MOLECULES. 375 



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 thousands of years, difference 

 of circumstances could produce difference of properties, these specimens of oxygen 

 would shew 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 vapour 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 compared 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 vibration, 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 Egyptians 

 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 



