SCIENCE IN THE ANCIENT WORLD 43 



from element to element seems to have been a way 

 of teaching the identity of the principles underlying 

 every phase of development from inorganic nature 

 to man. According to Empedocles, existence is 

 essentially one from end to end. 



Empedocles saw that, by imagining his four elements 

 united in different proportions, he could explain all 



the endless kinds of different substances 



The Atomists. , , . , T , 



known to mankind. Leucippus and 



Democritus developed this hypothesis into a theory of 

 atoms which we know best through the description 

 of it given at a later date by the Latin poet Lucretius. 



The ground on which the atomic theory of the 

 Greeks was founded was very different from the 

 definite experimental facts known to Dalton and 

 Avogadro when they formulated the atomic and mole- 

 cular theories of to-day. The modern chemists had 

 before them exact quantitative measurements of the 

 proportions in which chemical elements combined by 

 weight and by volume. These limited and definite facts 

 led irresistibly to the idea of atoms and of molecules, 

 and gave to them at once relative atomic and mole- 

 cular weights. The theory thus formulated was then 

 found to conform with all the rest of the countless 

 facts and experiences which had become the common 

 possession of science, to be supported by other relations 

 as successively they were discovered, and to serve 

 as a useful guide in the study and even the prediction 

 of new phenomena. 



But the Greeks had neither the definite experi- 

 mental facts to suggest the theory in the first place, 

 nor the power of testing by experiment the conse- 



