312 On Malaria. 



them at certain seasons of the year."* The possible origin of this 

 noxious principle on hills will be noticed hereafter. 



If I have given a probable conjecture of the material existence 

 of malaria, I may now proceed to show where it has been found to 

 reside. 



Morasses and jungle thickets, contain the materials for this pesti- 

 lential emanation in greater amount, than any other situations. Veg- 

 etable matters macerated in water, in every stage of existence, from 

 the incipient bud to the last point of decomposition, are here always 

 prepared to send forth such exhalations as the heat may disengage. 

 These are more virulent, as has been already said, in dry than in wet 

 seasons, from the concentration of the poison in a smaller volume of 

 water ; and more abundant from the greater amount of substances ex- 

 posed, which when submerged are inert. During great rains the air 

 has been found wholesome, where in succeeding drought the sickness 

 has been severe, and the mortality frightful. Upon corresponding 

 principles, countries are healthy during inundations. This is strik- 

 ingly exemplified in some of the departments in France, where the 

 lands are flooded every second or third year, when the water is drain- 

 ed off for tillage. The laborers enter upon the land as soon as the 

 waters are off, but not one half ever survive the cultivation of the 

 crop, and the lands are uninhabitable. Ponds, when full to the brim, 

 are not injurious, unless they form a marsh on their borders; but 

 when a drought exposes their muddy margins and bottoms, replete 

 with herbage and aquatic plants, the exhalations are winged with ma- 

 lignant diseases. That this poison becomes sublimated by drought 

 and heat, appears on the melancholy record of the sufferings of the 

 British army in Spain. After the battle of Talavera, when they re- 

 treated in the hottest weather upon the course of the Guadiana, the 

 country was so dry, that the river had ceased to be a continuous 

 stream, but stood in detached pools. " The soldiers then suffered 

 from remittent fevers of such destructive malignity, that the enemy, 

 and all Europe believed that the British host was lost." 



The causes which operate on ponds have a like effect upon canals. 

 Heat and moisture render the herbage on their edges tangled and lux- 

 uriant, the brooding dampness hovering over the half macerated 

 foliage, facilitates its decomposiuon ; then appear fever and ague and 

 lingering complaints, heat and drought as the summer wanes, disen- 



* Volney's View. 



