have been
equally wholesome.

MRS. B.

That is true, in a certain degree, with regard to those who have been
accustomed to them from birth; for we find that the natives of those
climates, which we consider as most deleterious, are as healthy as
ourselves; and if such climates are unwholesome to those who are
habituated to a more moderate temperature, it is because the animal
economy does not easily accustom itself to considerable changes.

CAROLINE.

But pray, Mrs. B., if the circulation preserves the body of an uniform
temperature, how does it happen that animals are sometimes frozen?

MRS. B.

Because, if more heat be carried off by the atmosphere than the
circulation can supply, the cold will finally prevail, the heart will
cease to beat, and the animal will be frozen. And, likewise, if the body
remained long exposed to a degree of heat, greater than the perspiration
could carry off, it would at last lose the power of resisting its
destructive influence.

CAROLINE.

Fish, I suppose, have no animal heat, but only partake of the
temperature of the water in which they live?

EMILY.

And their coldness, no doubt, proceeds from their not breathing?

MRS. B.

All kinds of fish breathe more or less, though in a much smaller degree
than land animals. Nor are they entirely destitute of animal heat,
though, for the same reason, they are much colder than other creatures.
They have comparatively but a very small quantity of blood, therefore
but very little oxygen is required, and a proportionally small quantity
of animal heat is generated.

CAROLINE.

But how can fish breathe under water?

MRS. B.

They breathe by means of the air which is dissolved in the water, and if
you put them into water deprived of air by boiling, they are soon
suffocated.

If a fish is confined in a vessel of water closed from the air, it soon
dies; and any fish put in afterwards would be killed immediately, as all
the air had been previously consumed.

CAROLINE.

Are there any species of animals that breathe more than we do?

MRS. B.

Yes; birds, of all animals, breathe the greatest quantity of air in
proportion to their size; and it is to this that they are supposed to
owe the peculiar firmness and strength of their muscles, by which they
are enabled to support the violent exertion of flying.

This difference between birds and fish, which may be considered as the
two extremes of the scale of muscular strength, is well w