6 Bacteria in Relation to Country Life 



which they produce. This work may, therefore, be 

 regarded as the starting point for much fruitful research, 

 the foundation of an extensive knowledge on the physi- 

 ology of bacteria, that is, the chemical changes involved 

 in their life-processes. Bacteria were to be distinguished, 

 henceforth, not by their appearance alone, but by the 

 chemical transformations of which they are capable. 

 They were to be regarded as chemical agents of wide 

 significance, builders and destroyers in vegetable and 

 animal substances, in organic and inorganic materials, 

 in the presence or absence of air. 



Bacteria as a cause of disease. — The study of bac- 

 teria, and of other microorganisms, as agents of decay, 

 putrefaction and fermentation, gained in interest with 

 the recognition that bacteria may also be the specific 

 cause of disease. As far back as 1762, the belief was 

 expressed by Plenciz, a Vienna physician, that disease 

 is the result of infection by animalcules; and, more 

 important still, that every disease has its particular 

 germ. The views of Plenciz met with no acceptance, 

 and were soon forgotten amid the clashing opinions on 

 spontaneous generation. Towards the middle of the 

 nineteenth century, the question of a living contagium 

 in animal diseases again attracted wide attention. 

 Bassi's demonstration, in 1837, that a certain disease 

 of the silkworm is due to a specific germ, was the first 

 of its kind, and carried much weight. 



The question was placed on a broader basis by 

 Henle's teachings on bacteria. He not only thought 

 that infectious diseases are caused by bacteria, but he 

 outlined the methods of inquiry and pointed out that 



