532 BACTERIOLOGY. 



which they are transferred to absolute alcohol for 1 to 2 

 days. In case the pieces of tissue are large it will be well 

 to make a second transfer to absolute alcohol. 



The tissues may be placed in alcohol containing 2. 5 per 

 cent, of the commercial formaldehyde (1 per cent, of the 

 gas). This may be especially desirable when the tissue 

 contains highly virulent organisms. 



Mercuric chloride. — A saturated aqueous solution of this 

 salt is used. An addition pf 5 per cent, of glacial acetic 

 acid may be made. The tissue is fixed in this solution in 

 from 4 to 12 hours. It may, however, be left in the liquid 

 for 24 hours. The. tissue must then be thoroughly washed 

 in running water. This is done by placing the bottle, the 

 mouth of which is covered with a piece of wire gauze, under 

 a hydrant for 12 to 24 hours. The pieces of tissue are then 

 placed in 70 per cent, alcohol for 24 hours, after which they 

 are transferred for a like period of time to 95 per cent, and 

 finally to absolute alcohoL 



A serious draw-back to the use of mercuric chloride is 

 its tendency to deposit a black, granular or semi-crystalline 

 precipitate in the tissue. The granules should not be con- 

 founded with micrococci. < 



Formaldehyde. — A 4 per cent, solution of formaldehyde 

 can be used for hardening tissue. This is prepared by 

 adding one part (10 c.c.) of the coran}ercial 40 per cent, 

 solution of formaldehyde to 9 parts (90 c.c.) of distilled 

 water. The tissue is allowed to remain in this solution for 

 4 to 12 hours. It is then transferred to 70 per cent, alcohol 

 for 24 hours, after which it is placed for a like period of 

 time in 95 per cent, and finally into absolute alcohol. 



It will be seen that no matter what solution is used for fixing, 

 eveBtually the tissues are placed in absolute alcohol. When thor- 

 oughly dehydrated the material is now ready for cutting direct, or 

 for imbedding and subsequent cutting. If the tissue is to be kept for 



