IMBEDDING OF TISSUE. 533 



some time it is advisable to preserve it in a dilute alcohol of about '70 

 per cent, strength. Many bacteria, however, are affected by pro- 

 longed sojourn in alcohol to such an extent that they will not readily 

 stain. This is notably true of the leprosy and tubercle bacilli. The 

 author prefers, therefore, to imbed the tissue in paraflSn and keep it 

 in this form. The parafBn with the tissue may be kept in bottles 

 rather than be put up into blocks. 



Imbedding in paraffin. — The first step toward imbedding' 

 in paraffin is to place the tissue from absolute alcohol into 

 toluol, for 24 hours, then for a like period into a strong 

 solution of paraffin in toluol. Xylol, chloroform or tur- 

 pentine may be used in place of toluol. 



The paraffin necessary for the next step is kept in a 

 melted condition in a suitable oven. An ordinary air-bath 

 may be used, although it is better to employ one with a 

 water-jacket. The temperature of the oven should be 

 about 50°, and is controlled by a thermo-regulator (p. 

 246). Two wide-mouth bottles should contain the necessary 

 soft and hard paraffin. The soft paraffin melts at from 38 to 

 4*2°, whereas the hard paraffin melts at about 46° The 

 latter is usually pi'epared by bringing together equal parts 

 38 and 52° paraffin. In very warm weather it will be neces- 

 sary to use two parts of the latter to one of the former. 



The tissue, after permeation with the toluol paraffin 

 mixture, is placed in a small wide-mouth bottle, or better 

 in the so-called tube vials. It is covered with melted soft 

 paraffin and placed in the oven for 12 to 24 hours. The 

 soft paraffin is then replaced by the melted hard paraffin 

 which is also allowed to act for 12 to 24 hours. The 

 tissue now thoroughly permeated with hard paraffin is 

 ready to be blocked. 



A rectangular trough, made by bringing together two 

 glass L's on a zinc plate, is filled with melted hard paraffin. 

 When the paraffin has slightly congealed on the bottom of 

 trough, the piece of tissue is introduced into the liquid by 

 means of a previously warmed pair of forceps. As soon as 

 the paraffin has cooled, so as to become opaque, the plate is 



