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LECTURE LVII. 



ON AQtrEOFS AND IGNEOUS METEORS. 



J- HE phenomena originating from the evaporation of water constitute a 

 large proportion of the subjects of meteorology: they are materially influenced 

 by the diversities of climates and winds, which we have lately considered; and 

 they appear to contribute to the electrical changes, which form a principal 

 part of luminous or igneous meteors : nor is the action of water wholly un- 

 concerned in many of the effects of subterraneous fires, which have also a 

 slight connexion with atmospherical electricity; and it has been conjectured 

 that the only igneous meteors, which appear wholly independent of any of 

 these phenomena, may originate from volcanic commotions in other worlds. 



The action of heat appears to detach continually from the surface of water, 

 and perhaps of every other liquid, and even solid, a certain quantity of va- 

 pour, in the form of an invisible gas; but when the space above the liquid is 

 already charged with as much vapour as can exist in it at the actual tempe- 

 rature, the vapour, thus continually thrown off, either remains suspended in 

 the form of visible particles, or falls back immediately into the liquid. This 

 is the simplest mode of explaining the continuance of evaporation, under the 

 pressure of any dry gas, however dense, and its apparent suppression in the 

 presence of moist air, however rare. Sometimes also, when the temperature of 

 the liquid is elevated, so that minute globules either of steam or of air rise 

 through it, some visible particles are projected upwards by each globule, and 

 continue to float in the air ; this appears, however, to be an irregularity un- 

 connected with the principal process of slow evaporation. 



The quantity of vapour, which can exist in the space above any portion of 

 water, has been supposed by Deluc, Volta, and Dalton, to be wliolly inde- 

 pendent of the nature, the density, or even the presence of the air or gas 



