In summer and
winter, at the pole and at the equator, we inspire an equal volume
of air; the cold air is warmed during respiration and acquires the
temperature of the body. In order, therefore, to introduce into the
lungs a given amount of oxygen, less expenditure of force is
necessary in winter than in summer, and for the same expenditure of
force more oxygen is inspired in winter. It is also obvious that in
an equal number of respirations we consume more oxygen at the level
of the sea than on a mountain.

The oxygen taken into the system is given out again in the same
form, both in summer and winter: we expire more carbon at a low than
at a high temperature, and require more or less carbon in our food
in the same proportion; and, consequently, more is respired in
Sweden than in Sicily, and in our own country and eighth more in
winter than in summer. Even if an equal weight of food is consumed
in hot and cold climates, Infinite Wisdom has ordained that very
unequal proportions of carbon shall be taken in it. The food
prepared for the inhabitants of southern climes does not contain in
a fresh state more than 12 per cent. of carbon, while the blubber
and train oil which feed the inhabitants of Polar regions contain 66
to 80 per cent. of that element.

From the same cause it is comparatively easy to be temperate in warm
climates, or to bear hunger for a long time under the equator; but
cold and hunger united very soon produce exhaustion.

The oxygen of the atmosphere received into the blood in the lungs,
and circulated throughout every part of the animal body, acting upon
the elements of the food, is the source of animal heat.

[Footnote 1: This account is deduced from observations made upon the
average daily consumption of about 30 soldiers in barracks. The food
of these men, consisting of meat, bread, potatoes, lentils, peas,
beans, butter, salt, pepper, &c., was accurately weighed during a
month, and each article subjected to ultimate analysis. Of the
quantity of food, beer, and spirits, taken by the men when out of
barracks, we have a close approximation from the report of the
sergeant; and from the weight and analysis of the faeces and urine,
it appears that the carbon which passes off through these channels
may be considered equivalent to the amount taken in that portion of
the food, and of sour-crout, which was not included in the
estimate.]

[Footnote 2: 17.5 ounces = 0.5 kilogramme.]

LETTER VII