REPORT ON THE LAWS OF CONTAGION. 89 



ticular part of a street ; or to a single building, as a house, a jail, 

 or a penitentiary. When spread over an extensive space, several 

 circumstances hav^e been observed to be favourable to their pro- 

 duction. Such are, situation with respect to the level of the sea, 

 or that of the surrounding country ; the form of the surface, as 

 inclined or flat ; the nature of the soil or substrata ; the quantity 

 and quality of the water ; the state of drainage and cultivation ; 

 the vicinity of forests, and of swamps and marshes. From 

 marshy ground exhalations almost constantly ascend, which give 

 rise to fevers of a peculiar type, called remittents when they oc- 

 casionally abate, and intermittents when the symptoms are ab- 

 sent for distinct intervals. In no instance has a remittent or 

 intermittent been communicated from one individual to another; 

 but intermittents are apt to pass into remittents, and the latter 

 to assume a continued type, when they become decidedly con- 

 tagious. 



2. Marshy exhalations, or miasms, as they may be exclusively 

 called (to distinguish them from animal contagions), are evolved 

 most abundantly in hot weather, from ground which is alter- 

 nately moist and dry, or barely covered Avith water ; not if en- 

 tirely or constantly inundated. Either fresh or sea- water is ade- 

 quate to their production ; but the alternation of the two has, 

 in certain situations, rendered miasms particularly virulent*. 

 Marshy ground, however, is not essential ; for the half-dried 

 gravelly beds of rivers have been observed to occasion fevers of 

 a severe typef. In a few instances newly broken ground is re- 

 corded to have had the same effectj. In general, miasms occupy 

 low situations, insomuch that no greater an elevation than the 

 upper stories of a house has afforded protection against them. 

 But this is not universal, for they have been known to rise to 

 considerable heights§, though in such instances the form of the 

 ground indicates that they have been carried up inclined planes, 

 by winds blowing from the place of their production. The 

 sphere of the activity of marsh miasms surpasses beyond com- 

 parison that of animal contagions, obviouslj' on account of the 

 infinitely greater quantity in which they are generated. The 



* Giorgini (Mem. read to the Royal Academy of Sciences in July 1825) 

 gives a frightful picture of the disease called Malatlie di Caltiva, caused by 

 marshes of this kind at the foot of the Ligurian Apennines. 



•|- Ferguson, Edinhuryh Transactions, ix. 273. 



X A remarkable instance is related in one of the latter volumes of Silliman's 

 American Journal. 



§ According to Monfalcon, {Hist, des Marais, Paris, 1824,) to 1400 or 1600 

 English feet. See also Ferguson, loc. cit. 



