44 AGRICULTURAL BACTERIOLOGY 



which were first discovered accidentally by Perkin in Eng- 

 land in 1856. They were, however, not used in the study 

 of the bacteria until 1876, when they were employed by 

 Weigert. They have been of the greatest service in the de- 

 velopment of bacteriology. 



The bacteria can be differentiated from the other mate- 

 rials in which they occur, either by a contrast in depth of 

 color, or by staining all or a portion of the bacteria one 

 color and the other materials another color. The first 

 method can be used in demonstrating the bacteria in milk ; 

 a film of milk stained with methylene blue will reveal the 

 bacteria as dark blue objects in a light blue field. The 

 casein and albumen form the background of the prepara- 

 tion. The organisms that cause tuberculosis may be de- 

 tected in sputum, milk, and in other materials by staining 

 them a bright red, while the other bacteria and the mate- 

 rial in which they are embedded are stained blue. The 

 tubercle bacilli are thus differentiated from the other bac- 

 teria and the background of the preparation in a striking- 

 way. This is made possible by a property of the tubercle 

 bacillus which will be discussed later. 



Stained preparations are usually employed for the study 

 of the morphology of bacteria, for the demonstration of 

 spores and flagella. The spores do not stain as easily as do 

 the vegetative cells. It is thus possible to have the spores 

 appear as a bright unstained area in a stained cell. If 

 the spores have been set free by the dissolution of the cell, 

 they will appear as bright oval bodies in the microscopic 

 field. 



The flagella are so minute that, unless their apparent di- 

 ameter is increased, they can not be seen with the micro- 

 scope. They can be made visible by precipitating some dye 

 on them and thus increasing their apparent diameter. 



The staining properties of bacteria aid in their classifica- 



