158 TREES AS A PROTECTION AGAINST MALARIA. 
sources of disease, situated to the windward of them, are very 
commonly admitted. 
It is argued that, in these cases, the foliage of trees and of 
other vegetables exercises a chemical as well as a mechanical 
effect upon the atmosphere, and some, who allow that forests 
may intercept the circulation of the miasmatic effluvia of 
swampy soils, or even render them harmless by decomposing 
them, contend, nevertheless, that they are themselves active 
causes of the production of malaria. The subject has been a 
good deal discussed in Italy, and there is some reason to think 
that under special circumstances the influence of the forest in 
this respect may be prejudicial rather than salutary, though 
this does not appear to be generally the case.* It is, at all 
* SanvaGnoui, Memorie sulle Maremme Toscane, pp. 213, 214. The sani- 
tary action of the forest has been lately matter of much attention in Italy. 
See Rendieonti del Congresso Medico del 1869 a Firenze, and especially the im- 
portant observations of Setmi1, J? Miasma Palustre, Padua, 1870, pp. 109 e 
seg. This action is held by this able writer to be almost wholly chemical, and 
he earnestly recommends the plantation of groves, at least of belts of trees, 
as an effectual protection against the miasmatic influence of marshes. Very 
interesting observations on this point will be found in EBERMAYER, Die 
Physikatischen Hinwirkungen des Waldes, Aschaffenburg, 1873, B. I., pp. 237 
et seq., where great importance is ascribed to the development of ozone by the 
chemical action of the forest. The beneficial influence of the ozone of the 
forest atmosphere on the human system is, however, questioned by some 
observers. See also the able memoir: Del Miasma vegetale e delle Malattie 
Miasmatiche of Dr. D. PANTALEONI in Lo Sperimentale, vol, xxii., 1870. 
The necessity of such hygienic improvements as shall render the new 
capital of Italy a salubrious residence gives great present importance to this 
question, and it is much to be hoped that the Agro Romano, as well as more 
distant parts of the Campagna, will soon be dotted with groves and traversed 
by files of rapidly growing trees. Many forest trees grow with great luxu- 
riance in Italy, and a moderate expense in plantation would in a very few 
years determine whether any amelioration of the sanitary condition of Rome 
can be expected from this measure. 
It is said by recent writers that in India the villages of the natives and the 
encampments of European troops, situated in the midst or in the neighbor- 
hood of groves and of forests, are exempt from cholera. Similar observations 
were also made in 1854 in Germany when this terrible disease was raging 
there. It is hence inferred that forests prevent the spreading of this malady, 
or rather the development of those unknown influences of which cholera is 
