Maiidicster Memoirs, Vol. Iv. (1910), No. 4- 9 



His most important publication, in connection with 

 the atomic theory, is a " Philosophical Essay Concerning 

 Light" (1776). This essay is very different from what it 

 purports to be. It contains only a fragment — all that 

 was ever published — of the essay on light that Higgins 

 had designed. The major part of the book is simply an 

 expansion and exposition of a " Syllabus of Chemistry," 

 which he had published earlier, in 1774 or 1775, and 

 which is also prefixed to the Essay. 



Higgins had gone to Newton for inspiration : the 

 "Philosophical Essay" is full of quotations from the 

 " Opticks." Nor need there be any wonder at Higgins 

 making his approach to the study of light by way of 

 chemistry, since Newton's views on chemical subjects are 

 to be found in the "Opticks" more than in any other of 

 his books. 



The discovery of new facts always gives a stimulus 

 to speculation. The impulse in Higgins' case came from 

 Joseph Priestley, who showed in the year 1775 that the 

 alkaline substance ammonia, and various acids, hydro- 

 chloric, for instance, can exist in the gaseous state. 

 Higgins thereupon proceeded to adapt the Newtonian 

 conception of a gas to the processes of chemistry. 

 Gaseous particles of the same kind were " mutually repul- 

 sive," but what should happen in case acid and alkali 

 were brought together? Higgins said that the acid 

 particles and the alkaline attracted one another, and 

 formed a neutral salt by combining /^^f/^V■/(^ with particle. 



Higgins laid great stress on this force of repulsion 

 between particles of the same kind. He thought an 

 acid and an alkali must combine with one another in 

 one proportion only, a combination of two particles of 

 acid and one of alkali, or two of alkali and one of acid, 

 being precluded, because the two similar particles 



