154 TEEES AS A PEOTECTIOlSr AGAIl^-ST MALAEIA. 



low grounds covered witla decaying vegetable matter, are BigUy 

 deleterious to human health, yet, in general, the air of the forest 

 is hardly chemically distinguishable from that of the sand-plains, 

 and we can as little trace the influence of the woods in the analy- 

 sis of the atmosphere, as we can prove that the mineral ingredi- 

 ents of land-springs sensibly affect the chemistry of the sea. I 

 may, then, properly dismiss the chemical, as I have done the elec- 

 trical, influences of the forest, and treat them both alikie, if not as 

 unimportant agencies, at least as quantities of unknown value in 

 our meteorological equation.* Our inquiries upon this branch of 

 the subject will accordingly be hmited to the thermometrical and 

 hygrometrical influences of the woods. There is, however, a spe- 

 cial protective function of the forest, perhaps in part of a chemi- 

 cal nature, which may be noticed here. 



Trees as a Protection agamst Mala/ria. 



The influence of forests in preventing the diffusion of mias- 

 matic vapors is not a matter of familiar observation, and perhaps 

 it does not come strictly within the sphere of the present inquiry, 

 but its importance will justify me in devoting some space to the 

 subject. " It has been observed " (I quote from Becquerel) 

 " that humid air, charged with miasmata, is deprived of them in 

 passing through the forest. Kigaud de Lille observed locahties 



* Schacht ascribes to the forest a specific, if not a measurable, influeace, 

 upon the constitution of the atmosphere. "Plants imbibe from the air car- 

 bonic acid and other gaseous or volatile products exhaled by animals or devel- 

 .oped 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 proportionately 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."— Xes Arbres, p. 111. 



See on this subject a paper by J. Jamin, in the B^vue 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, et seq. See also Al- 

 fred Maury, Les Forets de la Oaule, p. 107. 



