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of disposing of noxious matters—and from this its relation to 
health generally—the determination of the amount present at 
various times and localities is made an important subject of meteor- 
ological enquiry, and much valuable information has been obtained 
by comparing such determinations with statistics of disease at 
various seasons. 
Air is vitiated from three great causes— 
1. Respiration. 
2. Putrefaction. 
3. Manufactures and Combustion. 
The first of these sources of pollution is probably the most 
important. It is one which must exist, and therefore cannot be 
prevented, but which it is our duty to especially consider, in order 
that its injurious effects may be guarded against. In each one 
hundred parts of the air we breathe there are, as we have already 
seen, about twenty-one parts of oxygen; but after it has been 
breathed, the quantity is diminished, and we find in the air expelled 
from the lungs only about fifteen parts, so that six parts of oxygen 
are removed by the process of respiration. 
But expired air is not only in this way rendered deficient in 
oxygen, but it contains a correspondingly larger quantity of carbonic 
acid, or what is commonly called “choke-damp;” and in addition 
also to this increase of carbonic acid, there is an increase of the 
amount of organic matter, which is a most serious fact as regards 
health, for it has been shown that while air mixed with even one 
fiftieth of its volume of carbonic acid alone may be breathed, the 
same amount of air mixed with one quarter that amount of carbonic 
acid, but accompanied by the usual organic imputities resulting 
from respiration, provef much more poisonous. 
Dr. Ransome, in a paper read before the Manchester Literary 
and Philosophical Society, on the amount of organic matter in the 
human breath in health and disease, says—“In diseased states of 
the system there is much greater variation in the amount and kind 
of organic matter given off than in health;” and remarks—“ We 
cannot doubt that much of the disease which arises as a conse- 
hee ee 
