*oo NINETEENTH CENTURY. PT. HI. 



into which substances can be decomposed. Before then it 

 had been thought sufficient to say that a substance contained 

 sulphur, mercury, carbon, etc., without saying how much of 

 it there was. But after the discovery of oxygen, when the 

 ical nature of chemical change began to be understood, 

 chemists saw the importance of weighing accurately the 

 different elements into which a substance can be broken 

 up ; and when this had been done for some time, and a 

 great number of analyses had been made, it was seen that 

 any given cliemical compound ahvays contains the same elements 

 combined in the same proportion. 



Thus, for example, all water, whether it comes from rain, 

 snow, dew, steam, or exploded oxygen and hydrogen, will 

 always be found to contain two parts by weight of hydrogen 

 to sixteen parts by weight of oxygen ; so that if you decom- 

 pose 1 8 ounces of water you will collect 



2 volumes of hydrogen weighing I oz. each , . 2 ozs. 

 I volume of oxygen weighing 1 6 ozs. . . . 16 ozs. 



1 8 ozs. 



And this never varies. Again, if you take some ammonia 

 and decompose 1 7 ounces of it you will collect 



3 volumes of hydrogen weighing I oz. each . 

 I volume of nitrogen weighing 14 oz. . 



17 ozs. 



And this again never varies. Wherever you get ammonia 

 it will always be made up of these proportionate weights of 

 hydrogen and nitrogen. 



This combination of the different elements in fixed 

 quantities is called the law of definite proportions. It was 

 hinted at by four chemists before Dalton ; namely Proust, 

 Wenzel, Higgins, and Richter, but it was very little under 



