THE FURTHER GROWTH OF SCIENCE 97 



Again, the later years of the eighteenth century were im- 

 portant in the history of chemistry and physics. Such 

 generalizations as the Atomic Theory, and the theories of 

 Chemical Combination, Conservation of Energy, and Inde- 

 structibility of Matter were at least tentatively established. 

 When the nature of elements, compounds, and solutions 

 became sufficiently clear, chemistry could progress. When 

 the distinction between matter and energy was recognized, 

 the great advance made by physical science during the 

 nineteenth century became possible. 3 



The development of the atomic theory is an example of 

 how the theories of science may have their beginnings and 

 may reach their existing limits in philosophical speculation. 

 The doctrine that matter consists of invisible and indivisible 

 particles between which is a void appears in Hindu philos- 

 ophy at a remote period. Among the Greco-Roman philos- 

 ophers, Democritus and Lucretius were its ardent exponents. 

 During the Middle Ages theological beliefs sufficed. In the 

 seventeenth century Boyle and also Newton recognized its 

 advantages. The former had even grasped the idea of 

 permanent elements and changeable compounds, and had 

 defined chemistry as "the composition of substances." 

 But the Phlogiston-Theory intervened. 4 



3 These generalizations have persisted as the foundations of modern physico- 

 chemical theory, although the structure and possible divisibility of the atom 

 is a subject of present-day investigation. The facts discovered in connection 

 with radium have made the chemist skeptical regarding the permanency of 

 unchanging elements. The theory of the conservation of energy is, of course, 

 based upon experimentation within limited fields. Nevertheless, these funda- 

 mental hypotheses, which made their appearance toward the close of the 

 eighteenth century, have constituted the point of departure for subsequent 

 physico-chemical investigation. 



4 Robert Boyle (1627-1691) stated the principle that only tangible and 

 ponderable substances should be recognized as elements, a chemical element 

 being a substance from which other substances could be made. Georg Stahl 

 (1660-1734) developed the Phlogiston-Theory. Phlogiston was "the principle 

 of combustion," and was considered as a definite body and hence material. 

 All chemical action was explained as due to the presence of this all-important 

 substance. This concept delayed the establishment of the atomic theory, but 



