Page:Popular Science Monthly Volume 12.djvu/445

This page has been validated.
429
THE HYGIENIC INFLUENCE OF PLANTS.

the equator. The Germanic races, particularly, inevitably degenerate after living for a few generations in the tropics, and must be continually renewed by immigration if they desire to retain supremacy, as is proved by the case of the English in India. They will not be able to settle there and maintain the characteristics which have made them dominant, until means have been found of diminishing the heat of the body at pleasure, as we are able to maintain it in the north. At present our remedies against heat are baths, fans, and shade.

We lose the heat of our bodies in three different ways: by the medium in which we are, generally the air, and which can be warmed; by the evaporation of perspiration; and by radiation from bodies of a lower temperature, not taking into account a small portion of heat which goes off in mechanical labor. Under ordinary circumstances in temperate climates, we lose half the heat generated by radiation, one-fourth by evaporation, and one-fourth by the conducting medium in which we are. In proportion as any of these methods is diminished, one or both the others must be increased. As long as possible, our organisms are so obliging as to open and close the sluices themselves without our cognizance, provided that our regulating apparatus is in order, that we are not ill. It is only when our good servant the skin, under certain conditions, has come to an end of its powers, that we begin to feel that we must lend our aid. And thus we have found by experience that, in hot weather, shade helps the body to keep cool to the needful extent. The chief effect of shelter is to prevent the sun's rays from striking us directly; but, if this were all, it would be as cool in the height of summer in-doors, or even under the leaden roofs of Venice, which have driven many to frenzy and desperation, as under the shade of a tree or in a wood. It also makes a great difference whether the sun's rays fall on thick foliage or on a roof of slate or metal. A great deal of heat is neutralized by evaporation from the leaves; another portion by the decomposition of carbonic acid, just so much as is set free when we burn the wood and other organic combinations into the composition of which it enters. The heat produced by burning wood in a stove is derived from the sun; it is but the captured rays of the sun again set free by combustion. We learn from Ebermayer's work that the temperature of the trees in a forest, and even in the tops of them, is always lower than the air in the forest.

Besides this, shade in the open air always causes a certain draught which acts as a kind of fan. All must have noticed when walking in oppressive heat, when the air seems still as death, that a refreshing breeze arises as soon as a cloud casts a shade. The same thing may often be observed in summer in walking through a street with close rows of houses, when the air is still, and one side is sunny, the other in shade. On the sunny side there is not a breath of air, while on the other there may be a light breeze. This is easily explained; so far as