290 ON THE ORIGIN AKD HISTORY 



from its current is in a perpetual course of being refreslied and 

 renewed, can ever, by any degree of solar heat, be brought into 

 the state of morbific miasmata ; and the evil must therefore 

 reside in the half-dried and drying margin ; for the swamp is 

 no more than this mai'gin rolled up under another shape, and 

 it must be brought into the same degree of dryness, before it 

 can produce any morbific effects, i 



One only condition then seems to be indispensable to the 

 production of the marsh poison, on all surfaces capable of ab- 

 sorption ; and that is, the paucity of water, where it has pre- 

 viously and recently abounded. To this there is no exception 

 in climates of high temperature ; and from thence we may just- 

 ly infer, that the poison is produced at a highly advanced stage 

 of the drying process ; — but, in the present state of our know- 

 ledge, we can no more tell what that precise stage may be, or 

 what that poison actually is, the development of which must 

 necessarily be ever varying, according to circumstances of tem- 

 perature, moisture, elevation, perflation, aspect, texture, and 

 depth of soil, than we con define and describe those vapours 

 that generate typhus fevers, small-pox, and others diseases. 

 The marsh and the stagnant pool will no doubt be pointed out 

 as the ostensible sources from which this poison has ever 

 sprung ; but the marsh, it has been seen, is never pestiferous 

 when fully covered with water. At all other times it must 

 present a great variety of drying surface, and both the lake 

 and the marsh must ever possess their saturated, half-dried, 

 and drying margins. It is from these that the poison uni- 

 formly emanates, and never from the body of the lake or pool, 

 and I think it may even be fairly presumed that water, for as 

 long as it can preserve the figure of its particles above the sur- 

 face, is innoxious, and that it must first be absorbed into the 

 soil, and disappear to the eye, before it can produce any mis- 

 chievous, 



