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Although, however, it would be wrong to draw a fast line by 
which we may say how much ammonia (indicating organic matter) 
for instance, shall be present in a water to condemn it as absolutely 
injurious, or as certain to produce disease, yet it is well to act on 
the principle that a suspicious character is to be avoided ; and 
therefore, if the results of our analyses lead us to be suspicious, or 
to conclude that the sample is probably polluted with pernicious 
substances, we shall not be doing wrong if we advise the abandon- 
ment of the use of such water for drinking purposes. Probably 
we are sometimes unnecessarily suspicious, and this may arise from 
many causes. 
We now come to the second part of my subject; that of 
AIR IN ITS RELATIONS TO HEALTH OR DISEASE, 
I am disposed to think that impure air exists more abundantly 
than impure water; and considering the nature of the two media, 
may it not be anticipated that more disease is likely to be produced 
by the former than by the latter? Into the air, noxious gases and 
minute organisms, resulting from putrefactions, &c., immediately 
pass ; and we know that when the air enters the lungs, the blood 
is directly exposed to it, and that a part of the air (the oxygen) is 
carried by the venous blood to various parts of the body, where it 
meets with impurities due to dissociations caused by the muscular 
action going on in various ways. With these impurities the oxygen 
combines, and they are thus rendered easy of emission as products 
of respiration, &c. But during the passage of the air through the 
different parts of the organism, any living organic impurities present 
in it are likely to remain there to exercise their ferment actions, as 
already suggested in the case of impurities in water; but in this 
case (in the case of air) the impurities by being breathed are at 
once conveyed to the circulation, which is not the fact when we 
drink water. This, however, is only to be to a certain extent 
considered. 
The atmosphere has been well spoken of as a “catholic 
receptacle,” and may we not then be led to wonder at its peculiarly 
uniform condition, and its comparative freedom from deleterious 
a 
