'198 Dr. Macneven's Exposition of [Sept. 



6. Mr. Dalton, in 1804, turned his attention to the subject, and 

 was struck with the small number of proportions in which simple 

 substances are capable of uniting, and the constancy of these 

 proportions. Thus, if we represent the weight of carbon by 75, 

 we find that carbonic oxide and carbonic acid, the only two 

 compounds of carbon and oxygen, are composed as follows : 



Carbonic oxide of 75 carbon + 100 oxygen 

 Carbonic acid of 75 carbon + 200 oxygen 



So that the quantity of oxygen in carbonic acid is to that in 

 carbonic oxide as 2 to 1. 



If we represent the weight of nitrogen by 175 we find that all 

 the compounds of nitrogen and oxygen are composed as follows: 



Protoxide of nitrogen 175 nitrogen + 100 oxygen 

 Deutoxide of nitrogen 175 + 200 



Hyponitrous acid ... 175 + 300 



Nitrous acid 175 + 400 



Nitric acid 175 +500 



So that the quantity of oxygen in these compounds, supposing 

 the nitrogen to remain always the same, is as the numbers 

 1' 2, 3, 4, 5. 



Similar observations may be made respecting the composition 

 of the metallic oxides, the chlorides, the neutral salts, and all 

 chemical combinations with which we are acquainted. 



The fortunate thought occurred to Mr. Dalton " that those 

 proportional numbers represented the respective weights of the 

 atoms of the combining bodies : " that bodies combine either 



1 atom of one with one atom of another, or with 2 atoms, or with 

 3, 4, 5, or 6 atoms. According to this notion, if we represent 

 the weight of an atom of carbon by 0*75, an atom of oxygen will 

 be 1, and carbonic oxide will be a compound of 1 atom carbon 

 and 1 atom oxygen, and carbonic acid of 1 atom carbon and 



2 atoms oxygen. If the weight of an atom of nitrogen be 1"75, 

 and that of oxygen 1, then the compounds of nitrogen and 

 oxygen are composed as follows: 



Protoxide of nitrogen . . 1 atom nitrogen + 1 atom oxygen 

 Deutoxide of nitrogen. .1 +2 



Hyponitrous acid 1 +3 



Nitrous acid ...., 1 +4 



Nitric acid 1 +5 



The simplicity and beauty of this opinion made a speedy and 

 strong impression upon chemists in general. Its truth is now 

 universally admitted.* 



6. Mr. Higgins, Professor of Chemistry in Dublin, in a work 

 pubhshed by him in 1789, made a near approach to the atomic 



• Thomson, vol. iii. p. 19. Fifth Edition. 



