422 CHAPTER XXXIV. 



abridging the hardening, but there is risk of over-hardening ; 

 and G-OLGi thinks the results are never quite so delicate as 

 after hardening in the cold. 



(6) Impregnation. — As soon as the pieces of tissue have 

 attained the proper degree of hardening, they are brought 

 into a bath of nitrate of silver. The usual strength of this 

 bath is 0'75 per cent., but 0'50 per cent, may be taken for 

 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-hardened. The solution may be acidified (see 

 Ramon y Cajaf,, § 819). 



A large quanlity of solution should be taken for the bath. 



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

 bath an abundant precipitate 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 solution, until on being put 

 into a fresh quantity of it no further precipitate is formed. 

 Used solutions will do for this purpose. 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 normally necessary for impregnation by the 

 silver is from twenty-four to forty-eight hours (forty-eight 

 being quite exceptional). But tissues may remain in the 

 bath without hurt for days, weeks, or months. 



(e) 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 (see § 821), They are to be 

 washed very thoroughly in three or four changes of absolute 



