266 VIETVS, &C. PHYSIOGNOMY OF PLANTS. 



Lyctonia, as I have elsewhere shown to be probable, or 



whether, according to Otfried Miiller, 'the destruction of 



Lyctonia (Leuconia) refers to the Samothracian tradition of 



a great flood, which changed the form of that district/ is 

 a question which it is here unnecessary to decide." 



(9) p. 217 " Precipitation from the clouds" 



The vertical ascent of currents of air is one of the principal 

 causes of the most important meteorological phenomena. 

 Where a desert or a sandy surface devoid of vegetation is 

 surrounded by a high mountain-chain, the sea- wind may be 

 observed driving a dense cloud over the desert, without any 

 precipitation of vapour taking place before it reaches the 

 crest of the mountains. This phenomenon was formerly very 

 unsatisfactorily referred to an attraction supposed to be exer- 

 cised by the mountain-chain on the clouds. The true cause 

 appears to lie in the ascent from the sandy plain of a 

 column of warm air, which prevents the condensation of the 

 vesicles of vapour. The more barren the surface, and the 

 greater the degree of heat acquired by the sand, the higher 

 will be the ascent of the clouds, and the less readily will the 

 vapour be precipitated. Over the declivities of mountains 

 these causes cease. The play of the vertical column of air is 

 there weaker; the clouds sink, and their disintegration is 

 effected by a cooler stratum of air. Thus deficiency of rain 

 and absence of vegetation in the desert stand in a reciprocal 

 action to one another. It does not rain because the barren 

 and bare surface of sand becomes more strongly heated and 

 radiates more heat; and the desert is not converted into a 

 steppe or grassy plain because without water no organic 

 development is possible. 



(10). p. 218 " The indurating and heat-emitting mass of the 

 earth" 



If according to the hypothesis of the Neptunists (now long 

 since obsolete), the so-called primitive rocks were also pre- 

 cipitated from a fluid, the transition of the earth's crust from 

 a condition of fluidity to one of solidity, must have been fol- 

 lowed by the liberation of an enormous quantity of caloric, 

 which would have given rise to new evaporation and new 

 precipitations. The more recent these precipitations, the 



