this gas. From
the experiments of Dupuytren and Thenard, air that contains a thousandth
part of sulphureted hydrogen kills birds immediately. A dog perished in
air containing a hundredth part, and a horse in air containing a
fiftieth part of it."

[16] Sulphureted hydrogen gas is the deleterious agent exhaled from
privies or vaults, which have been so fatal, at times, to night men, who
have been employed to remove or cleanse them.--_Dr. Dunglison._

The preceding are far from being all the causes of atmospheric impurity.
Besides these, there are numerous exhalations, as well as gases, that
are poisonous. Some of these exhalations are more abundant in the
night, and about the time of the morning and evening twilight. "Hence
the importance," says a writer on health, "to those who are feeble, of
avoiding the air at all hours except when the sun is considerably above
the horizon."

Although the atmosphere, in its natural state, is not at all times
perfectly pure, still it is comparatively so, and especially in the
daytime. All, therefore, who would retain and improve their health,
should inhale the open air as much as possible, even though they can
not, like Franklin's Methusalem,[17] be always in it. This remark is
applicable to both sexes, and to every age and condition of life.

[17] Dr. Franklin, in his usual humorous manner, but with his accustomed
gravity, relates, in one of his essays, the following anecdote, for the
purpose, doubtless, of showing the influence of pure air upon health,
happiness, and longevity.

"It is recorded of Methusalem, who, being the longest liver, may be
supposed to have best preserved his health, that he slept always in the
open air; for when he had lived five hundred years, an angel said to
him, Arise, Methusalem, and build thee a house, for thou shalt live yet
five hundred years longer. But Methusalem answered and said, If I am to
live but five hundred years longer, it is not worth while to build me a
house. I will sleep in the air as I have been accustomed to do."

The following, from the pen of an American author[18] who has written
much and well on physical education, is pertinent to the subject under
consideration: "We breathe bad air principally as the production of our
own bodies. Here is the source of a large share of human wo; and to this
point must his attention be particularly directed who would save himself
from disease, and promote, in