466 Prof. J. C. Maxwell on Molecules. 



We can procure specimens of oxygen from very different 

 sources — from the air, from water, from rocks of every geo- 

 logical epoch. The history of these specimens has been very 

 different ; and if during thousands of years difference of cir- 

 cumstances could produce difference of properties, these speci- 

 mens 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 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 mo- 

 lecule. 



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 mole- 

 cules 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 re- 

 corded the fact that the two nations employed the same cubit as 

 a standard of length, we might prove it from the buildings them- 

 selves. 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 ob- 

 tained 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 



