MALARIA. 425 



tory organs is quite certain. What we really do know of it has refer- 

 ence 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 precautious 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 sub- 

 urban 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 um- 

 brageous trees interposed between the dwelling and the point of dan- 

 ger, 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 sleep- 

 ing-apartments should not be below the second story, and should bo 

 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 pi-edisposes the individual to malarial 

 invasion, personal hygiene is indispensable. It should of course be 



