48 HARDENING AGENTS. 



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

 for instance. See ante, 41. 



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

 as a hardening agent. Long immersion in osmic acid is sure to cause black- 

 ening, and may cause brittleness in the tissues. The strengths employed 

 for hardening vary from ^th per cent, to 1 per cent., and the tissues are left 

 in the solutions for twelve to twenty-four hours, seldom more. The chief 

 use of osmium in hardening is to enhance the energy of action of certain 

 mixtures into which it enters as a component, and serves to materially shorten 

 the time necessary for hardening. See the further information as to the 

 employment of this reagent given above, 25, 26, 27, 28, 29. 



74. Nitric Acid. Nitric acid is taken of a strength of from 3 per cent, 

 to 10 per cent, or more, and may be allowed to act for two or three weeks. 

 It gives, thus employed (10 per cent, to 12 percent.), very tough preparations 

 of brain. It is also conveniently used by employing a very short immersion 

 and completing the hardening with alcohol, in which case it is properly con- 

 sidering as a fixing agent. See the information given under this head, 37, 

 ante. 



Salts. 



75. Bichromate of Potash. Perhaps the most important of 

 all known hardening agents, sensu stricto. It hardens slowly, 

 much more so than chromic acid, but it gives an incompar- 

 ably better consistency to the tissues, and it has not the same 

 tendency to make them brittle if the reaction be prolonged. 

 They may remain almost indefinitely exposed to its action 

 without much hurt. 



The strength of the solutions employed is from 2 to 5 per 

 cent. As with chromic acid it is extremely important to 

 begin with weak solutions and proceed gradually to stronger 

 ones. About three weeks will be necessary for hardening a 

 sheep's eye in solutions gradually raised from 2 to 4 per cent. 

 Spinal cord requires from three to six weeks; a brain, at least 

 as many months. 



After hardening, the objects should be well soaked out in 

 water before being put into alcohol. They had better be kept 

 in the dark when in alcohol (see above, 30) . If you wish to 

 have a good stain with carmine, especially ammonia-carmine, 

 which is admirable for portions of nervous system so hardened, 

 you should not put the objects into alcohol at all, even for a 

 second, until they have been stained. 



You may stain either with carmine or haematoxylin. 



Bichromate objects have an ugly yellow colour which cannot be removed 

 bv soaking in water. It is said that it can be removed by washing for a 



