ON SOME OF THE STARS AND NERUL2E. 
561 
of these prismatic observations. Some comets have approached the sun sufficiently near 
to acquire a temperature high enough to convert even carbon into vapour*. Indeed 
for these comets a body of great fixity seems to be necessary. In the case of comets 
which have been submitted to a less fierce glare of solar heat, it may be suggested that 
this supposed difficulty is one of degree only ; for we do not know of any conditions 
under which even a gas, permanent at the temperature of the earth, could maintain 
sufficient heat to emit light, a state of things which appears to exist permanently in 
the case of the gaseous nebulse. 
If the substance of the comet be taken to be pure carbon, it would appear probable 
that the nucleus had been condensed from the gaseous state in which it existed at some 
former period. It would therefore probably consist of carbon in a state of excessively 
minute division. In such a form it would be able to take in nearly the whole of the 
sun’s energy, and thus acquire more speedily a temperature high enough for its conver- 
sion into vapour. In the liquid or gaseous state, or in a continuous solid state, this 
substance appears, from Dr. Tyndall’s researches, to be diathermanous. Still, under the 
most favourable of known conditions, the solar heat, to which the majority of comets are 
subjected, would seem to be inadequate to the production of luminous vapour of carbon. 
It should be stated that olefiant gas when burnt in air may give a similar spectrum of 
shaded bands. If the gas be ignited at the orifice of the tube from which it issues, the 
flame is brilliantly white, and gives a continuous spectrum. When a jet of air is directed 
through the flame it becomes less luminous, and of a greenish-blue colour. The spec- 
trum is now no longer continuous, but exhibits the bands distinctive of carbon. Under 
these circumstances, for obvious reasons, the bright lines of the hydrogen spectrum are 
not seen. In this way a spectrum resembling that of the comet may be obtained, with 
the difference that the fourth more refrangible band, which was not seen in the cometic 
spectrum, is stronger relatively to the other bands, than is the case when the spark is 
taken in olefiant gas. If we were to conceive the comet to consist of a compound of 
carbon and hydrogen, we should diminish in some degree the necessity for the excessively 
high temperature which pure carbon appears to require for its conversion into luminous 
vapour ; but other difficulties would arise in connexion with the decomposition we must 
then suppose to take place ; for we have no evidence, I believe, that olefiant gas or any 
other known compound of carbon can furnish this peculiar spectrum of shaded bands 
without undergoing decomposition. If, indeed, it were allowable to suppose a state of 
combustion, with oxygen or some other element, set up by the solar heat, we should have 
an explanation of a possible source of a degree of heat sufficient to render the cometary 
matter luminous, and which the sun’s heat would be directly inadequate to produce. 
* The comet of 1843 “ approached the luminous surface of the sun within about a seventh part of the sun’s 
radius. The heat to which the comet was subjected (a glare as that of 47,000 suns, such as we experience 
the warmth of) surpassed that in the focus of Parker’s great lens in the proportion of 24| to 1 without, or 
3| to 1 with the concentrating lens. Yet that lens so used melted cornelian, agate, and rock-crystal.” — Sir 
John Herschel, Outlines of Astronomy, 7th edit. p. 401. 
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