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Show 282 PHYSIOGNOMY OF PLANTS. and prevents the vesicles of vapor from being dissolved. The more complete the absence of vegetation, and the more the sand is heated, the greater is the height of the clouds, and the less can any fall of rain take place. When the clouds reach the mountains, these causes cease to operate; the play of the vertically-ascending atmospheric current is feebler, the clouds sink lower, and dissolve in rain in a cooler stratum of air. Thus, in deserts, the want of rain, and the absence of vegetation, act and react upon each other. It does not rain, because the naked, sandy surface, having no vegetable covering, becomes more powerfully heated by the solar rays, and thus radiates more heat; and the absence of rain forbids the desert being converted into a steppe or grassy plain, because without water no organic development is possible. (1°) p. 234.-" The mass of the ea1·th in solidifying and part·ing with its heat." ·If, according to the hypothesis of the Neptunista, now long since obsolete, the so-called primitive rocks were precipitated from a fluid, the transition of the crust of the earth from a fluid to a solid state must have been accompanied by an enormous Q.isengagement of heat, which would in turn have caused fresh evaporation and fresh precipitations. The later these precipitations, the more rapid, tumultuous, and uncrystalline they would have been. Such a sudden disengagement of heat might cause local augmentations of temperature independent of the height of the pole or the latitude of the place, and independent of the position of the earth's axis; and the temperatures thus caused would influence the distribution of plants. The same sudden disengagement of heat might also occasion a species of porosity, of which there seem to be indications in many enigmatical geological phenomena in sedimentary rocks. I have developed these conjectures in detail in a small memoir, "iiber ursprungliche Porositat." (See my work, entitled Versuche tiber die chemische Zersetzung des Luftkreises, 1799, s. 177; and Moll's Jahrbiicher der berg- und Hiittenkunde, 1797, s. 234.) According to the newer views which I now entertain, the shattered and fissured earth, with her molten ·interior, may long have maintained a high temperature on her oxidized surface, independently of position in |