their special tints more effectively. But speaking
generally, a stellar spectrum is like the solar spectrum. There is the
rainbow-tinted streak, which implies that the source of light is glowing
solid, liquid, or highly compressed vaporous matter, and athwart the
streak there are the multitudinous dark lines which imply that around
the glowing heart of the star there are envelopes of relatively cool
vapours.

We can understand, then, the meaning of the evidence obtained from the
new star in the Northern Crown.

In the first place, the new star showed the rainbow-tinted streak
crossed by dark lines, which indicated its sun-like nature. _But,
standing out on that rainbow-tinted streak as on a dark background, were
four exceedingly bright lines--lines so bright, though fine, that
clearly most of the star's light came from the glowing vapours to which
these lines belonged._ Three of the lines belonged to hydrogen, the
fourth was not identified with any known line.

Let us distinguish between what can certainly be concluded from this
remarkable observation, and what can only be inferred with a greater or
less degree of probability.

It is absolutely certain that when Messrs. Huggins and Miller made their
observation (by which time the new star had faded from the second to the
third magnitude), enormous masses of hydrogen around the star were
glowing with a heat far more intense than that of the star itself within
the hydrogen envelope. It is certain that the increase in the star's
light, rendering the star visible which before had been far beyond the
range of ordinary eyesight, was due to the abnormal heat of the
hydrogen surrounding that remote sun.

But it is not so clear whether the intense glow of the hydrogen was
caused by combustion or by intense heat without combustion. The
difference between the two causes of increased light is important;
because on the opinion we form on this point must depend our opinion as
to the probability that our sun may one day experience a similar
catastrophe, and also our opinion as to the state of the sun in the
Northern Crown after the outburst. To illustrate the distinction in
question, let us take two familiar cases of the emission of light. A
burning coal glows with red light, and so does a piece of iron placed in
a coal fire. But the coal and the iron are undergoing very different
processes. The coal is burning, and will presently be consumed; the iron
is not burning (except in