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chief impurities in vitiated air. It also seriously interrupts or interferes 
with the exhalation from the skin and lungs. When excessive moisture 
is associated with high temperature, we are cognizant of an oppressive 
and sultry feeling, and an enervation of mental and bodily vigour; with 
low temperature it is conducive to a damp, penetrating chilliness, which 
seems to search us through and through. Coughs, colds and rheu. 
matic troubles are common when this state of atmosphere prevails. 
When the air is too dry, the mucous membrane of the mouth, pharynx 
and nostrils become parched, and the use of the voice impaired or 
impossible. A general irritability of the system is a common result of 
too dry an atmosphere. 
CARBONIC ACID. 
Carbonic acid gas, until quite lately, has been regarded not only 
as the chief impurity, but as the only impurity of vitiated air, and the 
one constituent that it is necessary to determine when examining an 
air for hygienic purposes. Important as it is that carbonic acid should 
not be allowed to exceed a certain amount in an air we breathe, we 
now know also how detrimental organic matter is, and that we must 
look upon it as probably the much more dangerous to health of the 
two. Carbonic acid is always present in the atmosphere. Over the 
sea, on mountains and moors, and in localities far from contaminating 
sources, it varies from .03 to .04 per cent, by volume. It is the result 
of the union of carbon (or charcoal) with oxygen. It is formed in the 
process of combustion, in the respiration of animals, and by decay or 
putrefaction of organic matter in the air. The chemistry, as far as the 
result is concerned, is precisely the same in all of these. The burning 
of wood, coal or other material rich in carbon and hydrogen, is accom- 
panied by the development of heat and light. This is what is com- 
monly understood as combustion. The products are carbonic acid gas, 
and water. By estimating their amounts, the chemist can tell how 
much carbon and hydrogen the burnt material contained. And again, 
knowing the weights of carbon and hydrogen in a substance, the heat 
that will be generated by their combustion can be calculated with ac- 
curacy, since in their union with oxygen they always produce for a 
known weight a certain amount of heat. 
C 
RY 
