148 
certain distant stars, matter wliicli is ]3icked up and appro- 
priated by our system in its onward progress tlirougli the 
depths of space.^ This in itself removes any ground for 
regarding comets as possible non-partakers in the universal 
community of matter. The spectroscope has shown how- 
ever — and this is strictly to the point in the present in- 
quiry — that of whatever ingredients comets may be com- 
posed they certainly are not as a class destitute of volatiliz- 
able ingredients.*!' The physical condition of all terrestrial 
substances — and in view of the community of matter the 
physical condition also of extra-terrestrial materials is 
merely a question of temperature and pressure. In short, 
given a sufficiently high temperature and a sufficiently low 
pressure and anything will vaporise. 
The great periodic comet of our system, the body named 
after the illustrious Edmund. Halley pursues an orbital path 
of such eccentricity that it is sixty times more remote from 
the sun in aphelion than in perihelion and is therefore sub- 
jected three thousand six hundred times more to the in- 
fluence of the solar radiance in the latter position than in 
the former, a circumstance very suggestive of meteorological 
phenomena on a grand scale, when we consider that those 
terrestrial changes which to us seem so momentous are 
merely local circumstances resulting from local variations of 
presentation to the sun, from which, as a whole, the earth 
maintains .an all but constant distance. The great comet of 
1843 came from an aphelion point, at which the sun’s ap- 
parent magnitude would only be the two hundredth part of 
one degree, and the influence of his light and heat one ten 
thousandth part of that which we enjoy. But this same 
body in its peri helous passage almost grazed the very surface 
of the sun approaching the photosphere within one-seventh 
of the solar radius, J and being there exjDosed to the terrific 
* See ^"Nature,” Jan. 22ncl, 1874, p. 239. 
t See “Eoscoe’s Spectrum Analysis,” p. 290. 
X Herschel’s Outlines,” tenth edit., p. 400, 
