A multitude of corpuscles gravitate round the sun, and approach him with sufficient nearness to be attracted by his surface, and fall upon it. These are asteroids, which turn in whirling swarms around the sun. A shower of corpuscles, of meteorolites, may be always falling on his surface. Their fall would cause a great development of caloric, in consequence of the transformation of their enormous speed into heat, and this caloric would suffice, according to the authors of this theory, for the maintenance of solar radiation. Let us quote Professor Tyndall on this point:
"It is easy to calculate both the maximum and the minimum velocity imparted by the sun's attraction to an asteroid circulating round him. The maximum is generated when the body approaches from an infinite distance, the entire pull of the sun being then exerted upon it. The minimum is that velocity which would barely enable the body to revolve round the sun close to its surface. The final velocity of the former, just before striking the sun, would be 390 miles a second, that of the latter 276 miles a second. The asteroid, on striking the sun with the former velocity, would develop more than 9000 times the heat generated by the combustion of an equal asteroid of solid coal; while the shock, in the latter case, would generate heat equal to that of the combustion of upwards of 4000 such asteroids. It matters not therefore whether the substances falling into the sun be combustible or not; their being combustible would not add sensibly to the tremendous heat produced by their mechanical collision. Here then we have an agency competent to restore his lost energy to the sun, and to maintain a temperature at his surface which transcends all terrestrial combustion.
"The very quality of the solar rays—their incomparably penetrative power—enables us to infer that the temperature of their origin must be enormous; but in the fall of asteroids we find the means of producing such a temperature."—P. 423.
The fall of these asteroids on the surface of the sun would be followed by an increase in the bulk of that star, and there has been no such increase since the earliest period of its observation. Also, the augmentation of the sun's bulk by these foreign bodies, would have produced an accelerant motion in the orbits of all the stars, which, however slight, would be distinctly perceptible; whereas, for the 2000 years of celestial observation, whose records we possess, unbroken and perfect regularity in the progression of the stars of our solar world is registered.
There is another objection to this hypothesis. It is that it presupposes a solid and resistant medium in the sun. This medium does not exist, according to the new solar theory, which considers this star to be formed of vapour and of gas, or, at most, of a liquid sphere. Another proof that this resistant medium does not exist, is to be found in the fact that several comets, among others those of 1680, and of 1843, have passed so close to the sun at their perihelion, that their movements must have been greatly disturbed by the resistance of a dense medium. The movements of these comets, were, however, quite unaffected by this cause; they were observed to reappear at the moment indicated by the regular curve of their orbit.
The absence of a resistant medium in the sun has been regarded as so grave an objection by one of the authors of this theory, Mr. Thompson, that he has abandoned it, as incompatible with facts.
Another hypothesis has been proposed, for explaining the maintenance of solar heat. The substances which now form the sun have not always been collected together in their present state of aggregation. At first, his molecules were, relatively, extremely distant from one another, and formed a chaotic, or confused mass. Under the influence of attraction, they drew together by degrees, and agglomerated themselves into a nucleus, which has become the centre of attraction of the whole mass. This simply amounts to saying that the sun began by being in the state of nebulosity, and passed at a later period into the condition of adherent and continuous matter.
"The molecules of solar nebulosity," says Balfour Stewart, "precipitating themselves upon one another, produced heat; as, when a stone is thrown with force from the top of a precipice, heat is also the ultimate form into which the potential energy of the stone is converted."
This system of explanation of the primary origin of the planets is in general favour. Having drawn themselves together to form a continuous whole, the elements of the sun would have changed their physical condition, and the result of this change would have been an enormous escape of heat, sufficient to explain the origin of the solar focus. We know, in fact, that condensation of matter always accompanies an escape of heat; and it has been calculated that a diminution of only a thousandth part from the actual bulk of the sun would suffice to maintain the solar heat for 20,000 years.
M. Helmholtz, the author of this ingenious theory, has also calculated that "the mechanical force equivalent to the mutual gravitation of the particles of the nebulous mass would have been originally equal to 454 times the quantity of mechanical force actually disposable in our system," 453/454 of the force coming from the conatus to the gravitation would therefore have been already expended. The author adds that the 1/454 which remains of this original heat, would suffice to raise the temperature of a mass of water equal to the combined birth of the sun and the planets, to 28,000,000 of degrees centigrade; this is a quantity of heat equal to 2500 times that which would be engendered by the combustion of the entire solar system, supposing it to be turned into a mass of coal.
These calculations are, doubtless, most interesting, but their defect is that they rest upon the conception of the sun's nebulosity, an hypothesis which requires closer examination before it ought to be accepted as the basis of so important a deduction. Besides, if the sun were warmed by a physical cause not in action at the present time, his heat, however great it may be estimated to be, must necessarily have been diminishing as long as the sun has been in existence. Now, we repeat that it does not appear that the heat of the sun has ever suffered any diminution. The theory of nebulosity is therefore no more securely founded in principle than the other hypotheses which have preceded it.