482 Transactions. — Miscellaneous. 
in a medium more dense than itself, which is utterly out of the question. 
The most probable theory is that which ascribes the change in apparent 
bulk to the powers of the sun's rays to rarefy, distend, and render invisible 
a large portion of the comet, which portion, upon reaching a cooler region, 
begins again to condense like an evening cloud. 
Indeed, the extremes of heat and cold to which comets are subjected, 
render it easy to conceive of almost any change in their appearance. Even 
in the ease of Halley's comet, the heat and light of perihelion are to the 
heat and light of aphelion as 8,000 to 1. But that comet does not pass 
very near the sun (54,000,000 of miles), nor recede much beyond the orbit 
of Neptune. But the comet of 1843, like that of 1882, passed within 30,000 
miles of the sun's surface. Newton supposed his comet was subjected to a 
heat 2,000 times greater than that of red-hot iron. But the heat endured 
by the body mentioned must have exceeded this by about twenty-fold (for 
Newton's was 180,000 miles from the solar surface). This is a heat at which, 
of course, any terrestrial substance would be volatilized. On the other hand 
this same body will wander to regions where, under similar conditions, it 
would only receive one four hundredth part of the light and heat enjoyed 
by the earth. 
May it not be that such extremes of heat and cold, together with the 
almost total absence of pressure, produce conditions of matter unknown 
and unknowable to us? And may not these unknown and unknowable 
conditions lie at the bottom of some of the problems that are so perplexing 
to the human mind ? 
Yet, something is certainly known of the constitutional elements of a 
few comets. The spectroscope has shown the nucleus, or central part of the 
head of one, to be luminous gas ; while the outer part of the coma shone by 
reflected light. Another comet was found by Dr. Huggins to consist of 
volatilized (not burning) carbon,—the lines in the cometic spectrum agreeing 
exactly with the lines due to carbon in the spectrum of olefiant gas. This 
discovery, however, can only be regarded as adding another to the many 
problems connected with comets; for carbon is notable for its fixity at 
moderate temperatures, and the comet in question was in a temperate. 
region of space. Comets examined since 1868 show hydrogen and other 
elements associated with carbon. 
A peculiar relation is known to exist between comets and meteor 
systems, but it is still involved in obscurity. No less an authority than 
J. C. Adams, the English discoverer of Neptune, has computed the orbit of 
the November meteors and shown that they pass beyond the planet Uranus, 
and have a period of 33} years. He assigned to them exactly the same 
path as that already assigned to Temple’s comet! It is probable that the 
