Manchester Memoiis, J\>/. Iv. (191 1), No. 0. 15. 



The facts, as established by the note-books, are that 

 Dalton, for the purpose of his inquiry into the composition 

 of the atmosphere, was studying the combination of nitric 

 oxide and oxygen in the year 1803. He was at work on 

 the subject during March and April, and then again in 

 August. On the 4th of August he obtained the well- 

 known result that 100 measures of air could take 36, or 

 72, of nitric oxide.^' His first table of atomic weights 

 was drawn up by the 6th of September. 



The first case of combination in multiple proportions 

 observ^ed by Dalton must have seemed of great importance 

 to him. His observation of August 4th, regarding nitric 

 oxide and the oxygen of the air, is the first of the kind 

 which he recorded. It is difficult to suppose that he can 

 have known an earlier one. Yet Roscoe and Harden 

 think that this case was of comparative unimportance in 

 the development of the atomic theory. Their reason is 

 that the chemical compounds concerned are not sufficiently 

 represented in the first table of atomic weights. The 

 chemical changes, as Dalton understood them, may be set 

 out in the equations : — 



(i) NO + 0-=NO., (nitric acid). 

 (2) 2N0 + = NA ("'trous „ ). 



Certainly, if the whole matter turned on nitrous acid, 

 Roscoe and Harden argue, it is surprising that Dalton 

 ignored this substance in making up his table on September 

 6th. They suggest that the symbol for nitrous acid 

 which appears at the side of the table was added after- 

 wards, probably about the 12th October. Everyone must 

 admit this who inspects the original table, or the photo- 

 graph in Roscoe and Harden 's book. 



Dalton seems to have set aside the case of nitrous acid 



^1 Roscoe and Harden, op. ciL, pp. 34, 38. 



