4 Clarke, The Atomic Theory. 



Aristotelian logic must verify its premises. The authority 

 of evidence began to replace the authority of the schools. 



Early in the seventeenth century the atomic philosophy 

 of Epicurus was revived by Gassendi, who was soon 

 followed by Boyle, by Newton, and by many others. 

 One other important step was taken also. Boyle, in his 

 " Sceptical Chymist," gave the first scientific definition of 

 an element, a conception which was more fully developed 

 by Lavoisier later, but which received its complete modern 

 form only after Davy had decomposed the alkalies and 

 shown the true nature of chlorine. Without this pre- 

 liminary work of Boyle and Lavoisier, Dalton's theory 

 would hardly have been possible. An elementary atom 

 can be given no real definition unless we have some notion 

 of an element to begin with. But the strongest impulse 

 came from Newton, who accepted atomism in clear and 

 unmistakable terms. Coming before Newton, Descartes 

 had rejected the atomic hypothesis, holding that there 

 could be no vacuum in the universe, and making matter 

 essentially synonymous with extension. True, Descartes, 

 in his famous theory of vortices, imagined whirling particles 

 of various degrees of fineness ; but they were not atoms 

 as atoms and molecules are now conceived. It may be 

 dangerous to pick out landmarks in history, and to assert 

 that such and such a movement began at such and such a 

 time. Nevertheless, we may fairly say that the turning 

 point in physical philosophy was Newton's discovery of 

 gravitation, for that indicated mass as the fundamental 

 property of matter. For any given portion of matter 

 which we can segregate and identify, extension is variable 

 and mass is constant ; when that conclusion was 

 established, the dominance of atomism became inevitable. 

 Boyle, Newton, and Lavoisier were legitimate precursors 

 of Dalton, but whether Boscovich should be so considered 



