FIXING AND HARDENING AGENTS. 39 



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

 necessary to complete the hardening 1 . 



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. (These 

 precautions are peculiarly necessary in the case of chromic 

 acid.) 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. I 

 think it is frequently useful to add a little glycerin to the 

 hardening solution ; there is less brittleness and, I think, less 

 shrinkage. 



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. 



41. Action of light on alcohol containing chromic objects. When objects 

 that have been treated by chromic acid or a chromate are put into alcohol 

 for hardening or preservation, it is found that after a short time a fine pre- 

 cipitate is thrown down on the surface of the preparations, thus forming a 

 certain obstacle to the further penetration of the alcohol. Previous washing 

 by water does not prevent the formation of this precipitate, and changing 

 the alcohol does not prevent it from forming again and again. It has been 

 found by Hans Virchow (Arch./, mik. Anat., Bd. xxiv, 1885, p. 117) that 

 the formation of this precipitate may be entirely prevented by simply 

 keeping the preparations in the dark. The alcohol becomes yellow as usual 

 (and should be changed as often as this takes place), but no precipitate is 

 formed. If this precaution be taken, previous washing with water may be 

 omitted, or at all events greatly abridged. 



42. Chromic Acid and Spirit (UBBAX PKITCHABD, Quart. Journ. Mic. 

 Sci., 1873, p. 427). Chromic acid, 1 part ; water, 20 parts ; rectified spirit, 

 180 parts. Dissolve the chromic acid in the water first, and then add the 

 spirit (violent action will ensue if the dry chromic acid be added directly to 

 the spirit). The colour of the solution soon becomes brown. If after a 

 few days it turns semi-gelatinous, it should be changed for fresh. From a 

 week to ten days is required to harden such tissues as retina, cochlea, etc., 

 for which this fluid used to be considered particularly well adapted. 



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

 methylated spirit was much used by KLEIX in his investigations into the 

 structure of cells and nuclei, and found to give better results than the 

 ordinary reagents (including even osmic acid). 



Both these mixtures are seemingly irrational (see 40). MATER (Grund- 



