Radiation in determining the Site of Malaria. 327 



laria. I shall content myself, therefore, with quoting only 

 Dr. Ferguson, who observes in his History of the Marsh 

 Poison, " that the rarifying heat of the sun dispels the miasms 

 which create fevers and violent diseases, and that it is only 

 during the cooler temperature of the night, that they acquire 

 body, concentration, and power." 



Now surely any miasmatous effluvia liberated from ex- 

 posed vegeto-animal or other matters by the rays of the sun, 

 must exist in the atmosphere as much if not more during the 

 day-season than in the night; for it is more than probable that 

 nothing is given up by the ground after sun-set. How is it then, 

 we may ask, that the great potency of malaria at night, and 

 its comparative harmlessness during the day,have so constantly 

 forced themselves upon our notice? Is it not because the air 

 during the former period is cooled by radiation and rendered 

 incapable of retaining those matters which the warmer air of 

 the day-time held in perfect solution? A still atmosphere 

 containing miasmatous matters, therefore, becomes dangerous 

 to health in proportion as it reaches, by a gradual reduction 

 of temperature, such as ensues from radiation, its dew point : 

 for during the period when its temperature is elevated above 

 this point, the malarious matter is without any, or of but little 

 injurious agency ; while the nearer it approaches the point at 

 which moisture will be liberated from it, the more those ex- 

 traneous matters it may contain become developed, as is fully 

 shown by the much greater potency of odours at that period, 

 of which numerous instances might here be mentioned; but 

 it will be sufficient to recall what every one must have ob- 

 served during the summer months: after a hot day, if the air 

 at night remains still, or is favourable to the process of radia- 

 tion, it is truly astonishing how far odours will diffuse them- 

 selves, and how powerful they generally are : a few hours after 

 sun-set, on evenings favourable to the deposition of dew, many 

 effluvia become very perceptible, and are potent and concen- 

 trated in proportion to the stillness of the air and its approach to 

 the dew point. Winds, although they very often cause consider- 

 able reductions of temperature, are not so prejudicial, or so fre- 

 quently productive of ill effects upon the human body, as those 

 abstractions of caloric resulting from radiation ; and for this 

 reason, — because in the former instance the morbific particles 

 are dispersed, and so diluted by the aerial currents as to be 

 rendered incapable of exercising any injurious influence upon 

 the body, or only upon such as are rendered extremely sen- 

 sible to the exciting causes of disease ; whereas in the latter 

 instance they become often greatly accumulated, and so highly 

 prejudicial, that few escape. 



In 



