NEUROLOGICAL METHODS. 415 



material that has not been quite enough hardened, and solu- 

 tions of 1 per cent, may be used for material that has been 

 slightly over-hardeued. 



A relatively large quantity of solution should be taken 

 for the bath. 



The moment the pieces of tissue are put into the silver- 

 bath, an abundant yellow precipitate of chromate of silver 

 is formed. This of course weakens the bath pro tanto. It 

 is therefore well, before putting the pieces into the final 

 silver-bath, to first wash them well in a weaker silver solu- 

 tion, until on being put into a fresh quantity of it no farther 

 precipitate is formed. Used solutions will do for this pur- 

 pose. The final silver-bath in general needs no further 

 attention, unless it be that sometimes, in the case of tissues 

 that have taken up a great deal of bichromate of potash, 

 the solution may after six to ten hours become somewhat 

 yellow, in which case it should be changed for fresh. 



It is not necessary to keep the preparations in the dark 

 during the impregnation-bath ; in winter it is well to keep 

 them in a warm place. 



The time necessary for impregnation by the silver is from 

 twenty-four to forty-eight hours. The normal time is from 

 twenty- four to thirty hours, forty-eight being quite excep- 

 tional. By this is meant that the reaction is not obtained 

 in less time, but tissues may remain in the bath without hurt 

 for days, weeks, or months. 



(c) Preservation. As soon as a trial has shown that a 

 sufficiently satisfactory impregnation has been obtained, the 

 pieces are brought into alcohol. The alcohol is changed 

 two or three times, or even more, until it remains trans- 

 parent even after the preparations have been two or three 

 days in it ; for in view of good preservation it is necessary 

 that the excess of nitrate of silver should be washed out from 

 them thoroughly. 



Sections are now made. They are to be washed very 

 thoroughly in three or four changes of absolute alcohol. 

 They are then cleared, first in creosote, in which they should 

 remain only a few minutes, then in oil of turpentine, in 

 which they should remain for ten to fifteen minutes (they 

 may remain there for days without hurt). They are then 

 mounted in damar (rather than in balsam), and without a 



