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RESPIRATION. 
The process of respiration is really one of combustion, though un- 
accompanied by flame. Our food is rich in carbon and hydrogen. 
Starch and sugar, fats and albuminoids, of which our food consists, all 
contain large amounts of these elements. The blood which receives 
the digested food from the alimentary track is pumped from the right 
side of the heart into the lungs, where it is passed through countless 
small capillary tubes, with extremly thin membranous walls. It is here 
that it comes into close contact with the inspired a ; r, the oxygen of which 
it absorbs in large quantities. From the lungs it is then passed through 
the left side of the heart, and forced into the general circulation of the 
body. During its circulation, the absorbed or dissolved oxygen burns 
up the food material in the blood, forming carbonic acid and water, 
which are discharged chiefly on the reflow of the blood to the lungs 
though small quantities escape by way of the skin. The heat generated 
in the combustion of this food in the body to carbonic acid and aqueous 
vapor, is precisely equal to the amount that would have been produced 
if the food material had been burnt in the air ; and it is the heat so 
generated that maintains our body temperature. 
DECAY AND PUTREFACTION. 
Decay and putrefaction have been mentioned as the third source 
of carbonic acid gas in the atmosphere. These processes of the disin- 
tegration and dissipation ot organic matter, are really of the nature of 
slow combustion, usually brought about by the agency of microscopic 
plants, known as bacteria. Their products are much the same as 
those resulting from the combustion of fuel and of food. 
ORGANIC MATTER. 
Having now discussed the sources of the carbonic acid in the air, 
we must now speak of the organic matter, which is more especially 
present as the result of deficient ventilation. 
It has already been mentioned that the deleterious character of 
badly ventilated rooms is due rather to the organic matter than to the 
carbonic acid. It is therefore of great importance that we should learn 
somewhat of its origin and effects upon health. Organic matter, and 
