CH. xxxvii. LAW OF DEFINITE PROPORTIONS. 401 



stood, and some eminent chemists, such as Berthollet, even 

 doubted whether it was true. Dalton, however, made a re- 

 markable discovery which both proved the truth of the law 

 itself and showed that it meant a great deal more than had 

 been imagined. 



He found that not only are the elements in any one sub- 

 stance always in a fixed proportion, but that each element, 

 such as oxygen, has a weight of its own, and will only com- 

 bine with other elements in proportions of this fixed weight. 

 For example, oxygen will join itself to nitrogen in five dif- 

 ferent proportions, making five different substances, and in 

 each case the same fixed weight of oxygen is added. Thus, 

 if you decompose 2 2 -4 litres of 



Volumes of Weighing 



Nitrogen Oxygen Nitrogen Oxygen 



Nitrous oxide, you will get 2 . . I . .28 grammes 16 grammes 



Nitric oxide 2 . . 2 . . 28 32 



Nitrous acid 2 . . 3 . . 28 ,, 48 



Nitric peroxide 2 . . 4 . . 28 64 



Nitric acid 2 . . 5 . . 28 80 



So that each substance contains one more volume of oxygen 

 compared to the nitrogen than the one before it ; and this 

 volume always weighs 16 grammes, while each volume of 

 nitrogen weighs 14 grammes. 



Oxygen behaves in this way in all compounds, only join- 

 ing itself to other elements in weights of 16 or multiples 

 of 1 6. Thus, if you heat mercury as Lavoisier did, so that 

 it takes up oxygen out of the air, 200 parts by weight of 

 mercury will combine with 16 of oxygen and no more. If 

 you heat carbon with oxygen, 1 2 parts by weight of carbon 

 will take up 16 of oxygen to make carbonic oxide, or twice 

 16 = 32 to make carbonic acid, but it will not take up any- 

 thing between these weights. This same law holds true of 



