Contagious and Epizootic Diseases. 39 



wliicli at one stage of their existence are perfectly motion- 

 less, are at other stages endowed with powers of active 

 movement. The spores, like dried grain as compared with 

 the cereal plant, have a greatly enhanced vitality, can sur- 

 vive indefinitely without change, and in some cases resist 

 even a boiling temperature for a length of time. 



All bacteria live upon organic matter, and some use up a 

 large amount of oxygen by way of respiration — the amrohia 

 of Pasteur. Others can adapt themselves to a comparative 

 privation of oxygen, and some, it would appear, can live alto- 

 gether apart from the air, obtaining the oxygen necessary 

 to their existence from the decomposition of the nitroge- 

 nous animal or vegetable substances on which tliey feed : 

 These last are the anoerohia of Pasteur. A large class of 

 the air-breathing bacteria are mere scavengers (saprophytes) 

 feedinig upon decomposing organic matters and resolving 

 tlieir component parts into carbon dioxide and other simple 

 bodies which constitute food for plants. Thus they exer- 

 cise a most important function in nature, in transforming 

 into plant-food the products of vegetables and animals which 

 would otherwise accumulate in endless quantity. A fer- 

 menting manure-heap or a decomposing carcass or plant is 

 a grand exhibition of this beneficent work, and the nitrifi- 

 cation in soils is equally the work of these invisible servants 

 of nature. 



The products of bacteria growth are very numerous and 

 vary much with the species and the medium in which they 

 grow. The products of those growing in free air are, how- 

 ever, usually simple and comparatively harmless, while those 

 that have only a limited supply of air and that obtain their 

 oxygen by breaking up nitrogenous matters are usually, in 

 part at least, more complex in chemical composition and are 

 more likely to prove poisonous. Thus it is that disease- 

 germs increase in virulence and in their fatal power after 

 they have been grown for several generations in the tissues 



