76 PROFESSOR W. THOMSON ON THE 
At one-seventh of the Sun’s radius from his surface, this would be about 258 
miles per second ; and, therefore, a comet approaching so near the Sun, could not 
have a less velocity relatively to the resisting medium than 107 miles per second, 
and, if going against the stream, might have as great a relative velocity as 623 
miles. On the other hand, the great body of the meteors circulating round the 
Sun, and carrying the resisting medium along with them, may be moving through 
it with but small relative velocities ; the smaller for each individual meteor, the 
smaller its dimensions. The effects of the resistance must, therefore, be very 
eradual in bringing the meteors in to the Sun, even when they are very near his 
surface ; and we cannot tell how many years, or centuries, or thousands of years, 
each meteor, according to its dimensions, might revolve within a fraction of the 
Sun’s radius from his surface, before falling in, if it continued solid ; but we may 
be sure that it would so revolve long enough to take, in its outer parts at least, 
nearly the temperature of that portion of space; and therefore, probably, unless 
it be of some substance infinitely less volatile than any terrestrial or meteoric 
matter known to us, long enough to be wholly converted into vapour: (the mere 
fact of a comet* escaping from so near the Sun as has been stated, being enough 
to show that there is, at such a distance, no sufficient atmospheric pressure to 
prevent evaporation with so high a temperature). Even the planet Mercury, if the 
Sun is still bright when it falls in, will, in all probability, be dissipated in vapour 
long before it reaches the region of intense resistance; instead of (as it would 
inevitably do if not volatile) falling in solid, and in a very short time (perhaps a 
few seconds) generating three years’ heat, to be radiated off in a flash which 
would certainly scorch one half of the earth’s surface, or perhaps the whole, as 
we do not know that such an extensive disturbance of the luminiferous medium 
would be confined by the law of rectilineal propagation. Each meteor, when vola- 
tilized, will contribute the actual energy it had before evaporation to a vortex 
of revolving vapours, approaching the sun spirally to supply the place of the 
inner parts, which, from moving with enormously greater velocities than the parts 
of the Sun’s surface near them, first lose motion by intense resistance, emitting an 
equivalent of radiant heat and light, and then, from want of centrifugal force, fall 
in to the Sun, and, consequently, become condensed to a liquid or solid state at his 
surface, where they settle. The latent heat absorbed by the meteors in evaporation, 
and afterwards partially emitted in their condensation at a higher temperature, is 
* That a comet may escape with only a slight loss by evaporation, if the resistance is not too 
great to allow it to escape at all, is easily understood, when we consider that it cannot be for more 
than a few hours exposed to very intense heat (not more than two or three hours within a distance 
equal to the Sun’s radius from his surface). If it consist of a cloud of solid meteors, the smallest 
fragments may be wholly evaporated immediately ; but all whose dimensions exceed some very mo- 
derate limit of a few feet would, unless kept back by the resisting medium and made to circulate 
round the Sun until evaporated, get away with only a little boiled off from their surfaces, 
