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Dr. Reyiiolds on Meteors. 269 



the expansion of volume, specific levity, and subtilty of artifi- 

 cial gases, are in a direct proportion to the absolute quantity 

 of caloric they employ ; and the caloric is in the same pro- 

 portion to the insolubility of the substance with which it 

 unites. 



Sthly. When the specific gravity of bodies on the surface 

 of the earth, is reduced below that of the superincumbent at- 

 mosphere, they ascend to media of their own density, in obedi- 

 ence to the laws of Aerostatics ; thus we raise balloons by 

 filling them with light air, and the carbon of pit coal and com- 

 mon wood exposed to combustion, and water to the sun's rays, 

 will rise until they reach a medium of like specific gravity with 

 themselves. 



6thly. Mechanical agitation and division assist the solution 

 of soUds, by bringing fresh portions of the menstruum into suc- 

 cessive contact with their fragments, and thus exposing a lar- 

 ger surface. 



Under the second head I proceed to notice the situation of 

 of the earth's surface in respect to the sun, &c. The atmos- 

 phere is a thin, elastic, gravitating fluid, that completely en- 

 velopes the earth, to which it may be considered a kind of 

 appendage or external covering ; its base resting on the earth's 

 surface, is of an uniform density, growing rare as it recedes 

 therefrom, in a due ratio to the diminution of its gravitating 

 force, until it is lost in empty space. The atmosphere is esti- 

 mated on certain data to be about 44 or 45 miles high, but 

 we have good reasons to beheve it fills a much wider circle, 

 though too thin to reflect the rays of light above its reputed 

 height. 



The earth presents one whole hemisphere to the sun in un- 

 erring daily succession ; and those parts of it which have the 

 least protection against his rays, will, casteris paribus, suffer 

 the greatest intensity of their action. Within the tropics, the 

 atmosphere opposes less resistance to the sun's rays than in 

 the temperate zones ; and in both large tracts of cultivated 

 land, the summits and sides of great ranges of mountains, mar- 

 gin of oceans, rivers, &c. present an almost naked surface to 



