s constitution when taken into the lungs in the
act of breathing. When it is expelled from them, however, its
composition is found to be greatly altered. The quantity of nitrogen
remains nearly the same, but eight or eight and a half per cent. of the
oxygen or vital air have disappeared, and been replaced by an equal
amount of carbonic acid. In addition to these changes, the expired air
is loaded with moisture. Simultaneously with these occurrences, the
blood collected from the veins, which enters the lungs of a dark color
and unfit for the support of life, assumes a florid hue and acquires the
power of supporting life.

Physiologists are not fully agreed in explaining the processes by which
these changes are effected in the lungs. All, however, agree that the
change of the blood in the lungs is essentially dependent on the supply
of oxygen contained in the air we breathe, and that air is fit or unfit
for respiration in exact proportion as its quantity of oxygen
approaches to, or differs from, that contained in pure air. If we
attempt to breathe nitrogen, hydrogen, or any other gas that does not
contain oxygen, the result will be speedy suffocation. If, on the other
hand, we breathe air containing too great a proportion of oxygen, the
vital powers will speedily suffer from excess of stimulus.

The chief chemical properties of the atmosphere are owing to the
presence of _oxygen_. Nitrogen, which constitutes about four fifths of
its volume, has been supposed to act as a mere diluent to the oxygen.
_Increase_ the proportion of oxygen in the atmosphere, and, as already
stated, the vital powers will speedily suffer from excess of stimulus,
the circulation and respiration become _too rapid_, and the system
generally becomes highly excited. _Diminish_ the proportion of oxygen,
and the circulation and respiration become _too slow_, weakness and
lassitude ensue, and a sense of heaviness and uneasiness pervades the
entire system. As has been observed, air _loses_ during each respiration
a portion of its oxygen, and gains an equal quantity of _carbonic acid_,
which is an _active poison_. When mixed with atmospheric air in the
ratio of one to four, it extinguishes animal life. It is this gas that
is produced by burning charcoal in a confined portion of common air. Its
effect upon the system is well known to every reader of our newspapers.
It causes dimness of sight, weakness, dullness, a difficulty of
breathing, and ultimately _ap