this assumption added to our solar system, particularly as it aggregated
near the sun in the converging radii, and then it was clear that no such
mass of matter could be there without interfering demonstrably with the
observed course of the interior planets. So another source of the sun's
energy had to be sought. It was found forthwith by that other great
German, Helmholtz, who pointed out that the falling matter through which
heat may be generated might just as well be within the substance of the
sun as without--in other words, that contraction of the sun's heated
body is quite sufficient to account for a long-sustained heat-supply
which the mere burning of any known substance could not approach.
Moreover the amount of matter thus falling towards the sun's centre
being enormous--namely, the total substance of the sun--a relatively
small amount of contraction would be theoretically sufficient to keep
the sun's furnace at par, so to speak.

At first sight this explanation seemed a little puzzling to many laymen
and some experts, for it seemed to imply, as Lord Kelvin pointed out,
that the sun contracts because it is getting cooler, and gains heat
because it contracts. But this feat is not really as paradoxical as it
seems, for it is not implied that there is any real gain of heat in the
sun's mass as a whole, but quite the reverse. All that is sought is
an explanation of a maintenance of heat-giving capacity relatively
unchanged for a long, but not an interminable, period. Indeed,
exactly here comes in the novel and startling feature of. Helmholtz's
calculation. According to Mayer's meteoric hypothesis, there were no
data at hand for any estimate whatever as to the sun's permanency, since
no one could surmise what might be the limits of the meteoric supply.
But Helmholtz's estimate implied an incandescent body cooling--keeping
up a somewhat equable temperature through contraction for a time, but
for a limited time only; destined ultimately to become liquid, solid; to
cool below the temperature of incandescence--to die. Not only so, but
it became possible to calculate the limits of time within which this
culmination would probably occur. It was only necessary to calculate the
total amount of heat which could be generated by the total mass of our
solar system in falling together to the sun's centre from "infinity" to
find the total heat-supply to be drawn upon. Assuming, then, that the
present observed rate of heat-giving h