the actual nature of the elementary bodies which enter iuto the composition of stars? We find that the elementary bodies in these other suns are in the main identical with those which exist in our own sun and in the earth itself. If iron attracts iron by the law of gravitation in the solar system, why should not iron attract iron in the sidereal systems as well? But we are not dependent solely on this presumption for our knowledge of the important fact that the law of gravitation is not confined to the solar system. The movements of binary stars have been studied, and it has been invariably found that the phenomena observed are compatible with the supposition that the law of gravitation prevails throughout the universe.
It would not, however, be correct to assert, as has been sometimes done, that the facts of the binary systems actually prove that gravitation is the all-compelling force there as here. The circumstances do not warrant us in expressing the matter quite so forcibly. The binary stars are so remote that the observations which we are enabled to make are wanting in the almost mathematical precision which we can give to such work when applied to the bodies of our own system. It is quite possible for mathematical ingenuity to devise a wholly arbitrary and imaginary system of force, which might explain the facts of binary stars, as far as we are able to observe them, on quite another hypothesis than the simple law that the attraction between two particles varies with the inverse square of the distance. No one, however, will be likely to doubt that it is the law of gravitation, pure and simple, which prevails in the celestial spaces, and consequently we are able to make use of it to explain the circumstances attending the movement of Algol's dark companion.
