662 
Recent Researches on Malaria. [November, 
approximate calculation, two-thirds of the malarious regions 
occur on hills, and even on mountains. 
Experience teaches us, therefore, that malaria occurs only 
when the surface-soil dries up to such an extent that the 
atmospheric air is enabled to penetrate to a certain depth — 
i.e., to those strata which retain their moisture even in the 
hot season. The access of atmospheric air is of such im- 
portance that an old malarial soil can be covered over by 
natural or artificial deposits, and in a manner buried. It 
then produces no malaria as long as the superimposed depo- 
sits form a sufficiently compact layer. But the malaria re- 
appears when the swampy soil is separated from the 
atmosphere only by a thin loose stratum of soil, and espe- 
cially when by any contingency a direCt communication is 
opened up between the atmosphere and the malariferous 
subsoil. 
[Here belong, possibly, the pestilential outbreaks which 
have been recorded as following earthquakes.] 
From this point of view we may regard the appearance 
of malaria on turning up so-called virgin soils, in construct- 
ing railways, canals, harbours, fortifications, &c., in conse- 
quence of the inversion and disturbance of the earth 
occasioned. 
Such and similar observations show that the malarial 
germ inheres in the soil. They prove also that it requires 
the mediation of the atmosphere in order that it may be 
developed, or at least extend itself. The production of 
malaria can therefore occur almost everywhere — in swampy 
soils, and in soils apparently very dry ; in soils rich in 
organic putrescible matter, as well as in such as are almost 
free from such substances. 
The word malaria— bad air — was accordingly accepted al- 
most universally, in order to indicate the cause of intermittent 
and malignant fevers. In consequence of more recent in- 
vestigations we imply by it an agent which may infeCt every 
kind of soil, whatever its geological origin or chemical 
composition. 
The production of malaria takes place, however, only in 
certain well-defined conditions, and there are in this respeCt 
three factors, whose co-operation gives rise to the poison : — 
1. A fairly high temperature. 
2. Permanent moderate moisture of the soil. 
3. Access of the atmospheric air to the moist strata. 
[Unfortunately these are at the same time the elements 
of fertility, so that Nature seems to give man the grim 
choice between famine and pestilence.] 
