LUMINOUS APPEARANCE EXPLAINED. 



made on the duration of the morning and evening twilight, it may 

 be considered as proved, that beyond the elevation of thirty miles 

 there exists no atmosphere possessing any sensible mechanical 

 properties. We may safely conclude that at such elevations the 

 air must be so infinitely attenuated as to be divested of all 

 sensible resistance or inertia. The space there must, for example, 

 be more nearly a vacuum than any which could be produced 

 under the receiver of the most perfect air-pump ; how, then, can 

 we imagine such a compression of air so rarefied to be produced, 

 as would be necessary to evolve the enormous temperature requi- 

 site to render luminous the matter composing meteoric stones 1 



To this objection the following very plausible answer has 

 been made. It is known that the quantity of latent heat 

 contained in any proposed volume of air is greater the more 

 rare and attenuated the air is. This is easily proved. Every 

 one knows that when a volume of air at a given temperature 

 expands into a larger volume, its temperature falls, although no 

 heat has been abstracted from it. Since, therefore, it contains 

 the same absolute quantity of heat as it did before it expanded, 

 and since, nevertheless, its sensible heat is less, as is proved by its 

 fall of temperature, it follows that the portion of sensible heat 

 which has disappeared must have become latent, that is to say, 

 the air has augmented its latent at the expense of its sensible 

 heat. 



Since then highly rarefied air contains much more latent heat 

 than air of greater density, and since this excess of latent heat 

 is greater and greater as the rarefaction is greater and greater, 

 it follows that the latent heat of the air in the highest strata of 

 the atmosphere must be immeasurably greater than in the 

 inferior strata, and that the same degree of sudden compression 

 applied to it would develop a much greater amount of this 

 latent heat, and consequently produce a much greater elevation 

 of temperature than in the lower regions. 



It is, therefore, contended that not only notwithstanding the 

 rarefaction of the air in the higher strata, but' 'because of that 

 rarefaction, a meteorite rushing through it with a planetary 

 velocity, would, by the sudden compression of the air driven 

 before it, produce an elevation of temperature sufficient not only 

 to cause a superficial combustion of the meteorite, but to cause 

 its explosion by the sudden expansion and combustion of any 

 volatile or combustible matter which might form part of its 

 constituents. 



9. There is yet another supposition extremely plausible and 

 ingenious which was suggested by Poisson, the eminent French 

 geometer, to explain the evolution of light and heat observed 



137 



