quills; . . . paper, becoming soft and soppy by the loss of glazing, acts as a blotter; . . . metals are ever rusty; . . . and gunpowder, if not kept from the air, refuses to ignite."
But it is the direct effects of different hygrometric states which must here be more especially set down—the effects on the vital processes, and therefore on the individual activities, and, through them, on the social activities. There is good reason, inductive and deductive, for believing that the bodily functions are facilitated by atmospheric conditions which make evaporation from the skin and lungs tolerably rapid. That weak persons, whose variations of health furnish good tests, are worse when the air, surcharged with water, is about to precipitate, and are better when the weather is fine, and that such persons are commonly enervated by residence in moist localities but invigorated by residence in dry ones, are facts generally recognized. And this relation of cause and effect, manifest in individuals, is one which we may suspect holds in races—other things being equal. In temperate regions, differences of constitutional activity due to differences of atmospheric humidity, are less traceable than in torrid regions, the reason being, that the inhabitants are subject to a tolerably rapid escape of water from their surfaces, since the air, though well charged with water, will take up more when its temperature, previously low, is raised by contact with the body. But it is otherwise in tropical regions where the body and the air bathing it differ much less in temperature, and where, indeed, the air is often higher in temperature than the body. Here the rate of evaporation depends almost wholly on the quantity of surrounding vapor. If the air is hot and moist, the escape of water through the skin and lungs is greatly hindered; while it is greatly facilitated if the air is hot and dry. Hence, in the torrid zone, we may expect constitutional differences between the otherwise-allied inhabitants of the low, steaming tracts and the tracts which are habitually parched with heat. Needful as are cutaneous and pulmonary evaporation for maintaining the movement of fluids through the tissues, and thus furthering molecular changes, it is to be inferred that, other circumstances being alike, there will be more bodily activity in the people of hot and dry localities than in the people of hot and humid localities.
The evidence, so far as we can disentangle it, justifies this inference. The earliest recorded civilization grew up in a hot and dry region—Egypt; and in hot and dry regions also arose the Babylonian, Assyrian, and Phœnician civilizations. But the facts when stated in terms of nations are far less striking than when stated in terms of races. On glancing over the rain-map of the world, there will be seen an almost continuous area marked "rainless district," extending across North Africa, Arabia, Persia, and on through Thibet into Mongolia; and from within, or from the borders of, this district have come all the conquering races of the Old World. We have the Tartar race,
