312 On Malaria. 
them at certain seasons of the year.”* The possible origin of this 
noxious principle on hills will be noticed hereafter. 
If Ihave 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- 
Jential 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 
awater ; and more abundant from the greater amount of substances ex- 
posed, which when submerged are inert. During great rains the air 
das 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 
Jands 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 
lage, facilitates its decomposition ; then appear fever and ague and 
: ing complaints, heat and drought as the summer wanes, disen- 
* Volney’s View. 
