RECLAMATION OF MALARIOUS COUNTRIES. 787 



strate that among the conditions which assist the development of the 

 malarious ferment contained in the soil and the excessive air accumu- 

 lation 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 di- 

 rect 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 want- 

 ing, 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 ex- 

 hausted the humidity of the soil, so producing a purely hydraulic 

 amelioration, Avhich, 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 ameliorations, when the surface of the malarious soil is withdrawn 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 com- 

 bining 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 fri- 

 able 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. 



