AIE 47 



effects of a pure atmosphere otherwise than by supposing 

 that it interferes Avith the spontaneous generation of patho- 

 genic bacteria. 



In considering the influence of impure air on the produc- 

 tion of diseases now known to be caused by bacteria, it has 

 to be borne in mind that the pathogenic microbes, in 

 respect of their habit of life, fall into two great classes. 



The first of these is the class of obligatory bacteria, 

 which, whatever may have been their origin, are now 

 unable to multiply except in the animal body. All the 

 purely contagious diseases, that is to say, those that never 

 originate spontaneously or independently of connection with 

 an antecedent case, are caused by bacteria of this kind. 

 Glanders, tuberculosis, and cattle plague are examples. 

 The fact that the causal organisms of these purely con- 

 tagious diseases are unable to multiply outside the animal 

 body necessarily gives them a more or less restricted 

 distribution; they are not ubiquitous, but confined to the 

 neighbourhood of diseased animals or the places or premises 

 where diseased animals have been kept. 



The second great class of pathogenic microbes includes 

 the so-called facultative bacteria, which, like the obligatory 

 microbes, are able to multiply in the animal body and 

 induce disease, but are also capable of multiplying and 

 maintaining their existence in the outer world, in such 

 media as soil, water, and decaying animal or vegetable 

 matter. As might have been expected from their habit of 

 life, the bacteria of this class have a very wide, and some- 

 times a practically ubiquitous distribution, and, also as 

 might have been expected, the diseases which they induce 

 are not necessarily contagious or infectious, but may arise 

 sporadically. The majority of bacteria responsible for 

 suppuration and other unhealthy processes arising in con- 

 nection with wounds belong to this class, as do m all 

 probability also some of the microbes which are the cause 

 of 'colds,' bronchitis, and pneumonia. There are good 

 reasons for believing that some of the bacteria which belong 

 to this class are very frequently, if not constantly, present 



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