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the sources of air and water pollution, which by simple negligence 
may, and no doubt do, frequently exist. 
Impure water and air are not only the cause of epidemic 
diseases, but they frequently lay the foundation for those which 
become hereditary. And with regard to the former (as I wish to 
discuss the question chiefly from a chemical point, being that which 
I have especially studied), it will be easiest for me first, to show 
the substances which, when present in water, are liable to produce 
or to propagate such disease ; and then, by statistics, to trace the 
relation of polluted water to the production of it, so far as has 
been actually proved. 
In the first place, then, it may be stated that for several years, 
and especially during very recent years, physiologists and chemists 
have been investigating, not so much the treatment of disease, as 
the more important matters connected with its cause and preven- 
tion. They have examined the air and water where disease 
existed, with the view of determining what additional substances 
they contained and which were not present in ordinary fresh air and 
water. ‘Their experiments have been successful in many instances, 
and they have obtained conclusive results which show that many 
diseases are produced or spread in the form of epidemics by living 
particles of an exceedingly minute size; these being often conveyed 
long distances by the media of air and water, and entering the 
living organism, they are rapidly reproduced, and in this way 
seriously influence the ordinary vital functions. We find the 
action of these germs in the body to be one of fermentation ; 
such an action as takes place in the brewing of beer (though, 
unfortunately, the result cannot be so much appreciated)—and it 
is from this fact that certain diseases have been called zymotic. 
These diseases may again be classed into at least two different 
forms, according to the manner in which they are spread. The 
ordinary fevers may be taken as a class spreading by the germs 
passing directly into the blood through the lungs, or, by contagion, 
through the skin ; while cholera and typhoid fever may be taken 
as examples of another, in which the germs do not act by immediate 
contact with the blood, but by passing through the stomach into 
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