220 CLINICAL BACTERIOLOGY AND H^EMATOLOGY 



point. This paraffin should be obtained specially for the 

 purpose; the Cambridge paraffin is the best. It is made in 

 two varieties the soft, which melts at 48 C, and the hard, 

 which melts at 55 C. The amounts of each which should 

 be used for embedding depend upon the external tempera- 

 ture : in very hot weather hard paraffin may be used alone, 

 while under average circumstances a mixture of equal parts 

 of ea-ch is best. 



We shall no\v proceed to describe the various processes 

 seriatim. 



Dehydration. This is very simple. The blocks of tissue 

 are placed in weak spirit for a few hours or for a day, then 

 changed into stronger spirit, and so on until absolute alcohol 

 is reached. The slower this process is carried out the better 

 will be the results ; in practice the strengths of the successive 

 lots of spirit used may be 40 per cent., 75 per cent., and the 

 strong methylated spirit, and the block may remain in each 

 for twelve hours. Lastly, it goes into two successive lots 

 of absolute alcohol. 



In all cases the amount of fluid must be greatly in excess. 

 It is useless merely to cov-er the block w T ith the spirit. 



Clarification. In the next step the alcohol is removed from 

 the tissue and replaced by some fluid which will dissolve 

 paraffin. Fats are dissolved out from the tissues at this 

 stage. 



This step is also very simple. The blocks are passed 

 directly from absolute alcohol into chloroform, or, better, 

 ligroin, and allowed to remain there for twelve to twenty-four 

 hours, according to their size. It is not necessary to use a 

 preliminary bath of a mixture of alcohol and chloroform. 



It is a good plan to place the bottle containing the block 

 in a warm place with the cork out for an hour or so before 

 proceeding further, as by so doing the last traces of the 

 alcohol will be removed. 



Infiltration with Paraffin. This is the stage which presents 

 most difficulties to the home-worker, for it is necessary to 

 keep the block of tissue soaked in paraffin which is just 

 melted for at least twelve, and more often twenty-four, hours. 

 To do this properly involves the use of some sort of incuba- 

 tor. This might possibly be rigged up out of a tin biscuit- 

 box in the manner already described, though considerably 



