38 ESSENTIALS OF BACTERIOLOGY 



being slightly elevated) . The water that still adheres is dried 

 off in the air or gently over the flame, and when perfectly dry, 

 it 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 oneself and the window and scraping the sides care- 

 fully with the sharp point of the forceps, the side having the 

 specimen on it will, show the marks of the instrument. 



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 specimens 

 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 (Fig. 12) should be able to cut 

 sections 5-^ 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. 



Staining. All the various solutions should be in readiness, 

 best placed in the little dishes in the order in which they are to 



