34 CHAPTBE IV. 



Chromic acid is not a very penetrating reagent, and for 

 this reason, as well as for others, is now seldom used piire 

 for fixing. 



For "prolonged hardening it is generally employed m 

 strengths of ^ per cent, to \ per cent., the immersion lasting 

 a few days or a few weeks, according to the size and nature 

 of the object. Mucous membrane, for instance, will harden 

 satisfactorily in a few days ; brain will require some six 

 weeks. 



Large quantities of the solution must be taken (at least 

 200 grammes for a piece of tissue of 1 centimetre cube 

 Ranvier) . 



In order to obtain the best results you should not employ 

 portions of tissue of more than an inch cube. For a human 

 spinal cord you should take two litres of solution, and change 

 it for fresh after a few days. Six weeks or two months are 

 necessary to complete the hai-dening. 



I think it is frequently useful to add a little glycerin ; 

 there is less brittleness. 



The solution should be taken weak at first, and the strength 

 increased after a time. The objects should be removed from 

 the solution as soon as they have acquired the desired con- 

 sistency, as if left too long they will become brittle. They 

 may be preserved till wanted in alcohol (95 per cent.). It is 

 well to wash them out in water for twenty-four or forty-eight 

 hours before putting them into the alcohol. After a time 

 they generally become green in the alcohol. They may be 

 hleaclied if desired. 



Chromic acid is a most powerful and rapid hardening 

 agent. (By it you may obtain in a few days a degree of 

 hardening that you would hardly obtain in as many weeks 

 with bichromate, for instance.) It has the defect of a great 

 tendency to cause hrittleness. 



38. Chromic Acid and Alcohol (TJeban Pbitchaed, Quart. 

 Joiirn. Mic. ScL, 1873, p. 427). — Chromic acid, 1 part ; water, 20 parts ; 

 rectified spirit, 180 parts. Tor hai'dening such tissues as retina, 

 coclilea, etc. 



A mixture of 2 parts of \ per cent, chromic acid solution with 1 pai-t 

 of methylated spirit was once much used by Klein {Qnart. Journ. Mic. 

 Sci., 1878, p. 315). 



Both these mixtures are irrational (see § 37). A still more iri-ation^l 



