INTRODUCTION. 



slips of glass, when they may be made more conspicuous by 

 giving them an artificial color (staining) . Some of the substances 

 of which they are composed readily absorb certain dyes. For 

 this purpose the aniline dyes are used, and their employment 

 has been one of the irilportant factors in making progress in 

 bacteriology possible. 



With the microscope alone it is not usually f^ -.^ 



practicable to distinguish accurately between M^- 'h?h 

 various kinds of bacteria. Micrococci, for in- 

 stance, which are, in reality, entirely different 

 species, may look very much alike. But 

 differences usually become apparent when the 

 bacteria are grown artificially. The cultiva- 

 tion is done for the most part in test-tubes 

 containing some material which furnishes 

 suitable food. The nutrient materials most 

 used are meat-extract and peptone, which, 

 dissolved with salt in water, constitute nutrient 

 bouillon. Ordinary gelatin, or a vegetable 

 gelatin called agar-agar, may be added to the 

 bouillon when a solid culture-medium is de- 

 sired. Before these substances can be used 

 for the cultivation of bacteria all other bac- 

 teria which they may contain must be de- 

 stroyed by heat. 



Finally, the effects of bacteria in bringing 

 about disease may be tested on the lower 

 animals. The proof that a particular species of bacteria 

 causes a particular disease cannot be considered complete 

 unless the disease can be reproduced by introducing these 

 bacteria into some animal. 



Bacteriological Literature. — The student who wishes to 

 pursue bacteriological study in any direction farther than it is 

 possible for the limits of a short manual to go, may, besides 



Fig. 2.— Test- 

 tube containing 

 culture-medium. 



