[From the Encyclopaedia Britannica.] 



LXXIII. Atom. 



ATOM (aro/Ltos) is a body which cannot be cut in two. The atomic theory 

 is a theory of the constitution of bodies, which asserts that they are made up 

 of atoms. The opposite theory is that of the homogeneity and continuity of 

 bodies, and asserts, at least in the case of bodies having no apparent organisa- 

 tion, such, for instance, as water, that as we can divide a drop of water into 

 two parts which are each of them drops of water, so we have reason to believe 

 that these smaller drops can be divided again, and the theory goes on to assert 

 that there is nothing in the nature of things to hinder this process of division 

 from being repeated over and over again, times without end. This is the 

 doctrine of the infinite divisibility of bodies, and it is in direct contradiction 

 with the theory of atoms. 



The atomists assert that after a certain number of such divisions the parts 

 would be no longer divisible, because each of them would be an atom. The 

 advocates of the continuity of matter assert that the smallest conceivable body 

 has parts, and that whatever has parts may be divided. 



In ancient times Democritus was the founder of the atomic theory, while 

 Anaxagoras propounded that of continuity, under the name of the doctrine of 

 homo3omeria ('Oftoio//,e)oia), or of the similarity of the parts of a body to the 

 whole. The arguments of the atomists, and their replies to the objections of 

 Anaxagoras, are to be found in Lucretius. 



In modern times the study of nature has brought to light many properties 

 of bodies which appear to depend on the magnitude and motions of their 

 ultimate constituents, and the question of the existence of atoms has once more 

 become conspicuous among scientific inquiries. 



We shall begin by stating the opposing doctrines of atoms and of con- 

 tinuity before giving an outline of the state of molecular science as it now 

 exists. In the earliest times the most ancient philosophers whose speculations 



