622 On the Construction of Compound Molecules. 



The sharp angle at the top is determined by the fact that the 

 •c-force on the carbon atom at the apex must be zero, and 

 that this force is due principally to the two nearest carbon 

 atoms, the hydrogens having scarcely any effect upon the 

 carbon unless the distance is small. The angle is, therefore, 

 determined by the angle which the line of zero force for z 

 makes in PL IX. fig. 2. The two carbon atoms next below 

 the apex tend to separate horizontally from each other, and 

 would do so indefinitely were it not for the attraction of the 

 hydrogen atoms. The comparative size of this molecule and 

 CH 2 is shown in this figure. Before giving further details 

 of this molecule the compound C 18 H 12 s hown in fig. 15 will 

 be considered. It is formed by uniting four of the benzene 

 hexagons side by side. To bind this together the hydrogen 

 atoms form a straight line with atoms equally spaced except at 

 each end, where there is a triangle of atoms formed. The two 

 hydrogen atoms at either end are nearer to the carbon atoms 

 at the end than is the case at any other place, and these 

 atoms exert their attractive force upon the end carbon atoms. 

 This attractive force is transmitted through the straight chain 

 of hydrogen atoms to the other end. It is seen that the line 

 of hydrogen atoms acts like a cord under tension which is 

 anchored at the ends by the two triangles. The rule that a 

 chemist would follow to find the number of hydrogen atoms 

 required in this compound is to count the external points 

 completely around the hexagons. This number is 12, and it 

 is seen that the number of atoms required by my scheme for 

 this molecule is also 12. In the case of the benzene molecule 

 of fig. 14 it is now evident that the straight chain of hydrogen 

 atoms is reduced to two in number. 



It is clearly evident from this formation that the four 

 outside atoms of hydrogen will behave alike, in being 

 replaced by other atoms and by groups of atoms, and that the 

 two inner ones must act differently from the others. It 

 would break the whole tie-rod, so to speak, to remove an 

 inner atom, but only half of it to remove an outside 

 atom. 



The growth of this system may not only be accomplished 

 by adding hexagons on the side, but on the top and bottom 

 as well, the whole structure becoming in proportion more and 

 more carbon, less hydrogen being required for binding. The 

 ordinary rule of counting the points around the whole to find 

 the requisite number of hydrogen atoms agrees with the 

 scheme in the case of this larger growth. 



