OSMIO ACID. 45 



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

 necessary to complete the hardening. 



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. 



The reader's attention is called to the statements made in 

 30 concerning the action of light on the alcohol containing 

 chromic objects. 



Further directions for the employment of chomic acid will 

 be given in the special paragraphs. 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. 



71. Chromic Acid and Spirit (URBAN PEITCHAED, 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, &c., 

 for which this fluid is particularly well adapted. 



72. Chromo-osmic Acid (MAX FLESCH). Chromo-aceto-osmic 

 Acid (FLEMMING). Either of these mixtures may be used for 

 prolonged hardening, and are admirable. (See 34 and 35.) 



73. Chromic Acid and Platinum Chloride (MERKEI/S Solution). 

 The same remark applies to this excellent mixture. See 



42, ante. 



74. Picro-chromic Acid. This fixative may be found useful for 

 hardening objects that are only penetrable with difficulty. Some Tunicata, 

 for instance. See ante, 41. 



75. Osmic Acid. Osmic acid is much more useful as a fixing agent than 

 a hardening agent. Long immersion in osinic acid is sure to cause black. 



