288 THE POPULAR SCIENCE MONTHLY. 



Let us consider the properties of two kinds of molecules, thjse of 

 oxygen and those of hydrogen. 



We can procure specimens of oxygen from very different sources — 

 from the air, from water, from rocks of every geological epoch. The 

 history of these specimens has been very different, and, if, during thou- 

 sands of years, difference of circumstances could produce difference of 

 properties, these specimens of oxygen would show it. 



In like manner we may procure hydrogen from water, from coal, 

 or, as Graham did, from meteoric iron. Take two litres of any 

 specimen of hydrogen, it will combine with exactly one litre of any 

 specimen of oxygen, and will form exactly two litres of the vapor of 

 water. 



Now, if, during the whole previous history of either specimen, 

 whether imprisoned in the rocks, flowing in the sea, or careering 

 through unknown regions with the meteorites, any modification of 

 the molecules had taken place, these relations would no longer be 

 preserved. 



But we have another and an entirely different method of comparing 

 the properties of molecules. The molecule, though indestructible, is 

 not a hard, rigid body, but is capable of internal movements, and when 

 these are excited it emits rays, the wave-length of which is a measure 

 of the time of vibration of the molecule. 



By means of the spectroscope the wave-lengths of different kinds 

 of light may be computed to within one ten-thousandth part. In this 

 way it has been ascertained, not only that molecules taken from every 

 specimen of hydrogen in our laboratories have the same set of periods 

 of vibration, but that light, having the same set of periods of vibra- 

 tion, is emitted from the sun and from the fixed stars. 



We are thus assured that molecules of the same nature as those of 

 our hydrogen exist in those distant regions, or at least did exist when 

 the light by which we see them was emitted. 



From a comparison of the dimensions of the buildings of the Egyp- 

 tians with those of the Greeks, it appears that they have a common 

 measure. Hence, even if no ancient author had recorded the fact that 

 the two nations employed the same cubit as a standard of length, we 

 might prove it from the buildings themselves. We should also be 

 justified in asserting that at some time or other a material standard 

 of length must have been carried from one country to the other, or 

 that both countries had obtained their standards from a common 

 source. 



But in the heavens we discover by their light, and by their light 

 alone, stars so distant from each other, that no material thing can ever 

 have passed from one to another ; and yet this light, which is to us the 

 sole evidence of the existence of these distant worlds, tells us also that 

 each of them is built up of molecules of the same kinds as those which 

 we find on earth. A molecule of hydrogen, for example, whether 



