Dr. Reynolds on Meteors. . 269 
ihe expansion of volume, specific levity, and subtilty of arti- 
ficial gases, are in a direct proportion to the absolute quanti- 
ty of caloric they employ ; and the caloric is in the same pro- 
portion to the insolubility of the substance with which it 
unites. 
_ bthly. 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 obe- 
dience 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 solids, by bringing fresh portions of the menstruum into suc- 
cessive contact with their fragments, and thus exposing a lar- 
ger surface. : 
_ Under the second headI proceed to notice the situation of 
the earth’s surface in respect to the sun, Sc. he atmos- 
phere is a thin, elastic, gravitating fluid, that completely en- 
velopes the earth, to which it may be considered asa 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 believe 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, cxteris 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 
nd, the summits and sides of great ranges of mountains, mar- 
Bins of oceans, rivers, &c. present an almost naked surface to 
