306 On Malaria. 
France, in Holland, and in many parts of England, and on this conti- 
nent, in several-districts of New England, and in all the middle, west- 
ern, and southern portions of the United States. 
In vast tracts of France and Italy, where the people are not carri- 
ed off by violent attacks, the whole population exposed to this dis- 
ease, under its endless types and varieties, drag on a life of perpetua 
sickness ;: often of incurable intermittents, or a low constantly febrile 
state, with visceral affections ending in dropsy, or some other fatal 
termination. ‘'The countenances of these people are sallow, some- 
times'livid, and so emaciated as to give them the appearance of walking 
spectres.” » Nor are the effects on their mental condition less remarka- 
ble. . ‘* Apathy, recklessness, indolence, and melancholy,” extinguish 
even the natural.desire of improving their condition, or of prolonging 
their lives... The chronic forms into which this intractable fever runs, 
vary.in intensity, with varying climates. In the temperate regions, 
such as England, and the United States, as far south as the 40°th of 
N. lat. they appear in dyspepsia, low spirits, loss of appetite, languor, 
hypochondria, catarrh, imumerable nervous diseases, and consump- 
- oH. If the fact is satisfactorily established, that the foregoing dis 
eases are produced by pestiferous marsh exhalations, or malaria, it is 
of importance to ascertain what are its properties, and in what situa- 
tions, soils, winds, or other phenomena, it has its origin. ° 
» As this destructive influence is not cognizable by our senses, We 
‘oust rest content im the present state of evidence, with such a con- 
ception of it as results from weighing probabilities after “an attentive 
examination, and comparison of facts.—If I have quoted, or do here- 
after quote, extreme cases in support of probabilities, it is because 
they are conclusive to my mind in settling principles, from which, by 
analogical induction, parallels may be discovered in different climates, 
by making due allowance for the difference in quality and amount of 
materials, and of intensity and duration, in the degrees of heat. 
_» Chemical analysis has failed to discover malaria in any visible or 
tangible: form, either when escaping, or acting with its greatest malig- 
nancy. ‘There is an agent, or there are agents developed or created 
by the joint action of heat and moisture, aided perhaps by electricity 
or other subtle powers, upon decaying vegetables, which produce 
malaria. That these agents are aérial cannot be doubted, because 
the atmosphere is the medium through which they act. Chemistry, 
it has made us acquainted with several deadly gases; which 
