CHAPTER XXXI. 403 



812. Chromic Salts. The most commonly used is potassium 

 bichromate. The liquid of Erlicki has a more rapid action than 

 other solutions of chromic salts, but it has been generally abandoned 

 because of the alterations it very often produces. 



SAHLI (Ztschr. wiss. Mikr., ii, 1885, p. 1), after investigating the 

 action of the usual solutions, concludes that the best hardening 

 agent for fresh tissues is pure potassium bichromate, 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 to which it so fre- 

 quently gives rise. 



OBERSTEINER is of the same opinion, and recommends pure 

 bichromate for general hardening purposes ; whilst for the study of 

 delicate structural details he recommends fixing in Fol's modifica- 

 tion of Flemming's fluid (47) 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 the preparations for twenty-four hours or more with 

 80 to 90 per cent, alcohol, or better, for a few days with a formol 

 solution before putting them into the hardening fluid, or to add 

 formol (say 3 per cent.) to it, in order to avoid maceration of the 

 deeper layers of the tissues. In this case, however, the fluid must 

 be changed after twenty-four to forty-eight hours. 



Potassium bichromate should be employed 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 t and as much as 5 per cent, for the 

 cerebellum. 



Ammonium bichromate should be employed of half the strength 

 recommended for potassium bichromate at first ; it may be raised 

 to as much as 5 per cent, for cerebellum towards the end of the 

 hardening. 



NISSL (Enzycl. Mik. Technik., ii, 1910, p. 245) uses, for rapid 

 hardening, large quantities of Miiller's fluid 100 parts, formol 



3 parts, and enough glycerine to make the tissues float. If the 

 solution is often changed, even entire brains are in a few days 

 sufficiently hardened to be safely transferred into pure Miiller's 

 fluid, or potassium bichromate of about the same strength. 



BETZ'S method (Arch. mik. Anat., 1873, p. 101). Brain and 

 spinal cord are first hardened, for some days or weeks, in 70 to 80 per- 

 cent, alcohol containing enough iodine tincture to give it a light 

 brown coloration (as soon as the alcohol becomes colourless, more 

 iodine must be added.) They should then be definitely hardened 

 in 3 per cent, potassium bichromate for spinal cord, medulla 



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