382 CHAPTER XXX. 



though much more rapid than that of the salts, is much more 

 uneven, and frequently causes a disastrous friability of the 

 tissues. Osmic acid can hardly be used for objects of more 

 than a cubic centimetre in size at most. 



It has already been noted that the liquid of Erlicki has a 

 more rapid action than the other solutions of chromic salts. 

 SAHLJ, however (loc. cit. last ), after having studied the 

 action of the usual solutions, concludes that the best harden- 

 ing agent for fresh tissues is pure bichromate of potash, in 

 3 or 4 per cent, solution, the hardening being done in a cold 

 place. He rejects the liquid of Erlicki on account of the 

 precipitates it so frequently gives rise to (see 58). 



OBEBSTEINER is of the same opinion, recommending pure 

 bichromate for general hardening purposes; whilst for the 

 study of the most delicate structural relations he recommends 

 fixing in Fol's modification of Flemming's liquid ( 46) for 

 twenty-four hours, followed by washing with water and 

 hardening in 80 per cent, alcohol. 



In view of the slowness of penetration of chromic salts, it is 

 often advisable to treat preparations for twenty-four hours or 

 more with alcohol of 80 to 90 per cent before putting them 

 into the hardening liquid, in order to avoid maceration of the 

 deeper layers of tissue. 



BUECHABDT (La Cellule, xii, 2, 1897, p. 337) says that " according to the 

 unanimous judgment of all investigators the bichromates of potash and 

 ammonia should not be employed for the cytological study of nerve cells." 

 I have not noticed any such consensus of opinion of authors. 



Several observers have lately been using acetic alcohol. 

 So TIMOPEEW, Intern. Monatttschr., An at. u. Phys., xv, 18&8, 

 p. 259 ; Zeit.f. wiss. Mik., xvi, 1, 1899, p. 99 (CAKNOY'S second 

 formula, 83). 



OHLMACHER recommends his sublimate mixture, 84. 



For formaldehyde see 104, 692, and 752. 



Chromic acid is not much used alone (see 40). It forms part of some 

 of the mixtures mentioned below. A very little chromic acid (say one to 

 two drops of 1 per cent, solution for each ounce) added to bichromate solu- 

 tion will do no harm, and will quicken the hardening. 



Nitric acid has been, and still is, employed in strengths of 10 to 12 per 

 cent., and gives particularly tough preparations. 



Neutral acetate of lead in. 10 per cent, solution affords an excellent pre- 

 servation of ganglion cells, according to ANNA KOTLAEEWSKI (see Zeit. /. 

 wiss. Mile., iv, 3, 1887, p. 387). 



