LECTURE iv.] HISTORY OF CHEMISTRY. 57 



oxygen and nitrogen in the oxygen compounds of nitrogen. 24 

 He is acquainted with four of the latter. In nitric oxide he 

 finds 7 parts of oxygen for 5 of nitrogen ; its atom is, therefore, 

 the atom of the second order, derived from these elements. 

 In nitric acid, according to his view, there are 14 parts of 

 oxygen for 5 of nitrogen, or two atoms of the former gas for 

 one of the latter. In nitrous oxide, 7 parts of oxygen are com- 

 bined with 10 parts of nitrogen, and in this he therefore 

 assumes two atoms of nitrogen and one of oxygen. Nitrous 

 acid, however, is supposed to contain loj parts of oxygen for 

 5 of nitrogen, and in it he might have assumed two atoms of 

 nitrogen and three of oxygen. He prefers, however, to regard 

 this substance as a compound of nitric acid and nitric oxide. 



Further, he finds in ethylene 5.4 parts of carbon for i of 

 hydrogen, and in marsh gas the same quantity of carbon for 2 

 of hydrogen. On this account, he regards ethylene as consist- 

 ing of atoms of the second order, and assumes the atomic 

 weight of carbon to be 5.4. Carbonic oxide likewise consists 

 of atoms of the second order, since he finds in it 7 parts of 

 oxygen for 5.4 of carbon, while carbonic anhydride has atoms 

 of the third order, because it contains 14 parts of oxygen for 

 5.4 of carbon. 



But Dalton does not always adhere quite rigidly to his own 

 rules. Thus he regards sulphuretted hydrogen as consisting of 

 one atom of sulphur and three of hydrogen, and sulphuric acid, 

 of one atom of sulphur and three of oxygen, whereby he is led 

 to 13 as the atomic weight of sulphur. 



Hence there is, in my opinion, something haphazard in 

 these atomic weight determinations, quite apart from the rules 

 themselves. I shall return afterwards to the question whether 

 the latter are justifiable or not. The numbers advanced by 

 Dalton were thus relative in a twofold manner, if I may so ex- 

 press myself; they were affected by two unknown constants. 

 In the first place they were all determined with reference to an 

 arbitrary standard, and in the second, they were only relatively 



24 New System. I, 215 ; A.C.R. 2. 30-31. 



