ously
to account one 2,250-millionth part of the solar radiations; each of the
other planets and satellites takes a proportionate share; the rest,
being all but an infinitesmal fraction of the whole, is dissipated
through endless space, to serve what purpose we know not. Now, on the
late Sir William Siemens's plan, this reckless expenditure would cease;
the solar incomings and outgoings would be regulated on approved
economic principles, and the inevitable final bankruptcy would be staved
off to remote ages.

But there was a fatal flaw in its construction. He imagined a perpetual
circulation of combustible materials, alternately surrendering and
regaining chemical energy, the round being kept going by the motive
force of the sun's rotation.[1160] This, however, was merely to perch
the globe upon a tortoise, while leaving the tortoise in the air. The
sun's rotation contains a certain definite amount of mechanical
power--enough, according to Lord Kelvin, if directly converted into
heat, to keep up the sun's emission during 116 years and six days--a
mere moment in cosmical time. More economically applied, it would no
doubt go farther. Its exhaustion would, nevertheless, under the most
favourable circumstances, ensue in a comparatively short period.[1161]
Many other objections equally unanswerable have been urged to the
"regenerative" hypothesis, but this one suffices.

Dr. Croll's collision hypothesis[1162] is less demonstrably unsound, but
scarcely less unsatisfactory. By the mutual impact of two dark masses
rushing together with tremendous speed, he sought to provide the solar
nebula with an immense _original_ stock of heat for the reinforcement of
that subsequently evolved in the course of its progressive contraction.
The sun, while still living on its capital, would thus have a larger
capital to live on, and the time-demands of the less exacting geologists
and biologists might be successfully met. But the primitive event,
assumed for the purpose of dispensing them from the inconvenience of
"hurrying up their phenomena," is not one that a sane judgment can
readily admit to have ever, in point of actual fact, happened.

There remains, then, as the only intelligible rationale of solar
sustentation, Helmholtz's shrinkage theory. And this has a very
important bearing upon the nebular view of planetary formation; it may,
in fact, be termed its complement. For it involves the idea that the
sun's materials, once enormously