HARDENING ANIMAL TISSUES. 191 



means. Rutherford advises brain to be soaked in the 

 following solution : 



Chromic acid 15 grains. 



Bichromate of potash 31 grains. 



Water 43 ounces. 



Small pieces are first immersed for twenty-four hours in 

 methylated spirit, drained on blotting-paper, and then 

 soaked for five or six weeks in a large quantity of the 

 above solution, changing it several times in that period. 

 Adipose tissue may be hardened in methylated spirit ; 

 liver prepared for cutting by soaking in alcohol, commenc- 

 ing with weak methylated spirit and finishing in absolute 

 alcohol ; lung in chromic acid, muscle in chromic acid, as 

 well as tongue, stomach, and spinal cord the last in one 

 part of the stock solution diluted with nine of water, as it 

 is extremely liable to over-hardening. The actual opera- 

 tion of hardening may be performed in the following 

 manner : Cut the substance kidney, for instance into 

 pieces half an inch square, and about the length suitable 

 for the well of the microtome. Place these in methylated 

 spirit diluted with an equal bulk of water for three days, 

 drain well upon bibulous paper, and then immerse in a 

 solution of chromic acid prepared by diluting one part of 

 the stock solution with seven parts of water. Allow the 

 pieces to remain in this for three days, then pour it off and 

 replace it by fresh. At the end of ten days a piece may 

 be abstracted and a rough section cut in order to see whether 

 it has become sufficiently hardened; if not, the chromic 

 acid must be again poured off and the pieces covered with 

 fresh solution, making an examination of it with the razor 

 every three days. Always harden insufficiently in the 

 chromic acid, and when just under that degree required, 

 take it out and put into methylated spirit to cleanse. 



