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 con- 

 verted into a steppe or grassy plain, because without water no organic 

 development is possible. 



( 10 ) p. 234. " The mass of the earth in solidifying and parting 

 with its heat." 



If, according to the hypothesis of the Neptunists, 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 disengagement 

 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 tempera- 

 ture 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, " liber 

 ursprungliche Porositat." (See my work, entitled Yersuche iiber 

 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 



