Page:Popular Science Monthly Volume 9.djvu/449

Rh organs is quite certain. What we really do know of it has reference more particularly to its mode of action. It is most dangerous when the sun is down, and it seems almost inert during the day. It appears providential that the same agency which is so potent in its production should be the principal instrument of its destruction. It loves the ground, where in many regions it is so concentrated and deadly as to destroy the incautious sleeper on the earth almost as quickly as the most noxious gas. Hence it is generally regarded as having a specific gravity heavier than that of air, but this is by no means certain. It is doubtless rendered heavy by combining with night-fogs and dews, but upon their being dissipated by the sun it rises into the air and probably becomes innocuous by wide diffusion and dilution. It is intercepted by impediments, such as walls and groves of leafy trees, which obstruct the winds that bear it. Perhaps the latter also neutralize it by absorption. It is likewise neutralized and probably absorbed in passing over a considerable body of water—especially salt-water. The distance necessary to effect this result naturally varies with circumstances—force of winds, concentration, intensity, and abundance of the poison itself. According to Blane, in the channel between Beveland and Walcheren, 3,000 feet of water rendered it inert. In China, three-quarters of a mile, and in the West Indies, one mile, have been required to be effectual.

Recognizing the facts mentioned, the precautions to be observed against malaria are quite obvious. In built-up cities we are protected by pavements and sewers to a great extent, and probably also by the character of the atmosphere, which is artificially warmed by radiation at night, and impregnated with gases which, though injurious in other ways, are antagonistic to malarial emanations. But in malarial suburban and country districts it is otherwise. There certain precautions are necessary. If possible, elevation of a dwelling-place, at least 500 feet above the source of the miasm, is to be recommended in temperate climates, and from 1,500 to 2,000 feet in the tropics. If this be not practicable, thorough subsoil drainage, filling up of low and moist grounds, covering the earth with closely-cut herbage, belts of umbrageous trees interposed between the dwelling and the point of danger, but at a sufficient distance to permit free ventilation, and the access of sunlight; doors and windows opening principally away from the malarial quarter; the house, if possible, to be raised on pillars or arches a few feet above the ground, otherwise a sub-cellar thoroughly cemented—all these are measures of primary importance. The sleeping-apartments should not be below the second story, and should be provided with open fireplaces in which on damp or chilly nights a little fire may be kindled. Exposure to the open air after sunset, or until several hours after sunrise, should be avoided. As whatever tends to lower the vital powers predisposes the individual to malarial invasion, personal hygiene is indispensable. It should of course be