ost critical problem of physics
and chemistry, that the extremes of pressure and temperature in the
heavenly bodies, and the prevalence of other physical conditions not
yet successfully imitated on earth, promise the greatest progress.
It fortunately happens that astrophysical research is now at the
very apex of its development, founded as it is upon many centuries
of astronomical investigation, rejuvenated by the introduction
into the observatory of all the modern devices of the physicist,
and strengthened with instruments of truly extraordinary range
and power. These instruments bring within reach experiments that
are in progress on some minute region of the sun's disk, or in
some star too distant even to be glimpsed with ordinary telescopes.
Indeed, the huge astronomical lenses and mirrors now available
serve for these remote light-sources exactly the purpose of the
lens or mirror employed by the physicist to project upon the slit
of his spectroscope the image of a spark or arc or vacuum tube
within which atoms and molecules are exposed to the influence of
the electric discharge. The physicist has the advantage of complete
control over the experimental conditions, while the astrophysicist
must observe and interpret the experiments performed for him in
remote laboratories. In actual practice, the two classes of work
must be done in the closest conjunction, if adequate utilization
is to be made of either. And this is only natural, for the trend
of recent research has made clear the fact that one of the three
greatest problems of modern astronomy and astrophysics, ranking
with the structure of the universe and the evolution of celestial
bodies, is the constitution of matter. Let us see why this is so.

TRANSMUTATION OF THE ELEMENTS

The dream of the alchemist was to transmute one element into another,
with the prime object of producing gold. Such transmutation has been
actually accomplished within the last few years, but the process
is invariably one of disintegration--the more complex elements
being broken up into simpler constituents. Much remains to be done
in this same direction; and here the stars and nebulae, which show
the spectra of the elements under a great variety of conditions,
should help to point the way. The progressive changes in spectra,
from the exclusive indications of the simple elements hydrogen,
helium, nitrogen, possibly carbon, and the terrestrially unknown
gas nebulium in the gaseous nebulae,