that this rapid hardening does not always 

 give the best results. Sahli is of opinion that it does not 

 (see Zeit. wiss. Mik., 1885, p. 3), and other authors are of 

 the same opinion. 



On the other hand, the slowness of the action of chromic 

 salts at the normal temperature is such that decomposition 

 may be set up before the hardening fluid has had time to do 

 its work. For this reason voluminous preparations that are 

 to be hardened in the slow way should be put away in a 

 very cool place — best of all in an ice safe. A hemisphere 

 will require eight or nine months for hardening in this waj'. 



Very large quantities of liquid should be taken, and be 

 changed, at first, every day for fresh. 



Eeagents (except osmic acid) should at first be taken as 

 weak as is consistent with the preservation of the tissue, and 

 be changed by degrees for strongei-. 



See also Pfistee in Neurol. Centralh., xvii, 1898, p. 64.3 (Zeit. whs. 

 Mik., XV, 1899, p. 4.94). 



740. The Reagents to he employed. — Those most used are 

 formal, and the chromic salts. 



I3ui!CHAUDT {La Cellule, xii, ] 897, p. 337) says that "accord- 

 ing to the unanimous judgment of all investigators, the 

 bichromates of jiotash and ammonia should not be employed 

 for the cytological study of nerve-cells." NissL [Enrycl. 

 mil-. Technil-, ii, p. 253) holds that, for this purpose, only 

 alcohol, formol, sublimate, and occasionally nitric acid, are 

 admissible. But this does not refer to hardening for purposes 

 oi fibre-anatomy, nor indeed in an absolute sense to cyto- 

 logical studies. It means that these are the reagents best 

 fitted for producing a " Nervenzellenrequivalentbild," that 

 , is — a standard and regularly ohtairahle fixation, always 

 amenable to certain current stains, of the whole of the various 

 kinds of nerve-cells found in nervous centres. For fibre- 

 anatomy he himself uses liichromate of potash. 



741. Formol. — Formol gives much h, iter penetration than 



