NEUROLOGICAL METHODS. 419 



time into the silver solution for thirty-six to forty-eight 

 hours. It is important to hit off the proper duration of the 

 first impregnation in the bichromate. If it has been too long 

 (four days) or too short (one day), the second impregnation 

 will not succeed. In this case a third impregnation must be 

 resorted to, the objects being again treated with the weak 

 osmium-bichromate mixture, and afterwards again with the 

 silver solution. 



This modification of the original process is, perhaps, the 

 most important that has hitherto been made. 



750. KALLIUS (Anat. Hefte, x, 1894, p. 527; Zeit. f. wiss. Mile., xi, 2, 



1894, p. 154) states that he has often found it advantageous to employ 

 bichromate of ammonia or of soda instead of the bichromate of potash, and 

 to perform all the reactions in the dark. Preparations made by the 

 ammonia or soda salt rarely require a double impregnation. 



751. BOEHM, and afterwards OPPEL (Anat. Anz., v, 1890, p. 143, and 

 vi, 1891, p. 165; Zeit. f. wiss. Mik., vii, 2, 1890, p. 222, and viii, 2, 1891, 

 p. 224), have modified the hardening part of the process by taking instead 

 of bichromate of potash (slow process) the one an 0'5 per cent, solution of 

 chromic acid (forty-eight hours), the other a solution of neutral chromate 

 of potash of from 0'5 per cent, to as much as 10 per cent. This is for 

 liver. 



BERKLEY (Anat. Anz., 1893, p. 772) fixes pieces of liver for fifteen to 

 thirty minutes in warm half-saturated solution of picric acid, and hardens 

 for forty-eight hours in a stove in the dark in a "sunned" ( 336) mixture 

 of sixteen parts 2 per cent, osmic acid and 100 parts saturated solution of 

 bichromate. 



752. Formaldehyde Mixtures. STRONG (Anat. Anz., x, 15, 



1895, p. 494) states that formaldehyde can with advantage 

 be substituted for the osmic acid in the osmio-bichromic 

 mixture of GOLGI'S rapid process. He adds from 2 '5 to 5 per 

 cent, of "formaline" to the 3'5 to 5 per cent, bichromate 

 solution. 



The advantage is stated to be that the stage of hardening 

 favourable for impregnation lasts longer ; in other words, the 

 formaldehyde bichromate does not over-harden. 



DURIG (ibid., p. 659) obtained the best results by means 

 of 3 per cent, bichromate solutions containing 4 to 6 per cent, 

 of formaldehyde, hardening therein for three days, and then 

 performing double impregnation by RAMON Y CAJAI/S process. 



