42 METHODS OP MICROSCOPICAL EESEARCH. 



alcohol is changed at first, the more perfect, as well 

 as the more rapid, will be the hardening. 



Nitric Acid 



should be used in an aqueous solution of from 5 per 

 cent, to 15 per cent., and the specimens may remain 

 in it from fourteen to twenty-one days. When it 

 shall prove desirable to complete the hardening of 

 a specimen in spirit it may be removed from the 

 nitric acid and placed in the spirit after a much less 

 prolonged immersion in the acid. Great care must 

 be taken to remove from the specimen all trace of 

 the acid by prolonged soaking in water, frequently 

 changed, before placing it in alcohol. 



Chromic Acid. 



Make a 1 per cent, solution in water and reduce 

 its strength to J, , or such other percentage as may 

 be required. Specimens must be immersed in large 

 quantities of this solution, in which they may remain 

 for days or even weeks, the length of time during 

 which they should be subjected to its action being 

 determined by the size and nature of the specimen. 



Mucous tissues harden rapidly in this medium. 

 Nervous tissues, spinal cord and brain require weeks. 

 Bone from which the muscles (but not the perios- 

 teum) have been removed should be steeped in a very 

 weak solution, commencing with a ~ per cent, and 

 increasing to -J- per cent, with frequent, changes 

 during a period of ten days. It is then to be de- 

 calcified by immersion in chromic and nitric fluid 

 until a needle can be passed through it. It is then 



