:} 1 CHAPTER IV. 



Chromic acid is not a very penetrating reagent, and for 

 tliis reason, as well as for others, is now seldom used pure 

 for fi,t'iuij. 



Fur jtrulongt'd harJt'iiiug it is generally employed in 

 Mivnmlis of % per cent, to i 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 

 1 Janvier). 



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 hardening. 



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 

 bleached 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 brittleness. 



38. Chromic Acid and Alcohol (URBAN PRITCHARD, Quart. 

 Journ. Mic. Sci., 1873, p. 427). Chromic acid, 1 part ; water, 20 parts ; 

 rectified spirit, 180 parts. For hardening such tissues as retina, 

 cochlea, etc, 



A mixture of 2 parts of A per cent, chromic acid solution with 1 part 

 <>t' methylated spirit was once much used by KLEIN (Quart. Journ. Mic 

 Sci, 1878, p. 315). 



these niixiuivs are irrational (see 37). A still more irrational 



