ATOM. 479 



their various periods of vibration either identical, or so nearly identical that our 

 spectroscopes cannot distinguish them. We might argue from this that these 

 molecules are alike in all other respects, as, for instance, in mass. But it is 

 sufficient for our present purpose to observe that the same kind of molecule, 

 say that of hydrogen, has the same set of periods of vibration, whether we 

 procure the hydrogen from water, from coal, or from meteoric iron, and that 

 light, having the same set of periods of vibration, comes to us from the sun, 

 from Sinus, and from Arcturus. 



The same kind of reasoning which led us to believe that hydrogen exists 

 in the sun and stars, also leads us to believe that the molecules of hydrogen 

 in all these bodies had a common origin. For a material system capable of 

 vibration may have for its periods of vibration any set of values whatever. 

 The probability, therefore, that two material systems, quite independent of each 

 other, shall have, to the degree of accuracy of modern spectroscopic measure- 

 ments, the same set of periods of vibration, is so very small that we are 

 forced to believe that the two systems are not independent of each other. 

 When, instead of two such systems, we have innumerable multitudes all 

 having the same set of periods, the argument is immensely strengthened. 



Admitting, then, that there is a real relation between any two molecules 

 of hydrogen, let us consider what this relation may be. 



We may conceive of a mutual action between one body and another 

 tending to assimilate them. Two clocks, for instance, will keep time with each 

 other if connected by a wooden rod, though they have different rates if they 

 were disconnected. But even if the properties of a molecule were as capable 

 of modification as those of a clock, there is no physical connection of a suffi- 

 cient kind between Sirius and Arcturus. 



There are also methods by which a large number of bodies differing from 

 each other may be sorted into sets, so that those in each set more or less 

 resemble each other. In the manufacture of small shot this is done by making 

 the shot roll down an inclined plane. The largest specimens acquire the greatest 

 velocities, and are projected farther than the smaller ones. In this way the 

 various pellets, which differ both in size and in roundness, are sorted into 

 different kinds, those belonging to each kind being nearly of the same size, 

 and those which are not tolerably spherical being rejected altogether. 



If the molecules were originally as various as these leaden pellets, arid 

 were afterwards sorted into kinds, we should have to account for the dis- 



