Art. LX.—The Maintenance of the Sun's Heat.
[Read before the Auckland Institute, 29th July, 1885.]
The maintenance of the sun's heat: of what possible interest is this? perhaps you will say. Nevertheless, it is recorded that, centuries ago, a man was tending a flock of sheep, when he saw “a flame in the midst of a bush, and the bush burned with fire and was not consumed.” And this man, although he lived at a time and in a country where no spirit of inquiry existed, yet thought that this was a matter of the greatest interest, for he said, “I will now turn aside, and see this great sight, why the bush is not burnt.” Why, then, should we not turn aside for a few minutes, and ask ourselves, How is it that that great blazing mass, which daily lights and warms us, burns, and is not consumed? But it may be said that there is here a pure assumption introduced, to lend a fictitious interest to the subject; that there is nothing more remarkable in the existence of a vast mass of matter at a white heat than at any other temperature, and that the extraordinary statement that the sun is not consumed has no basis in fact, or, at any rate, cannot be proved.
The first point, therefore, for us to consider is, whether the sun does behave in a manner altogether different from ordinary fire, or a white-hot ball; whether he keeps on shining longer and more fiercely than a fire of his size could do. The solution of this question is by no means difficult, though it involves the use
of some very large figures. It is evident that we must first find out how much heat the sun loses in an hour, or a week, or a year, and then compare this amount with the quantity which could be evolved by a hot or burning body as big as the sun.
We are all of us fully aware that the sun radiates heat, but there are probably only a few here who have ever thought about the quantity of heat thus radiated, or who have any definite idea of the enormous thermal loss which the sun daily undergoes.
To learn how much heat the sun loses in a given time, we must measure the amount of radiation on a given area of the earth's surface. Since the radiation is going on simultaneously in all other directions, it is clear that every square mile on the surface of a sphere of which the radius is 95,000,000 miles will be equally warmed; we must therefore, to find the total radiation, multiply the number we have obtained by the number of square miles on the surface of this sphere, that is, by 108,000 million million. Many measurements of the solar radiation have been made with more or less perfect apparatus. The first, which were carried out at the Cape of Good Hope by Herschel, led to the conclusion that the solar radiation on a square mile would raise 47,500,000 lbs. of water from freezing to boiling in an hour. In obtaining this result, however, no account was taken of the large amount of heat absorbed by the atmosphere, an amount which varies with the humidity of the air, and with the obliquity of the sun's rays. Allowing for this atmospheric absorption, and basing our calculation on the experiments of Violle, which are probably the most exact, we find that the solar radiation per square mile per hour, just outside the earth's atmosphere, would raise 85,500,000 lbs. of water from freezing to boiling. Multiplying this number by 108,000 million million, we obtain an expression for the total hourly solar radiation which is, according to Tyndall, sufficient to boil 700,000 millions of cubic miles of ice-cold water per hour. If, now, we consider how this loss would affect the temperature of a hot body of the mass of the sun which received no heat from any source, we find that it would result in a fall of the sun's temperature of at least 2°C. annually, or 10,000° in 5,000 years; yet all the evidence accumulated by geologists goes to show that in bygone ages the sun's rays were no hotter than they now are. If, on the other hand, we suppose that the sun is a vast burning mass, we find that if it were made of solid coal, and were burning at a rate sufficient to yield this enormous supply of heat, it would be all consumed in 6,000 years. As no apparent diminution of the solar radiation has taken place for thousands of years, I was justified in comparing the sun to the bush in the desert, which burnt, yet was not consumed.
Two well-known hypotheses have been set up to account for the maintenance of the sun's heat; the one ascribes it to a great
shower of meteorites, the other to the gradual contraction of the sun's mass. To appreciate the meteoric theory we must remember that whenever the motion of a body is destroyed, and no other motion set up in its place, heat is evolved; thus, when I bring this hammer on this piece of lead its motion is stopped, and the lead thereby becomes hot. (Exp. with thermopile.)
The heat thus generated is proportional to the mass of the moving body and the square of its velocity, and is so great that if the earth fell into the sun, the heat generated would be equal to that obtained from the combustion of 5,600 worlds of solid carbon. There is, therefore, nothing improbable about this meteoric theory, and its supporters go so far as to point to the zodiacal light as material evidence of it, saying that this light is emitted by a vast meteoric cloud. The adequacy of this theory, as regards the possible supply of heat, is well brought out in the following table, which is due to Sir W. Thomson:—
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It is, however, very doubtful whether there is any such supply of meteoric matter as is required by this hypothesis. The earth encounters but little, and there is no valid reason to suppose that the zodiacal light results from meteoric matter.
The second hypothesis, which is due to Helmholtz, refers the sun's heat to the simple contraction of its mass; and, in order to show the sufficiency of this theory, it has been calculated that the contraction of the sun from a nebula the size of the orbit of Neptune to its present bulk would yield a sufficient heat to maintain the present rate of radiation for 120,000,000 years, while a contraction of the sun's diameter of about 300 feet per annum would make up the yearly loss. The chief obstacle to the acceptance of this explanation of the origin of the sun's heat is the fact that the heat due to contraction would be set free throughout the sun's mass, and that it is almost impossible to imagine it reaching the surface in time to prevent the sun's surface from becoming cold.
Either of the two hypotheses which I have briefly put before you account fairly well for the fact that, in the period of a few thousand years during which some sort of written record has been kept, no diminution of the sun's heat has been observed;
but both of them place a limit to the solar life. In the case of the meteoric theory, it cannot be supposed that the supply of meteors is inexhaustible; we must look forward to the time when every stone wandering in the planetary spaces shall have fallen into the sun, and when, therefore, the supply of heat shall cease, a time to be followed at no great distance by the dying away of the solar light. Since a mass of matter cannot go on contracting for ever, it is evident that the shrinkage theory, like the meteoric, cannot invest the sun with the attribute of permanence. In this respect they both fail to commend themselves to the mind, as has been said by Sir W. Siemens: “If either of these hypotheses could be proved, we should only have the satisfaction of knowing that the solar waste of energy by dissipation into space was not dependent entirely upon loss of its sensible heat, but that its existence as a luminary would be prolonged by calling into requisition a limited, though may be large, store of energy in the form of separated matter. The true solution of the problem will be furnished by a theory, according to which the radiant energy which is now supposed to be dissipated into space and irrecoverably lost to our solar system, could be arrested and brought back in another form to the sun itself, there to continue the work of solar radiation.”
In accordance with this idea, Sir W. Siemens propounded a theory regarding the conservation of the sun's heat, which I will endeavour to explain to you. In order to understand this theory we must suppose that the planetary system is immersed in a rarified atmosphere, consisting mainly of hydrogen, marsh gas, carbonic oxide, water vapour, etc.; that this is no unreasonable assumption is made clear to us by the fact that it has been proved by Maxwell, Clausius, and Thomson that it is impossible to assign a limit to a gaseous atmosphere in space. The nature of this interplanetary atmosphere is, moreover, made known to us by the meteorolites which frequently find their way to the earth; these meteorolites contain gases hidden in their pores, which, being other than oxygen or nitrogen, must, one would think, have been derived from the interplanetary spaces. These gases are those just enumerated. Further proof, if any be needed, of the existence of gaseous matter in interstellar space, is furnished by spectrum analysis, which tells us that the nucleus of a comet contains carbon, hydrogen, nitrogen, and probably oxygen.
Having arrived at a conception of an interplanetary atmosphere, we have next to think of the action of the sun upon it. Let us first investigate the action of any revolving body upon the gaseous medium in which it is placed. (Exp., wheel and candles.)
We thus see that the sun must act like a great fan, projecting the gases from its equator, and drawing them in at its poles.
Let us think of the stream of hydrogen, oxygen, marsh gas, etc., arriving near the sun at its poles; the rise of temperature will evidently bring about combustion, with its accompanying great development of heat. The result of the combustion, the aqueous vapour and the carbon dioxide, will flow to the solar equator, and be projected into space. Thus it would appear that the constitution of the interplanetary atmosphere would be gradually altered; but Sir W. Siemens here steps in with the suggestion that the solar radiation would bring back the combined materials to their original condition of separation, thus enabling them again to flow towards the sun, and by their second combustion supply the central power with further energy. It remains to show how this could take place.
There is no fact better known to students of chemistry than the decomposition of substances by heat. Nearly all organic substances and many metallic salts are resolved into simple compounds by exposure to heat, while such stable bodies as the metallic oxides, and even water itself, are broken up at a high temperature. The explanation of this very general phenomenon is as follows:—The substances are made up of particles, which are all exactly alike, and all complex, being themselves formed by an aggregation of atoms. These atoms, within the particle or molecule, are subject to definite periodical motions or vibrations, which increase in amplitude with the temperature. It is therefore evident that, as the motions of the atoms within the molecule gradually increase in violence, the time must arrive when the cohesive forces which hold them together must be overcome, and the atoms flying off in different directions will either remain at large, or will come into contact with others derived from other particles, forming, in the majority of cases, simpler aggregations. The destruction of the particles is, in fact, not unlike that of a fly-wheel which is rotated more and more rapidly, until at length the centrifugal force overcomes the cohesion of the iron, and the wheel flies to pieces.
Now, it has been shown by Tyndall and others, that vapour of water and other gaseous compounds possess a remarkable power of absorbing the vibrations of radiant heat, the violence of the atomic vibrations becoming thereby greatly augmented. Nevertheless, under ordinary circumstances, no decomposition is apparent. At low pressures, however, the decomposition is greatly increased, and it is reasonable to suppose that, at the extremely low pressure which reigns in the interplanetary spaces, the destruction of the molecules would be considerable.
Here, then, we have an hypothesis which explains how the solar radiant energy is not lost, but gathered up by the particles of matter distributed in space, to be poured again into the sun by the great gaseous current which circulates among the planets.
Let me, in conclusion, sum up the main conditions of this hypothesis:—
That aqueous vapour and carbon compounds are present in stellar or interplanetary space.
That these gaseous compounds are capable of being dissociated by radiant solar energy while in a state of extreme attenuation.
That these dissociated vapours are returned to the sun, and exchanged for recombined vapours by the centrifugal action of the sun.
As Sir W. Siemens has remarked: “If these conditions could be substantiated, we should gain the satisfaction that our solar system would no longer impress us with the idea of prodigious waste through dissipation of energy into space, but rather that of well-ordered, self-sustaining action, capable of perpetuating solar radiation to the remotest future.”