the Molecule and Chemical Combination. 535 



that contact along an edge of a cell is analogous to what is 

 described on the valency theory as union by a single bond; 

 while the fourfold connexion, i. e. contact along two lines, is 

 analogous to the state known as union by a double bond. 



If we use the modified notation previously described, the 

 number of bonds between two atoms is equal to the number 

 of electrons their two cells have in common. 



There is nothing in the ordinal valency theory analogous 

 to the cells having three electrons in common, though there 

 is on the modification I gave of that theory in the paper on 

 chemical combination already referred to. 



The three-electron contact has to be invoked (if we suppose 

 the one-electron contact to be too fragile) to account for the 

 existence of compounds like C10 2 , which are inexplicable on 

 the theory of valency if chlorine has any odd valency. On 

 the theory we are discussing there are 3 cells in this com- 

 pound, and these have to be formed from 19 electrons : this 

 could be done by making the cell surrounding one of the 

 oxygen atoms have a double contact with the other oxygen 

 cell, while one of these oxygen cells has a threefold contact 

 with the chlorine cell. 



Another example where the threefold contact might come in 

 is the well-worn one of the benzene ring.- In benzene C G H G 

 we have on our theory to make up 6 cells and we have 

 30 electrons with which to do it. The simplest and most 

 symmetrical way of doing this is to have the six cells in 

 contact round a ring with threefold contact between each 

 two. As two opposite triangular faces of the twisted cell 

 represented in fig. 1 are inclined to each other, this could 

 be done without introducing much strain in the system. 

 With this arrangement we have complete symmetry, and it 

 is analogous to the Armstrong and Baeyer or central theory 

 of the benzene ring. The analogue on our theory to Kekule's 

 conception of the constitution of the benzene ring would 

 consist of three sets of pairs of cells, the cells in one pair 

 having fourfold contact with each other, but only double 

 contact with a cell in a neighbouring pair. It has often been 

 pointed out that it is difficult to explain on this view why we 

 do not get, when we replace two atoms of hydrogen by two 

 atoms of chlorine, more isomers than have been observed. 



Let us consider from the point of view of this theory the 

 changes which must take place in the disposition of the cells 

 if one of the carbon atoms takes up another atom in addition 

 to the hydrogen atom already attached to it. Let us suppose 

 that this is another atom of hydrogen. This atom will 

 introduce another electron in the system which will have to 



2N2 



