From photographs taken at Kenwood Observatory, Chicago, March 25,
1895, and kindly loaned by Professor George E. Hale, of the Chicago
University.]
In the ordinary incandescent electric lamp, the brilliant light is
produced by a glowing filament of carbon. The powerful current of
electricity experiences so much resistance as it flows through this
badly conducting substance, that it raises the temperature of the
carbon wire so as to make it dazzlingly white-hot. Indeed the carbon
is thus elevated to a temperature far in excess of that which could
be obtained in any other way. The reason why carbon is employed in
the electric lamp, in preference to any other substance, may be easily
understood. Suppose we tried to employ an iron wire as the glowing
filament within the well-known glass globe. Then when the current was
turned on that iron would of course become red-hot and white-hot;
but ere a sufficient temperature had been attained to produce the
requisite illumination, the iron wire would have been fused into drops
of liquid, the current would have been broken, and the lamp would have
been destroyed. Nor would the attempt to make an incandescent lamp
have proved much more successful had the filament been made of
any other metal. The least fusible of metals is the costly element
platinum, but even a wire of platinum, though it would stand much
more heat than a wire of iron or of steel, would not have retained the
solid form by the time it had been raised to the temperature necessary
for an incandescent lamp.
There is no known metal, and perhaps no substance whatever, which
demands so high a temperature to fuse it as does the element carbon.
A filament of carbon, and a filament of carbon alone, will remain
unfused and unbroken when heated by the electric current to the
dazzling brilliance necessary for effective illumination. This is
the reason why this particular element is so indispensable for our
incandescent electric lamps. Modern research has now taught us that,
just as the electrician has to employ carbon as the immediate agent in
producing the brightest of artificial lights down here, so the sun in
heaven uses precisely the same element as the immediate agent in
the production of its transcendent light and heat. Owing to the
extraordinary fervor which prevails in the interior parts of the sun,
all substances there present, no matter how difficult we may find
their fusion, would have to submit to be melted, nay, even to be
driven off into vapor. If submitted to the heat of this appalling
solar furnace, an iron poker, for instance, would vanish into
invisible vapor. In the presence of the intense heat of the inner
parts of the sun, even carbon itself is unable to remain solid.
It would seem that it must assume a gaseous form under such
circumstances, just as the copper and the iron and all the other
substances do which yield more readily than it to the fierce heat of
their surroundings.
The buoyancy of carbon vapor is one of its most remarkable
characteristics. Accordingly immense volumes of the carbon steam
in the sun soar at a higher level than do the vapors of the other
elements. Thus carbon becomes a very large and important constituent
of the more elevated regions of the solar atmosphere. We can
understand what happens to these carbon vapors by the analogous case
of the familiar clouds in our own skies. It is true, no doubt, that
our terrestrial clouds are composed of a material totally different
from that which constitutes the solar clouds. The sun evaporates the
water from the great oceans which cover so large a proportion of our
earth. The vapor thus produced ascends in the form of invisible gas
through our atmosphere, until it reaches an altitude thousands of
feet above the surface of the earth. The chill that the watery vapor
experiences up there is so great that the vapor collects into little
liquid beads, and it is, of course, these liquid beads, associated in
countless myriads, which form the clouds we know so well.
We can now understand what happens as the buoyant carbon vapors
soar upwards through the sun's atmosphere. They attain at last to an
elevation where the fearful intensity of the solar heat has so
far abated that, though nearly all other elements may still remain
entirely gaseous, yet the exceptionally refractory carbon begins to
return to the liquid state. At the first stage in this return, the
carbon vapor conducts itself just as does the ascending watery vapor
from the earth when about to be transformed into a visible cloud.
Under the influence of a chill the carbon vapor collects into a myriad
host of little beads of liquid. Each of these drops of liquid carbon
in the glorious solar clouds has a temperature and a corresponding
radiance vastly exceeding that with which the filament glows in the
incandescent electric lamp. When we remember further that the entire
surface of our luminary is coated with these clouds, every particle
of which is thus intensely luminous, we need no longer wonder at that
dazzling brilliance which, even across the awful gulf of ninety-three
millions of miles, produces for us the indescribable glory of
daylight.
_Sir Robert Ball will contribute a series of articles on "The Marvels
of the Universe." Six or eight of these articles may be expected
during the coming year_.
[Illustration: THEOLOGICAL SEMINARY BUILDINGS, ANDOVER,
MASSACHUSETTS.]