46 HARDENING AGENTS. 



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. See the 

 further information as to the employment of this reagent given above, 

 26, 27, 28. 



76. 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 percent, to 12 per cent.), 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- 

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

 ante. 



Salts. 



77. 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 incom- 

 parably better consistency to the tissues, and it has not the 

 same tendency to make them brittle if the reaction be pro- 

 longed. 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 by soaking in water. It is said that it 

 can be removed by washing for a few minutes in a 1 per 

 cent, solution of chloral hydrate. Gierke, however, says that 

 this treatment is prejudicial to the preservation of the tissues. 



