302 Professor William J. Pope [May 1, 



define the saturation- capacity or the atom-fixing power of the atoms 

 of the other elements. It will be clear that for rough diagrammatic 

 purposes we may provide the spheres representing the atoms with as 

 many wooden pegs as the element itself exhibits units of valency ; 

 compound molecules can then be represented by fitting the atoms 

 together by means of the pegs representing the number of valency 

 units possessed by the various constituent atoms. By so doing a 

 great advance is made upon the atomic theory of Dalton's time and a 

 mental picture is obtained of the way in which the atoms are con- 

 nected together within the molecule itself. 



During the early part of the nineteenth century it became evident, 

 principally from the work of Liebig and Wohler in Germany and 

 of Faraday at the Royal Institution, that substances exist which 

 possess totally different properties, but nevertheless have the same 

 molecular composition ; as this became slowly realised, the atomic 

 theory was naturally called upon to furnish some adequate explana- 

 tion. In view of the proven identity of molecular composition the 

 required explanation could only be sought for in differences in the 

 atomic arrangement within the molecules of the several substances. 

 That such differences can be successfully illustrated by the aid of 

 the atomic models will be seen on considering some specific case. 

 Ordinary ethyl alcohol and methyl ether differ greatly from each 

 other — the first is a liquid whilst the second is a gas at ordinary 

 temperatures — but possess the same molecular composition, the mole- 

 cule in each case consisting of two atoms of carbon, six of hydrogen 

 and one of oxygen. These two substances have to be represented on 

 the assumption that hydrogen is monovalent, carbon tetravalent and 

 oxygen divalent. By joining wooden spheres together in the order 

 shown in the figures — in which the valencies of the component atoms 

 are carefully respected — diagrammatic representations are obtained 

 which illustrate to the chemist the differences existing between ethyl 

 alcohol and methyl ether. Substances related to each other in this 



H H H H 



II II 



H— C— 0— O— H H— C-0— C-H 



II II 



H H H H 



Ethyl alcohol Methyl ether 



way are said to be isomeric : they have the same molecular composi- 

 tion but different molecular constitutions. The step in advance which 

 is involved in thus writing molecular constitutions or in constructing 

 molecular models was taken by Kekule in 1858. 



Two great stages in the development of chemical theory have 

 now been indicated. First, that contributed by Dalton, who regarded 

 constancy of molecular composition as characteristic of a chemical 

 substance; secondly, that further stage, attained as a result of the 

 labours of Liebig, Wohler, Faraday, Frankland and Kekule, which 



