ioo THE POPULAR SCIENCE MONTHLY. 



entirely through this very limited region ; and, even when we have 

 proved this, we should have only made an infinitesimal advance to a 

 proof that gravitation is absolutely universal. 



I do not for a moment assert that our ordinary statement of the 

 law of gravitation is untrue. I merely say that it has not been proved, 

 and we may also add that it does not seem as if it ever could be proved. 

 Most people who have considered the matter will probably believe 

 that gravitation is universal. Nor is this belief unnatural. If we set 

 aside comets' tails, and perhaps one or two other slightly doubtful 

 matters, we may assert that we always find the law of gravitation to 

 be true whenever we have an opportunity of testing it. These oppor- 

 tunities are very limited, so that we have but very slender supports for 

 the induction that gravitation is universal. But it must be admitted 

 that an hypothesis which has practically borne every test which can be 

 applied has very strong grounds for our acceptance : such, then, are 

 the claims of the law of gravitation to be admitted to a place among 

 the laws of Nature. 



The wondrous series of spectroscopic researches by which Mr. Hug- 

 gins has so vastly extended our knowledge should also be here referred 

 to. Mr. Huggins has shown that many of the substances most abun- 

 dant on the 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 nebula?. 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 favor of the universality of gravitation. 



In what has hitherto been said, we have attempted to give an out- 

 line 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 scope of our telescopes. The ray of light is no 

 doubt but a feeble one, but we must take Avhatever information we 

 can obtain, even though it may fall far short of that which an intel- 

 lectual curiosity will desire. The question now before us may be 

 simply stated : Are Ave entitled to suppose that the part of the uni- 

 verse 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 alto- 

 gether exceptional ; that there are stars in other parts quite unlike 

 our stars, composed of different materials, acted upon by different 

 laws, of which we have no conception ? The presumption is, that the 

 materials of which our system is composed are representative of the 

 materials elsewhere. This presumption is strengthened by the very 

 important considerations now to be adduced. 



