Methods of Determination of Pathogenic Microbes. 103 



characterized, the first of these means is often suffi- 

 cient; but in other cases the germs to be determined 

 must be subjected to the test of the three reactions 

 which we have just mentioned. Thus, for example, 

 the bacillus of human tuberculosis and that of avian 

 tuberculosis possess identical physical characters, re- 

 act in the same way to coloring matters, and give 

 cultures difficult to differentiate ; on the other hand, 

 they have different effects on experimental animals. 

 In default of this last test we might have concluded 

 that the two germs were identical when they are 

 really quite distinct in some of their physiological 

 properties. 



I. Examination, investigation, and staining of bacteria. 

 The first examination of a liquid, from a bacterio- 

 logical point of view, should always be made without 

 the help of reagents; we thus see the germs in their 

 real form and size. Desiccation, or the addition of 

 staining solutions often change the morphological 

 character of microbes. It is in fresh preparations, 

 also, that we are in a position to judge of the mobil- 

 ity or immobility of the beings which we wish to 

 study. 



Microbes being naturally colorless, little refringent, 

 and consequently difficult to distinguish, staining 

 methods have been adopted in order to bring them 

 into relief in the media to be examined. 



For this purpose we have recourse to the colors de- 

 rived from coal — aniline colors — which have an in- 

 tense staining power. Without entering further into 

 the details of this subject, we may say that, with the 

 stains employed, we often associate adjuvant sub- 



