FIRST TABLE OF ATOMIC WEIGHTS. 273 



know from himself, in the year 1804. He proved that 

 neither of these gases contained any thing besides carbon 

 and hydrogen, and ascertained, by exploding with oxygen 

 in a Volta's eudiometer, that if we reckon the carbon in 

 each the same, then carburetted hydrogen contains exactly 

 twice as much hydrogen as olefiant gas does, and that 

 "just half of the oxygen expended on its combustion was 

 applied to the hydrogen and the other half to the charcoal. 

 This leading fact afforded a clue to its constitution." 

 "Whereas in the case of olefiant gas two parts of oxy- 

 gen are spent upon the charcoal and one part upon the 

 hydrogen. 



The atomic weight of nitrogen (azote = 4*2) was doubt- 

 less obtained from the consideration of the composition of 

 ammonia, whose atomic weight is given in the Table at 5*2. 

 Ammonia was discovered in 1774 by Priestley; but the 

 composition was ascertained by Berthollet in 1775 by 

 splitting it into its constituent elements by means of elec- 

 tricitv, when he came to the conclusion that it contained 

 193 parts by weight of hydrogen to 807 parts by weight 

 of nitrogen. Dalton assumed that this substance is a 

 compound of one atom of hydrogen with one of nitro- 

 gen, and hence he obtained for the atomic weight of azote 



— =4*2: and 4*2+ i = '5 , 2 as the atomic weight of 



193 T T J & 



ammonia. It is also probable that Dalton made use of 

 the composition of the oxides of nitrogen for the purpose 

 of obtaining the atomic weight of nitrogen. If we take 

 the numbers obtained partly by Davy and partly by him- 

 self, as given on page 318 of the l New System/ as repre- 

 senting the composition of the three lowest oxides, it ap- 

 pears that the mean value for nitrogen is 4*3 when oxygen 

 is taken as 5*5. In all probability the number in this 

 Table (4*2) was obtained from an experiment of Dalton's 

 made at an earlier date. 



