produced in various ways. This, says Dr. Comstock, is
immediately destructive to animal life, and will not support combustion.
It exists in stagnant water, especially in warm weather, and is
generated by the decomposition of vegetable products. Dr. Arnott
expresses the conviction that the immediate and chief cause of many of
the diseases which impair the bodily and mental health of the people,
and bring a considerable portion prematurely to the grave, is the poison
of atmospheric impurity, arising from the accumulation in and around
their dwellings of the decomposing remnants of the substances used for
food and in their arts, and of the impurities given out from their own
bodies. If you allow the sources of aerial impurity to exist in or
around dwellings, he continues, you are poisoning the people; and while
many die at early ages of fevers and other acute diseases, the remainder
will have their health impaired and their lives shortened.

There are many instances on record where the progress of an epidemic has
been speedily arrested by ventilation. A striking instance is given by
the writer last quoted. "When I visited Glasgow with Mr. Chadwick," says
he, "there was described to us one vast lodging-house, in connection
with a manufactory there, in which formerly fever constantly prevailed,
but where, by making an opening from the top of each room through a
channel of communication to an air-pump common to all the channels, the
disease had disappeared altogether. The supply of pure air obtained by
that mode of ventilation was sufficient to dilute the cause of the
disease, so that it became powerless."

Sulphureted hydrogen gas is also exceedingly poisonous to the lungs and
to every part of the system. When pure, this gas is described as
instantly fatal to animal life. Even when diluted with fifteen hundred
times its bulk of air, it has been found so poisonous as to destroy a
bird in a few seconds. "This gas," says Dr. Dunglison, in his Elements
of Hygiene, "is extremely deleterious.[16] When respired in a pure state
it kills instantly; and its deadly agency is rapidly exerted when put in
contact with any of the tissues of the body, through which it penetrates
with astonishing rapidity. Even when mixed with a portion of air, it has
proved immediately destructive. Dr. Paris refers to the case of a
chemist of his acquaintance, who was suddenly deprived of sense as he
stood over a pneumatic trough in which he was collecting