l86 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



paraffin, and finally soaked in a mixture of hard and soft paraffin 

 kept just at the melting-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 temperature : in 

 very hot weather hard paraffin may be used alone, while under 

 average circumstances a mixture of equal parts of each is best. 



We shall now 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 cover the block with 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, 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 proceed- 

 ing 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 incubator. This might possibly 

 be rigged up out of a tin biscuit-box in the manner already 

 described, though considerably more heat would be necessary, as 

 the paraffin melts at about 50" C. But the writer has often 

 embedded the blocks by placing them in bottles containing the 

 paraffin at such a distance from the fire that the paraffin is never 



