[126] Travels in Italy, chap. iii.
[127] Le Alpi che cingono l'Italia, i, p. 377.
[128] "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."—Clavé, Études sur l'Économie Forestière, p. 13.
This opinion of the modification of the atmosphere by vegetation is contested.
[129] Schacht ascribes to the forest a specific, if not a measurable, influence upon the constitution of the atmosphere. "Plants imbibe from the air carbonic acid and other gaseous or volatile products exhaled by animals or developed 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 evaporation; 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 Appendix, [No. 23].
[130] Composition, texture and color of soil are important elements to be considered in estimating the effects of the removal of the forest upon its thermoscopic action. "Experience has proved," says Becquerel, "that when the soil is bared, it becomes more or less heated [by the rays of the sun] according to the nature and the color of the particles which compose it, and according to its humidity, and that, in the refrigeration resulting from radiation, we must take into the account the conducting power of those particles also. Other things being equal, silicious and calcareous sands, compared in equal volumes with different argillaceous earths, with calcareous powder or dust, with humus, with arable and with garden earth, are the soils which least conduct heat. It is for this reason that sandy ground, in summer, maintains a high temperature even during the night. We may hence conclude that when a sandy soil is stripped of wood, the local temperature will be raised. After the sands follow successively argillaceous, arable, and garden ground, then humus, which occupies the lowest rank. If we represent the power of calcareous sand to retain heat by 100, we have, according to Schubler,
| For [silicious?] sand | 95.6 |
| " arable calcareous soil | 74.8 |
| " argillaceous earth | 68.4 |
| " garden earth | 64.8 |
| " humus | 49.0 |
"The retentive power of humus, then, is but half as great as that of calcareous sand. We will add that the power of retaining heat is proportional to the density. It has also a relation to the magnitude of the particles. It is for this reason that ground covered with silicious pebbles cools more slowly than silicious sand, and that pebbly soils are best suited to the cultivation of the vine, because they advance the ripening of the grape more rapidly than chalky and clayey earths, which cool quickly. Hence we see that in examining the calorific effects of clearing forests, it is important to take into account the properties of the soil laid bare."—Becquerel, Des Climats et des Sols boisés, p. 137.
[131] "The Washington elm at Cambridge—a tree of no extraordinary size—was some years ago estimated to produce a crop of seven millions of leaves, exposing a surface of two hundred thousand square feet, or about five acres of foliage."—Gray, First Lessons in Botany and Vegetable Physiology, as quoted by Coultas, What may be learned from a Tree, p. 34.
[132] See, on this particular point, and on the general influence of the forest on temperature, Humboldt, Ansichten der Natur, i, 158.