strate that among the conditions which assist the development of the malarious ferment contained in the soil and the excessive air accumulation of that ferment in the air, there are three of primary importance, as their concurrence is indispensable to the production of malaria. These are (1), a temperature not lower than 20° C.; (2), a moderate degree of permanent humidity in the malarious soil; and (3), the direct action of the oxygen of the air on the strata of the soil which contains the ferment. If one only of these three conditions be wanting, the development of malaria becomes impossible. Now, this is an important point in the natural history of malaria, as giving us the key to the chief part of the soil reclamation attempted by man.
First, let us take Nature's amelioration of the malarious countries, suspending as she does for a longer or shorter time the production of malaria. Winter, for example, causes in all these countries a purely thermic amelioration—that is, it suspends the production of malaria simply by making the temperature fall below the minimum required for the development of the poison. In fact, there are often, even in winter, sudden outbreaks of malaria when a sirocco-wind raises the temperature above this minimum. Again, during a very warm and dry summer, malaria is not developed, because the sun's rays have exhausted the humidity of the soil, so producing a purely hydraulic amelioration, which, as in the Roman Campagna, in 1881-'82, may last for a considerable time; easily to be dissipated, however, by one steady shower. Finally, there may occur in nature purely atmospheric ameliorations, when the surface of the malarious soil is withdrawn from the direct action of the oxygen of the air by means of natural earth-coverings formed by alluvial deposits of healthy soil, or by means of the "earth-felt" wrought up from the soil by the roots of herbage in a natural meadow.
In their various attempts to suspend the development of malaria from the soil, men have tried to imitate Nature—to eliminate, that is to say, one of the three conditions indispensable to the multiplication of the specific ferment contained in that soil. Naturally enough, they have never attempted thermic ameliorations, such as Nature effects in winter, because it is not in their power to control the sun's rays. They have had to restrict their efforts to either hydraulic or atmospheric ameliorations; but sometimes they have succeeded in happily combining the one and the other—that is, in eliminating at once the humidity of the soil and the direct action of the oxygen of the air upon it.
Hydraulic amelioration has assumed many forms, according to the nature and site of the malarious soil. Drainage, in which the ancient Romans excelled us, has been practiced in Italy both in deep and friable soils and in subsoils compact and almost impermeable, in which latter the "cunicular" drains of the Etruscans, Latins, Volscians, and Romans might even nowadays be studied with advantage.
