574 SCIENCE PROGRESS 



entirely with atomistic conceptions of matter as unnecessary 

 and unproved speculations, always failed to render intelligible 

 the simple laws of chemical combination for which that theory 

 had primarily been created to account. Of chemical origin, its 

 development proper proceeded almost entirely along chemical 

 lines, from the recognition and differentiation of the various 

 kinds of atoms by their weights, to the architecture of complex 

 substances, the number and relative arrangements of the atoms 

 therein, the manner in which they were held together by units 

 of combining power or valency, and to the beautiful space 

 chemistry of the particular varieties of complexes, in which the 

 same atoms may be grouped in two ways absolutely identical 

 on any two-dimensional representation, but in space of three 

 dimensions having the relations to one another of an object to 

 its mirror image. But although this is the real and only Atomic 

 Theory, it is not the Atomic Theory about which much, perhaps 

 most, has been said and written. 



Like the Atomic Theory, the Molecular Theory, which in the 

 past has so often usurped its name, originated in the laws of 

 chemical combination, for the one special case of gases. Gay 

 Lussac's law of combining volumes led Amedeo Avogadro 

 to his famous generalisation that equal volumes of all gases, 

 under the same condition of temperature and pressure, contain 

 the same number of molecules. It is not a general theory, and 

 indeed we know now cannot apply to some states of matter, 

 though the labours of van't Hoff and succeeding physical 

 chemists of the present era have extended it to the interior of 

 liquids with suitable important and extensive modifications. 

 But, quite apart from this, the conception of the molecule is a 

 distinct and independent development of mathematical and 

 experimental physics. 



The molecule is not the atom, though in certain special and 

 rather exceptional cases, that of the monatomic gases, such as 

 helium and mercury vapour, it happens that the same particle is 

 both the atom and the molecule of the element in question. This 

 one case, which also includes that of the vapours of most 

 metallic elements, when these are gasified at sufficiently high 

 temperature and often when they are dissolved in mercury to 

 form liquid amalgams, furnishes a common point of contact 

 between conceptions which are by their nature and origin very 

 largely distinct. 



