PRINCIPLES OF CHEMISTRY 107 



school, coming about the same time as the discovery of radium, 

 we should be perhaps justified in saying that the opening of the 

 twentieth century is a new epoch in chemistry. For in these last 

 few years chemists have had to get accustomed to the idea that 

 the atoms of Dalton, previously supposed to be indestructible, 

 are complex structures all of which can be broken up, some even 

 undergoing spontaneous disintegration. 



Leaving generalities we may now give a brief statement of 

 the fundamental principles accepted generally by chemists at 

 the present day, in order that what follows may be intelligible 

 to the general reader. 



First of all great principles in which chemistry is concerned is 

 the doctrine of the Conservation of Mass. This means that 

 though matter may be transformed in appearance and qualities 

 none of it is lost or destroyed. When a candle is burned it 

 slowly disappears, but when arrangements are made for catching 

 and weighing the gaseous products of its combustion these are 

 found to be made up of the carbon and hydrogen of which the 

 wax is composed, together with oxygen taken from the air. Or 

 when limestone is heated in a limekiln the lime which remains 

 always weighs fifty-six pounds for every hundred pounds of 

 limestone burnt, if the latter is free from impurities. The carbon 

 dioxide, commonly called carbonic acid, which escapes can be 

 shown to represent the missing forty-four pounds, and if com- 

 bined again with the lime will reproduce the original substance. 

 Experiments of this kind were originally published by Black in 

 1777. 



It is true that elaborate experiments have been made in 

 recent times to test the validity of this principle, but none of the 

 results observed so far have shaken confidence in its soundness. 



The practice of weighing carefully led to the discovery, early 

 in the nineteenth century, of the Law of Constant Proportions. 

 Here again is a fundamental principle. The law states that any 

 given chemical compound is always composed of the same 

 elements united in the same proportions. This proposition was 

 finally established by a French chemist, Proust, early in the 

 nineteenth century, notwithstanding much controversy and 

 criticism. It is of course a principle on which rests the whole of 

 quantitative analytical chemistry, and from the practice of 

 which daily evidence of its truth is supplied. In plain language 

 it means that, for example, water is always composed of one 



