Manchester Memoirs, Vol. lit. (1908), No. 10 17 



molecule would itself be the ultimate formative crystalline 

 element. Where an atom has a higher valency, it must, 

 according to any formula of spacial chemical constitution, 

 aggregate more atoms around it and in touch with, it in 

 the molecule : it must, on that account alone, itself occupy 

 or exist in a larger central space. In this way greater 

 atomic volume would be in general a result of greater 

 valency, while the atomic volume will always be nearly 

 the same in similar surroundings : the very striking 

 recent investigations to ascertain how far the structure 

 of the crystal is determined by the arrangement of the 

 atoms in its molecule on the basis that equi-valent 

 atoms require about the same atomic volume, are known 

 to all of us here.* 



The contrast has recently been sketched by Professor 

 Voigt in eloquent terms between this domain of the 

 properties of crystals, where all is definite, orderly arrange- 

 ment, and that of liquids and gases where physical 

 properties are merely average values which belong in the 

 statistical sense to crowds of jostling molecules. But 

 even here of course the regularity is limited ; the 

 molecules become confined more or less securely in 

 definite positions by the mutual forces of cohesion, but 

 not so firmly as to prevent them from taking part in the 

 conduction of heat and other modes of equalisation or 

 dissipation of energy ; the very bonds of cohesion are 

 themselves functions of the temperature. 



The tendency of most physicists would still probably 

 be to take comfort from a remark of Helmholtz, 

 published in one of his letters, to the effect that organic 

 chemistry progresses steadily and surely, but in a manner 

 which, from the physical standpoint, appears not to be 

 describable as quite rational. Yet as time goes on it 



* Barlow and Pope : Cf. Trans. Chem. Soc, 1906. 



