578 SCIENCE PROGRESS 



the conception of compounds must be extended to include not 

 only compounds of atoms of different elements, but also com- 

 pounds of atoms of the same element. To the chemist oxygen 

 and ozone are such compounds, elementary oxygen being 

 oxygen in the special so-called "nascent" condition. 



In general the chemist usually has in mind the molecule with 

 the fewest number of atoms to represent the composition and 

 chemical properties of the compound with which he deals. Such 

 a molecule may or may not in fact exist, or it may exist only 

 over a very limited range of physical conditions far removed 

 from those for which his other knowledge of the compound has 

 been derived. But however artificial or special the conditions 

 under which the molecular weight has been determined, the 

 tendency is irresistible to look upon such molecules as having 

 a real general existence. In similar way the physicist extends 

 his molecular conceptions derived from the study of liquids and 

 gases to matter in general. 



It is interesting therefore to note that the molecular con- 

 ception, either as a physical or chemical unit, fails completely in 

 the case of crystalline solids. This, the second advance referred 

 to previously, follows from the recent application of the X-rays 

 to crystalline structure by Prof. Bragg and his son, who have 

 shown that any atom of chlorine in the interior of a rock-salt 

 crystal, for example, is fixed in space symmetrically with refer- 

 ence to several sodium atoms around it and with equal justice 

 may claim any one of them as its partner in the hypothetical 

 molecule NaCl. It seems therefore that such molecules cannot 

 really exist in crystalline solids. Between the crystal as a whole 

 and the single constituent atoms of which it is composed there 

 are no intermediate or penultimate particles that can be con- 

 sidered to have a separate existence. 



With so much of historical comment on past developments 

 let us refer to a few aspects of more modern advances, some of 

 which are set forth by Sir J. J. Thomson in his recent Romanes 

 Lecture. 1 



The first striking point is the enormous advance in sensitive- 

 ness attained when electrified atoms, or gaseous ions, commenced 

 to be studied. " An unelectrified atom is so elusive that unless 

 more than a million million are present we have no means 



1 The Atomic Theory, Romanes Lecture, 1914, by Sir J. J. Thomson, O.M. 

 Oxford: Clarendon Press, 1914. Price is. bd. net.) 



