56 ESSENTIALS OF BACTERIOLOGY 



end being slightly elevated). The water that still adheres 

 is dried off in the air or gently over the flame, and when per- 

 fectly dry, the cover-glass is placed upon the drop of Canada 

 balsam which has been put upon the glass slide. 



In placing the cover-glass in the staining solutions one 

 must be careful to remember which is the spread side, by 

 holding it between one's self and the window and scraping the 

 sides carefully with the sharp point of the forceps, the side 

 having the specimen on it will show the marks of the instru- 

 ment. 



Little glass dishes, about one-half dozen, should be at hand 

 for containing the various stains and decolorants. 



Tissue Preparations. In order to obtain suitable speci- 

 mens for staining, very thin sections of the tissue must be 

 made. 



As with histologic preparations, the tissue must be hardened 

 before it can be cut thin enough. Alcohol is the best agent 

 for this purpose. 



Pieces of the tissue one-quarter inch in size are covered with 

 alcohol for twenty-four to forty-eight hours. 



When hardened, it must be fixed upon or in some firm 

 object. A paste composed of 



Gelatin i part 



Glycerin 4 parts 



Water 2 " 



will make it adhere firmly to a cork in about two hours, or it 

 can be embedded in a small block of paraffin and covered 

 over with melted paraffin. Celloidin may be used as an 

 embedding agent, and formalin is useful to harden tissue 

 quickly. 



Cutting. The microtome should be able to cut sections 

 ToW inch in thickness; this is the fineness usually required. 



The sections are brought into alcohol as soon as cut, unless 

 they have been' embedded in paraffin, when they are first 

 washed in chloroform to dissolve out the paraffin. 



