the law of gravitation to be admitted to a place among the laws of Nature.
The series of spectroscopic researches by which Sir William Huggins has so vastly extended our knowledge should be referred to. Sir William Huggins has shown that many of the substances most abundant on earth are widely spread through the universe. Take, for instance, the metal iron and the gas hydrogen. We can detect the existence of these elements in objects enormously distant. Both iron and hydrogen exist in many stars, and hydrogen has been shown, in all probability, to be an important constituent of the nebulæ. That the rest of the sidereal system should thus be composed of materials known to be to a large extent identical with the materials in the solar system is a presumption in favour of the universality of gravitation.
In what has hitherto been said, we have attempted to give an outline of the facts so far as they are certainly known to us. Into mere speculations we have no desire to enter. We may, however, sketch out a brief chapter in modern sidereal astronomy, which seems to throw a ray of light into the constituents of the vast abyss of space which lies beyond the range of our telescopes. The ray of light is no doubt but a feeble one, but we must take whatever information we can obtain, even though it may fall far short of that which an intellectual curiosity will desire. The question now before us may be simply stated. Are we entitled to suppose that the part of the universe accessible to our telescopes is fairly typical of the other parts of the universe, or are we to believe that the system we know is altogether exceptional; that there are stars in other parts quite unlike our stars, composed of different
