32 ESSAYS ON BACTERIOLOGY. 



slower, the discovery of new bacteria has become 

 of less importance. The germ -theory of disease has 

 been absolutely established, the specific germs of cer- 

 tain infectious diseases have apparently been settled, 

 and there is a general agreement as to the probable 

 bacterial origin of the others whose specific germs are 

 not yet known. The grounds for this belief are well 

 understood and need not be reviewed here. 



The question is often asked why the bacteria of cer- 

 tain diseases, as measles and scarlatina, which are com- 

 ' mon enough and are evidently infectious, have not 

 been discovered. There is, of course, every reason to 

 believe that these maladies are due to living poisons, 

 and it does seem strange that they have thus far 

 baffled investigation. The more strange does it seem 

 at first sight, in view of the fact that the methods of 

 bacteriological research are now so well understood 

 and the cultivation of many germs so comparatively 

 easy. 



There are, however, some marked differences be- 

 tween the poisons of measles, scarlatina, smallpox, 

 and many other infectious diseases. For instance, 

 tbey are apparently much more readily disseminated 

 through the air; they are said to be more "volatile." 

 The opinion, therefore, has grown that these differ- 

 ences are indicators of others more profound, and 

 which make it impossible to cultivate them by the or- 

 dinary methods. Until, therefore, further progress 

 has been made and perhaps radically new methods in- 



