CHEMISTRY OF THE EARTH 93 



It has been said above that two carbon 

 atoms can unite and so partially saturate 

 each other, and leave over four positions 

 which can be occupied by four hydrogen 

 atoms so giving a compound of two carbon 

 and four hydrogen atoms. But how, it may 

 be asked, is it known that this compound does 

 not consist of one carbon atom and two 

 hydrogen atoms, the relative proportion of 

 carbon and hydrogen would be just the same, 

 and then carbon would be a dyad, since it 

 unites with two hydrogen atoms ? The 

 proof lies in the weight of the gas formed 

 relatively to that of hydrogen, when equal 

 volumes are weighed. All gases contain in 

 equal volumes, under like conditions, the same 

 number of molecules, as is proven by many 

 concordant facts in physics and chemistry. 

 This gives a criterion, for it is obvious that, 

 if in one case the gas molecule contained one 

 atom of carbon and two of hydrogen and in 

 the other, two atoms of carbon and four of 

 hydrogen (since the weight of the volume of 

 gas is the united weight of all the molecules), 

 then in the second case the gas must be twice 

 as heavy as the first. The latter result is the 

 one found experimentally, the density, or 

 weight per equal volume, of the gas compared 

 to hydrogen shows that its molecule must 



