SOME ASPECTS OF THE ATOMIC THEORY 575 



The Molecular Theory of the physicist, as it now appears, 

 involves nothing of the conception of ultimateness implied in 

 the Atomic Theory. It is based on underlying principles which 

 apply to discrete freely moving particles in general, be they 

 molecules in the ordinary chemical sense or microscopically 

 visible masses, so long as they are free to move about as 

 individuals, as in a gaseous or fluid medium, or even, possibly, 

 in free space, as in a nebula. It was something of a lucky, if 

 confusing, accident that in the simple gases on which Gay 

 Lussac experimented and Avogadro theorised, the molecules 

 are composed of so few, often only two, individual atoms, and 

 that since then even certain monatomic molecules should have 

 been found to be capable of existence. 



Of course it may be urged that the conception of ultimate- 

 ness no longer, with recent acquisitions of knowledge, does 

 apply to the chemist's atom. But it must be understood that 

 the atomic theory of Dalton has always been concerned with 

 the ultimate particles of the elementary substances, rather than 

 with the ultimate particle of matter in general, so that the new 

 knowledge in fact involves only confirmation of old ideas. 

 The atom of uranium is still the ultimate particle of the element 

 uranium. Its disintegration during radioactive change into an 

 atom of radium, three atoms of helium, and two electrons, is 

 in no way inconsistent with the statement that the atom of 

 uranium is the ultimate particle of the element uranium. 

 Indeed, such evidence proves positively that no smaller particle 

 of the element uranium than the single Daltonian atom can be 

 or ever will be known. For could the term atom have any 

 other meaning than this ? If the chemist's atom ever had 

 meant the ultimate particle of matter in general without reference 

 to one and one only particular element, how could it ever have 

 been applied except to the one kind of atom, or one kind of 

 matter, hydrogen, the lightest particle of matter then and for 

 that matter still known to science? 



It is only comparatively recently with the study of radio- 

 active changes, and the recognition of the profound difference 

 between, for example, the disintegration of a radium atom and 

 the decomposition of a water molecule, that the true conception 

 of the atom as distinct from that of the molecule has become 

 at all general. One would search in vain for any such 

 distinction in, for example, Sir Arthur Rucker's Presidential 



