THE REIGN OF LAW 161 



In his analyses of air Dalton made use of the old 

 nitric oxide method. In 1802 this led to an inter- 

 esting discovery. If in a tube .3 of an inch wide he 

 mixed 100 parts of common air with 36 parts of 

 nitric oxide, the oxygen of the air combined with 

 the nitric oxide, and a residue of 79 parts of atmos- 

 pheric nitrogen remained. And if he mixed 100 

 parts of common air with 72 of nitric oxide, but in 

 a wide vessel over water (in which conditions the 

 combination is more quickly effected), the oxygen 

 of the air again combined with the nitric oxide and 

 a residue of 79 parts of nitrogen again resulted. But 

 in the last experiment, if less than 72 parts of nitric 

 oxide be employed, there will be a residue of oxygen 

 as well as nitrogen ; and if more than 72, there will 

 be a residue of nitric oxide in addition to the nitro- 

 gen. In the words of Dalton, " oxygen may com- 

 bine with a certain portion of nitrous gas [as he 

 called nitric oxide], or with twice that portion, but 

 with no intermediate portion." 



Naturally these experimental facts were to be ex- 

 plained in terms of the ultimate particles of which 

 the various gases are composed. In the following 

 year Dalton gave graphic representation to his idea 

 of the atomic constitution of chemical elements and 

 compounds. 



O Hydrogen (D Nitrogen 



O Oxygen Carbon 



(DO Nitric oxide () Carbonic acid 

 GCD0 Nitrous oxide 



Much against Dalton' s will his method of indicating 

 chemical elements and their combinations had to 



