12 Meldrum, Development of the Atomic Theorr. 



He thought an element R must form oxides in the order 

 RO, RO„, RO3, etc. Thus he regarded sulphurous acid 

 virtually as SO, and sulphuric acid as SOo."'' He recog- 

 nised five oxides of nitrogen, and regarded them as NO, 

 NO„, NO.„ NO4, and NOe."' These ideas of chemical 

 composition are based on the assumption that similar 

 atoms repel one another, an assumption which is also the 

 basis of his system of chemical dynamics. His argument 

 was that because of this force of repulsion, the compound 

 RO is more stable than RO2, RO, than RO,., and so on. 



The line of thought thus opened by the Higginses 

 afterwards proved extremely valuable, but it was not 

 followed up at the time. William Higgins' book, pub- 

 lished in 1789, and re-published in 1791, was read as a 

 contribution to the phlogiston and anti-phlogiston con- 

 troversy. That was the absorbing topic in science then, 

 and nothing else could be duly attended to. 



The history of this eighteenth century movement 

 proved a difficult problem in the succeeding century. It 

 occupied the attention at different times of such persons 

 as William Charles Henry, R. Angus Smith, and, in 

 collaboration, Roscoe and Schorlemmer. There was also 

 a long and doubtful controversy regarding the relative 

 merits of William Higgins and John Dalton, the discussion 

 of which is left to a future paper. 



Angus Smith's estimate of Bryan Higgins is a vastly 

 different one from that advanced in this paper. His main 

 conclusions are, that Bryan Higgins' " opinions on atoms 

 might have been held by the ancients,"'* and " that his 

 theory was not clear, or he would have been led by it to 



--Ibid., pp. 36-37. 



•'" Ibid., pp. 132-135, 165. 



■^* R. Angus Smith, "Memoir of Dalton,'' p. 175. 



