1 8 HISTORY OF SCIENCE. 



colony on the coast of Thrace, has an interest for us, as that of the 

 founder of a philosophy which has provided the theoretical basis for all 

 the physical science of the present day. Leucippus declined to admit 

 that fire, air, earth, and water were the true elements ; and he substi- 

 tuted the notion of all things being formed of particles so small, that 

 they could individually be neither seen nor felt. These particles he con- 

 ceived to be, nevertheless, of various forms and sizes ; and he supposed 

 that their different arrangements and movements gave rise to all the 

 various properties of bodies. It would appear, therefore, that Leu- 

 cippus regarded matter as consisting of particles of various kinds, and 

 intervening spaces in which these particles moved in various ways. 



These doctrines were greatly developed by DEMOCRITUS (B.C. 470 

 361), the pupil of Leucippus; he was also a native of Abdera, but, 

 like Pythagoras, he extended his opportunities of acquiring knowledge 

 by long residence in Persia, Syria, and Egypt. Though he left behind 

 him many treatises, not one has survived ; but the principles he taught 

 have been transmitted to us in a few distinct propositions. One of 

 these asserts that nothing car. be made from nothing, and that all change 

 consists only in the composition and separation of particles. This is 

 identical with the modern principle of the indestructibility of matter. 

 Democritus asserted also that there exists nothing but atoms and 

 empty spaces, and that the atoms have no internal actions, but affect 

 each other simply by impulse and pressure. At the present day the 

 most general formulas by which we express the facts of nature involve 

 the supposition of the existence and motions of atoms. Such are the 

 theories of sound, heat, light, and chemistry. Strange that, three and 

 twenty centuries ago, the fundamental theories of modern science 

 should have in a manner been anticipated by the philosopher of 

 Abdera ! The reader may wonder how it happened that a theory 

 which has proved so fruitful in the hands of the moderns did not lead 

 in ancient times to results of corresponding importance. It should be 

 observed that we are in possession of an immense store of facts of 

 which the ancients had no conception, and that for many branches of 

 science it is the power of our modern mathematical analysis which 

 has given strength to the atomic theory. Besides, in the history of 

 Grecian philosophy we observe that a remarkable change was some 

 time afterwards given to the direction of men's thoughts, by a few 

 master-minds who appeared among the Athenians. 



The reader may remark, by what we have related of the specula- 

 tions of the earlier Grecian philosophers, that one great question with 

 them was concerning the stuff from which things are made. One 

 thought this universal principle was water, another air, and so on , but 

 all agreed that the key to all knowledge lay in its discovery. EMPE- 

 DOCLES of Agrigentum (B.C. 460) introduced a new idea into philosophy. 

 He adopts the hypothesis of invariable and indestructible particles, but 

 attributes a special action to fire or heat as the active principle. He 



