Meteoric masses become luminous and kindle at heights which
p 118
must be regarded as almost devoid of air, of occupied by an atmosphere that
does not even contain 1/100000th part of oxygen. The recent investigations
of Biot on the important phenomenon of twilight* have considerably lowered
the lines which had, perhaps with some degree of temerity, been usually
termed the boundaries of the atmosphere; but processes of light may be
evolved independently of the presence of oxygen, and Poisson conjectured
that a?‘roliteswere ignited far beyond the range of our atmosphere.
Numerical calculation and geometrical measurement are the only means by
which as in the case of the larger bodies of our solar system, we are
enabled to impart a firm and safe basis to our investigations of meteoric
stones.
[footnote] *Biot, 'Trait?? d'Astronomie Physique' (3??me ??d.), 1841, t.
i., p. 149, 177, 238, 312. My lamented friend Poisson endeavored, in a
singular manner, to solve the difficulty attending an assumption of the
spontaneous ignition of meteoric stones at an elevation where the density of
the atmosphere is almost null. These are his words: "It is difficult to
attribute, as is uaually done, the incandescence of a?‘rolites to friction
against the molecules of the atmosphere at an elevation above the earth
where the density of the air is almost null.
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