ORIGIN OF LIFE. 21 
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 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 com- 
paring 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 mole- 
cule. . . . 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 
