STRENGTHS OF THE REAGENTS. 387 



sistent with the preservation of the tissue,, and be changed by 

 degrees for stronger. 



Osmic acid may be taken of 1 per cent, strength, and will 

 harden small pieces of tissue sufficiently in five to ten days 



(EXNER). 



Bichromate of potash should be taken at first of not more 

 than 2 per cent, strength ; this is then gradually raised to 3 

 or 4 per cent, for the, cord and cerebrum, and as much as 5 

 per cent, for the cerebellum. Obersteiner begins with 1 per 

 cent., and proceeds gradually during six to eight weeks to 2 

 or 3 per cent. (This is at the normal temperature; at a tem- 

 perature of 35 to 45 C. the hardening can be got through 

 in one or two weeks.) 



Bichromate of ammonia should be taken of half the 

 strength recommended for bichromate of potash, or even 

 weaker at first ; it may be raised to as much as 5 per cent, for 

 cerebellum towards the end of the hardening. 



Chromic acid is not much used alone. See 69. 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 

 solution 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. Perhaps a weaker solution 

 might give good results, but I cannot find that any such have been tried. 



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

 preservation of ganglion-cells, according to ANNA KOTLAREWSKI (see Zeit. 

 f. wiss. MiJc., iv, 3, 1887, p. 387). 



TEZEBINSKI (Virchoiv's Arch., 1887, p. 1 ; Zeit.f. wiss. Mik., iv, 4, 1887, 

 p. 497) finds that as regards the faithful preservation of ganglion-cells (of 

 the spinal cord of the rabbit and dog) the best results are obtained by 

 hardening for eight days in 10 per cent, solution of corrosive sublimate, 

 followed by hardening in alcohol containing 0'5 per cent, of iodine. 



DIOMIDOFF (ibid., p. 499) also obtained very excellent results by harden- 

 ing small pieces of brain (as suggested by GAULE, OGATA, and BECHTEREFF) 

 for from five to nine days (not more in any case) in 7 per cent, sublimate 

 solution, and then putting the tissues for twenty-four hours into 50 per 

 cent, alcohol, and for the same time into 70 per cent, and 96 per cent, 

 alcohol successively. (This process produces artificial "pigment spots," 

 similar to those produced by solution of Erlicki ; they may be dissolved 

 out by prolonged treatment with warm water, or in five minutes by strong 

 solution of LUGOL.) The tissues are of a good consistence for cutting. 

 Chloride of zinc has been recommended for some purposes (see below, 

 780). 



The next following paragraphs give in detail some methods of hardening 

 recommended by some of the most competent workers. 



