142 CHEMICAL AND METEOROLOGICAL INFLUENCE OF FORESTS. 
upon good authority, that a similar increase in the frequency 
and violence of hail storms in the neighborhood of Saluzzo 
and Mondovi, the lower part of the Valtelline, and the terri¬ 
tory of Verona and Vicenza, is probably to be ascribed to a 
similar cause.* 
Chemical Influence of the Forest. 
We know that the air in a close apartment is appreciably 
affected through the inspiration and expiration of gases by 
plants growing in it. The same operations are performed on 
a gigantic scale by the forest, and it has even been supposed 
that the absorption of carbon, by the rank vegetation of earlier 
geological periods, occasioned a permanent change in the con¬ 
stitution of the terrestrial atmosphere, f To the effects thus 
produced, are to be added those of the ultimate gaseous decom¬ 
position of the vast vegetable mass annually shed by trees, and 
of their trunks and branches when they fall a prey to time. 
But the quantity of gases thus abstracted from and restored 
to the atmosphere is inconsiderable—infinitesimal, one might 
almost say, in comparison with the ocean of air from which 
they are drawn and to which they return; and though the 
exhalations 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 chemi¬ 
cally distinguishable from that of the sand plains, and we can 
as little trace the influence of the woods in the analysis of the 
atmosphere, as we can prove that the mineral ingredients of 
* Le Alpi che cingono VItalia, i, p. 377. 
t “ Long before the appearance of man, * * * they [the forests] 
had robbed the atmosphere of the enormous quantity of carbonic acid it 
contained, and thereby transformed it into respirable air. Trees heaped 
upon trees had already filled up the ponds and marshes, and buried with 
them in the bowels of the earth—to restore it to us after thousands of ages 
in the form of bituminous coal and of anthracite—the carbon which was 
destined to become, by this wonderful condensation, a precious store of 
future wealth.” —Clave, Etudes sur VEconomie Forestiere , p. 13 . 
This opinion of the modification of the atmosphere by vegetation is 
contested. 
