266 SCIENCE PROGRESS. 



quite independent both of the chemical nature of the gas 

 and of the number of atoms in its molecule. If we take a 

 certain volume of hydrogen and convert it into water gas, 

 H 2 0, the volume remains unaltered. Further if we suppose 

 that by adding ethylene C 2 H 4 we could convert the water 

 into alcohol gas QH^O, the volume would still remain un- 

 altered. By adding ethylene once more we may suppose 

 that butyl alcohol, C 4 H 10 O, is formed. The volume of butyl 

 alcohol gas would be precisely that of the hydrogen from 

 which we started, and so on. 



Just as the presence of additive properties in matter 

 forms the foundation of the Atomic Theory, so the appear- 

 ance of colligative properties forms the basis of the Molecu- 

 lar Theory. Colligative properties may be explained on 

 the assumption that there exist certain independent groups 

 of atoms, the molecules, which behave entirely in virtue of 

 their number and irrespective of their nature and chemical 

 composition. These colligative properties therefore lead 

 to the molecular theory, and every such property may be 

 made use of in order to determine the relative complexity 

 or number of the molecules. 



For example, according to Avogadro's law, equal volumes 

 of gases, under the same conditions of temperature and 

 pressure, contain the same number of molecules. This 

 law may also be stated in another way, namely : — 



Under the same conditions of temperature and pressure 

 the molecular weights of gases are directly proportional to 

 their densities. . 



It is thus easy to determine the molecular weight of 

 gases from their densities, the molecular weight of hydrogen 

 being chosen = 2 for reasons which are well known to 

 chemists. 



If it were found necessary to determine the molecular 

 weight of a gas without a knowledge of its density, this 

 could be done by measuring the rate of alteration of its 

 volume energy with temperature. The volume energy of a 

 gas is the product of its volume and pressure, pv ; and it 

 we choose as the unit of volume of a gas, the volume in litres 

 occupied by the molecular weight expressed in grams, and 



