WHAT IS A MOLECULE? 691 



other, is as old as philosophy, 'was first applied to its explanation by 

 the English chemist Dalton in 1807. He suggested that the ultimate 

 particles of matter, or atoms between which union is assumed to take 

 place, have a definite weight ; in other words, that they are distinct 

 masses of matter. In the combination of the two elements in ques- 

 tion, therefore, an atom of iron unites with an atom of sulphur to 

 form a molecule of sulphide of iron ; and the union takes place in the 

 proportion by weight of fifty-six to thirty-two, simply because these 

 numbers represent the relative weights of the two sorts of atoms. 

 Now, Dalton may be wrong, and there may be no such things as 

 atoms ; but every science postulates fundamental principles, of which 

 the only proof that can be offered is a certain harmony with observed 

 facts ; and the chemist assumes the reality of atoms and molecules 

 because they enable him to explain what Avould otherwise be a chaos 

 of unrelated facts. The combining proportions of substances, then, 

 indicate their relative molecular weights ; and, bearing this in mind, 

 we must turn again for a moment to the physical side of the question, 

 to inquire whether, and in what way, the physicist can determine the 

 weight of a molecule. 



Water, alcohol, and ether expand when heated, like other forms of 

 matter, but they do so very unequally. Their vapors, on the other 

 hand, are expanded by heat at exactly the same rate under like con- 

 ditions. The theory supposes that the molecules which are close to- 

 gether in the liquids become widely separated when these are converted 

 into vapors ; and the action of the particles on each other becomes 

 less and less as they are driven farther apart by heat, until at last it 

 is inappreciated. When the molecules of the vapors in question are 

 thus freed from other influences, it is found that heat acts in an exactly 

 similar manner upon each of them ; and this is found to be true of all 

 gaseous bodies. The obvious explanation in the case before us is, that 

 there are the same number of particles within a given space in the 

 vapors of all three liquids. This is the law of Avogadro, which is 

 formulated as follows : " Equal volumes of all substances, when in the 

 form of gas, contain the same number of molecules " ; and we shall see 

 how simply this conception is applied for the purpose of determining 

 the molecular weights of all bodies which are capable of being vapor- 

 ized. It will be understood that we are still dealing, as in the case of 

 chemical combination, with relative weights only. We have no means 

 of ascertaining the absolute weight of a molecule of any substance ; 

 but we can state with perfect accuracy what relation these weights 

 bear to one another. For this purpose, the molecule of hydrogen, 

 which is the lightest body known to science, has been selected as the 

 unit. Calling the weight of a litre of hydrogen one, we find by the 

 balance that a litre of oxygen weighs sixteen ; and as, by Avogadro's 

 law, both litres contain the same number of molecules, the molecule 

 of oxygen is sixteen times heavier than that of hydrogen. The mo- 



