156 TREES AS A PROTECTION AGAINST MALARIA. 
almost say—in comparison with the ocean of air from which they 
are drawn and to which they return; and though the exhala- 
tions from bogs, and other low grounds covered with decaying 
vegetable matter, are highly deleterious to human health, yet, 
in general, the air of the forest is hardly chemically distin- 
guishable from that of the sand plains, and we can as little 
trace the influence of the woods in the analysis of the atmos- 
phere, as we can prove that the mineral ingredients of land- 
springs sensibly affect the chemistry of the sea. I may, then, 
properly dismiss the chemical, as I have done the electrical, 
influences of the forest, and treat them both alike, if not as un- 
important agencies, at least as quantities of unknown value in 
our meteorological equation.* Our inquiries upon this branch 
of the subject will accordingly be limited to the thermometrical 
and hygrometrical influences of the woods. There is, however, 
a special protective function of the forest, perhaps, in part, of 
a chemical nature, which may be noticed here. 
Trees as a Protection against Malaria. 
The influence of forests in preventing the diffusion of mias- 
matic vapors is not a matter of familiar observation, and per- 
* Schacht ascribes to the forest:a specific, if not a measurable, influence 
upon the constitution of the atmosphere. ‘‘ Plants imbibe from the air car- 
bonic acid and other gaseous or volatile products exhaled by animals or de- 
veloped by the natural phenomena of decomposition. On the other hand, the 
vegetable pours into the atmosphere oxygen, which is taken up by animals 
and appropriated by them. The tree, by means of its leaves and its young 
herbaceous twigs, presents a considerable surface for absorption and evapora- 
tion; it abstracts the carbon of carbonic acid, and solidifies it in wood, fecula, 
and a multitude of other compounds. ‘The result is that a forest withdraws 
from the air, by its great absorbent surface, much more gas than meadows or 
cultivated fields, and exhales proportionally a considerably greater quantity 
of oxygen. The influence of the forests on the chemical composition of the 
atmosphere is, in a word, of the highest importance.’’—Les Arbres, p. 111. 
See on this subject a paper by J. Jamin, in the Pevue des Deux Mondes for 
Sept. 15, 1864; and, on the effects of human industry on the atmosphere, an 
article in Aus der Natur, vol. 29, 1864, pp. 443, 449, 465, e¢ seg. See also 
ALFRED Maury, Les Moréts de la Gaule, p. 107. 
