l66 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



oil of cloves, two needles being used for the purpose, and a slip of 

 paper insinuated beneath it. This strip of paper is then drawn 

 slowly out of the liquid, and any folds or creases which may be in 

 the section straightened out with the needles, the excess of the 

 oil of cloves being allowed to drop off whilst this is taking place. 

 The strip of paper is then inverted (the section remaining adherent 

 to the under surface), placed upon a clean slide, and pressed firmly 

 upon it ; the pressure squeezes out the greater part of the oil, so 

 that the section adheres to the slide, and the paper can be stripped 

 cautiously from it. A drop of balsam is then applied, the section 

 covered with a cover-glass, and examined under the microscope. 



It is a great advantage to rinse the section in distilled or clear 

 rain-water after removing it from the haematoxylin. 



The solution of haematoxylin is best bought ready made, as its 

 preparation is somewhat difficult. Delafield's solution is the best 

 for general work. 



A counterstain is not really necessary for diagnostic purposes, 

 and its omission hastens the process somewhat. 



THE PARAFFIN PROCESS 



Tissues which are to be cut in paraffin may be hardened in any 

 of the fluids mentioned above. They are then dehydrated, cleared 

 in chloroform or other fluid which mixes with alcohol and dissolves 

 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. 



