ARCHITECTURE OF MOLECULES 217 



One of these is obviously a reflection of the other, and is not 

 superposable upon it in such a way that the letters coincide. 



The use of models assists materially in the consideration of 

 the problems arising out of this hypothesis. One of the first 

 questions which arise relates to the direction in which the 

 several valencies of an atom of carbon may be supposed to be 

 exerted. If the direction of these be supposed to be absolutely 

 fixed, then it can be shown that : 



1. Two carbon atoms cannot unite by two bonds, nor by three, 

 because that would involve the distortion of the atom. 



2. Three or more carbon atoms cannot unite to form a ring 

 for the same reason. 



But inasmuch as carbon atoms do certainly combine together 

 to form rings or closed chains, and therefore the direction of the 

 valency must be drawn from the normal, there must be some- 

 thing analogous to the action at the pole of a magnet, that is 

 there is a certain field. It appears, however, that though two 

 carbon atoms may be apparently united by two or more units of 

 valency, in all such cases the combination is not only not more 

 secure but is decidedly more easily broken up than when one 

 valency of each atom is employed. 



A modification of the hypothesis of the tetrahedral carbon is 

 based on the idea that the relative force of attraction between 

 two units of valency depends on the distance through which they 

 have to act. By assuming that the combination between two 

 carbon atoms is not in the direction of the solid angles of the 

 tetrahedron, but that the attraction between the two is 

 in the direction of the normals to the faces of that figure, 

 it is obvious that the most intimate union is that in which 

 two of these faces are placed parallel to, and probably very 

 near, each other. A less intimate union occurs when the centres 

 of gravity of two faces of one atom attract two faces of another. 

 The two tetrahedra have then a common edge, the two pairs of 

 faces forming equal angles with each other. And, lastly, three 

 faces of one may attract equally three faces of the other, and so 

 cause the two tetrahedra to be applied to each other by one of 

 their solid angles. These three positions correspond to combina- 

 tion by single, double, and triple bonds. According to this 

 assumption it is only possible for the atoms to touch each other 

 when united by the single bond, as shown at a in the following 

 diagram (Fig. 60). 



