Malaria. — I spoke of dry heat as a tonic; it is also anti-putrid, it parches foliage to crumbling dust, 
and mummifies the camel that drops exhausted in the desert ; but moist heat favours putrefaction, and 
when vegetable and animal substances decay in stagnant water that process of decay engenders a subtle 
influence that has never been seen. It is called a miasm or malaria, and is supposed to be an exhalation of poisonous gas that rides on the wings of vapour and falls with the dew. It is the penalty of misapplied water, the price demanded for the fertility conferred upon a land by great rains and rivers. Malaria is most virulent as the equator is approached, where it helps to check a too abundant population. The effects of this telluric poison vary in the same country, and in India, malarious fever is different, in one of its great territorial divisions, from what it is in another. This may depend on some peculiarities in the fauna, the flora, and the soil of each malarial nursery-ground. It is not, however, given to all paludal chemistry to evolve malaria, for the water-logged marshes of Taiti and of New Caledonia have been well turned up without causing intermittent fever. French pathologists say it is because these marshes rest on coral reefs, but this will not explain why the paludal districts of Monte Video and Buenos Ayres are free from ague, and why so paludal a country as Paraguay gives rise to rare and easily curable intermittents. In like manner it has been also occasionally remarked in India, that the wind may blow from certain marsh lands 
without giving rise to fever. This shows that we have still much to learn respecting the etiology of ague but it does not justify the belief that malaria is a myth, and that all intermittent and remittent fevers are due to the sole action of cold on the human body. If water would only persist in covering the jungles, marshes, and river banks of India, there would be little malaria — I say little, for it is the belief of many Indian doctors that water may hold the quid ignotum of malaria in solution, and then let it loose on the air; but, as a rule, when the water drains away, it leaves uncovered the decaying vegetable refuse of a previous season, and exposes the mass of animal and vegetable things with which teams the mud of tropical lands. This decomposes, and when the scorching sun has cracked the surface into innumerable fissures many feet deep, malaria rises from the ground. Hence it is the natural produce of alluvial plains; it lies heavily on clay, is less noxious on a sandy and gravelly soil, and it is still less frequently met with on a ferruginous bed. The great forests that are now being ruthlessly cut down, used to let pass the pure air, while they retained and decomposed the poisonous miasmata. When freshwater and seawater fauna and flora decay in diluted seawater, the telluric poison is still more virulent. Man contaminates the ground on 
which he has continued to live for centuries; the dust accumulated by successive generations of men cannot be turned up without its disengaging malaria more or less detrimental to health. This applies principally to venerable towns like Jerusalem or Rome; but it also holds good to a limited extent in the vast plains of Bengal, inhabited by millions from countless ages, the hotbed of malaria and the birthplace of cholera.