110 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



steps are usually necessary: spreading, drying, fixing, 

 staining, washing, and mounting. A small amount of the 

 organism to be studied, usually bacteria or yeasts, may be 

 mixed in a drop of water on a clean cover glass and spread 

 over the surface in a uniform layer. The glass should be 

 sufficiently clean and free from grease or oil so that there 

 will be no tendency for the water to round up in the form 

 of a drop. If the organism to be studied has been growing 

 in a liquid medium, such as broth, the water may not be 

 necessary, but it may be spread directly in its nutrient 

 medium upon the cover glass. After spreading, the mate- 

 rial is allowed to dry in the air, or it may be gently heated, 

 using care that the organisms are not overheated. The 

 preparation is then fixed by passing it rapidly two or three 

 times through the flame of the Bunsen burner. This hard- 

 ens the protoplasm of the organisms and causes them to 

 stick firmly to the glass, so firmly in fact that they may 

 be washed vigorously without being loosened. A drop of 

 the stain to be used is then spread over the surface, 

 allowed to act for the desired length of time, then washed 

 off by means of the tap. The cover glass may then be 

 dried and inverted over a drop of water or a drop of 

 balsam on a glass slide and is then ready for microscopic 

 observation. 



In many laboratories the cover glass is dispensed with, 

 the mount being made directly upon the glass slip or slide. 

 When dried after staining, the immersion oil is placed 

 directly upon the mount and examination made without 

 the interposition of the cover glass. 



For staining particular parts of the cell, and for demon- 

 strating peculiarities of staining reactions, ^certain special 

 methods have been devised. Of these, four are of sufficient 

 importance to deserve comment in this connection. They 

 are the spore stain, the stain for acid fast bacteria, the Gram 

 stain, and the stain for flagella. 



