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.