THE SUN'S HEAT.

Progress, Volume I, Issue 5, 1 March 1906, Page 99

THE SUN'S HEAT.

How does the sun maintain its heat ? This has always been a problem of intense interest to the human mind. It was at first supposed, naively, that the sun's heat was maintained by ordinary chemical combustion ; that the sun was a burning fire, and that when the coal, or what not, which the sun was composed, was consumed, there would be an end of light and heat and life. This belief was shown to be untenable, among others by Professor Tait who said : " Take (in mass equal to the sun's mass) the most energetic chemicals known to us and the proper proportion for giving the greatest amount of heat by actual chemical combination and, so far as we yet know their properties, we cannot see the means of supplying the sun's present waste for even

5,000 years It is quite obvious that the heat of the sun cannot possibly be supplied by any chemical process of which we have the slightest conception, . . . This question is quite unanswerable, unless there be chemical agencies at work m the sun of a far more powerful order than anything we meet with on the earth's surface." Next it was supposed that the meteorites falling into the sun could generate heat sufficient to maintain its energy. This also was disproved. Finally then came Helmholtz's theory, based upon the nebular hypothesis, that the heat of the sun might be maintained by its own contraction from a nebular condition. It is not too much to say that in recent years this has been the accepted theory of science, It has always been burdened, however, by the fact that, on this basis, the sun could not have maintained its energy and have illuminated the earth in the past for a time sufficient to account for the observed geological changes. Professor Young says m *-is " General Astronomy," " No conclusion of geometry is more certain than this that the contraction of the sun to its present size, from a diameter even many times greater than Neptune's orbit cannot have been emitting heat at its present rate for more than 18,000,000 years, if its heat has really been generated in this mamier. Finally, Lord Kelvin has calculated the energy lost in the concentration of the sun from a condition of infinite dispersion with the conclusion that it is " on the whole probable that the sun has not illuminated the earth for 100,000,000 years, and almost certain that he has not done so for 500,000,000 years. As for the future we may say with equal certainty that inhabitants of the earth cannot continue to enjoy the light and heat essential to their life for many million years longer, unless sources now unknown to us are prepared 111 the great store houses of creation." We shall see that m radioactivity we have, probably, an additional store of energy. We know that there exists in the sun enormous quantities of the element helium. We know, also, that helium is a decomposition product from radioactive substances, — and finally we know that radioactive substances generate enormous quantities of heat. It is, therefore, possible, and even likely, that there exist in the sun's mass large quantities of radioactive matter, and on this supposition it is easily possible to increase to an enormous extent the duration of the sun's age and heat in the past, and its maintenance for untold millions of years m the future. It may be shown that the presence of 3.6 grams of radium in each cubic metre of the sun's mass is sufficient to account for its present rate of emission of energy, or, calculated m another way, that 2.5 parts by weight of radioactive matter in a million would keep the sun going. Rutherford concludes that if the energy resident in the atoms of the elements is available in the sun that the time during which the sun may continue to radiate at its present rate may be as much as 500 times longer than the maximum limit afforded by Lord Kelvin. We see thus that the depressing conclusion of the older science that the earth must come to an end in a time short in comparison with its past duration, was unwarranted. It may, however, be objected to this conclusion that if the sun possesses radioactivity, this radioactivity ought to be perceptible on earth. But this is not so, for even the most penetrating rays, the gamma-rays, would be practically stopped and absorbed by the earth's atmosphere which is equivalent to 30 inches of mercury.