LABORATORY TECHNIC 45 



frequently made in the text to Gram's stain, and it is 

 desirable that the reader be familiar with the term 

 and its significance. It is a combination of aniline 

 oil, water, and gentian violet, which stain can be fixed 

 into some bacteria by after-treatment with iodin solu- 

 tion, so that alcohol will not wash it out. The test is 

 of great importance in determining certain species. 



Animal Inoculation. Another method of studying 

 bacteria is by injecting them into susceptible animals. 

 Thus can be discovered their power of producing 

 disease, its severity, called virulence, and the nature 

 of their action. When the presence of bacteria in 

 morbid matter cannot be demonstrated by stain 

 or by cultural methods, it may sometimes be shown 

 by injecting the suspected material into animals. If 

 the animal fall sick or die one can then obtain cul- 

 tures of the germs for study. The value of this 

 method of discovering bacteria is increased by the 

 development of changes in the animal's organs peculiar 

 to certain germs. Thus the tubercle bacillus, an organ- 

 ism not easy to find by direct examination, produces 

 definite alterations of organs and special kinds of 

 inflammation by which its presence is indicated and 

 from which it can be obtained. This is also true 

 for other bacteria streptococci, anthrax, and glanders 

 bacilli. 



Protozoa. The study of protozoa varies according 

 to the source. The parasite of malaria may be found 

 by direct microscopic examination of the fresh blood. 

 This is also true of the organism of sleeping sickness. 

 The protozoa causing dysentery require the mainten- 

 ance of a definite temperature for a long time, and 



