Compound Molecules with Theoretical Atoms. 619 



similarly to any other, and no differences would be observed 

 in the replacement of one hydrogen atom by a bromine atom, 

 for instance, from that of any other. 



The compound C 2 H 4 is shown in fig. 8. The atoms all lie 

 in one plane, the axes of rotation being in this plane parallel 

 to each other. It may be formed by uniting together two 

 molecules of 0H 2 . We may picture the forces upon the 

 atoms in this molecule by imagining the two upper hydrogen 

 atoms H 3 and H 4 to be connected together by an elastic band 

 under tension, and similarly the lower pair Hi and H 2 . The 

 angle is such that the force between the upper hydrogen H 3 

 and its opposite lower hydrogen H 2 is nil and stable for 

 displacements. The force of repulsion between the two 

 carbon atoms Oj and C 2 may be represented by elastic bands 

 under tension fixed ;it points to the left of Ci and right 

 of C 2 . It is evident that if C^ is connected to H x and H 3 , 

 and C 2 to H 2 and H 4 , the forces represented by these bands 

 will cause the plane figure to be the stable form. Any dis- 

 placement of tlie pair HxH 3 out of the plane, by a twist say, 

 brings into play a restoring torque to return them to this 

 plane. 



The form in which chemists, following van't Hoff's tetra- 

 hedral conception of the carbon atom, usually write the 

 structural formula lor C 2 H 4 is 



H/ Mi 



with the two carbon atoms nearer together than the pair- of 

 hydrogens. The representation in fig. 8, however, changing 

 the positions of the hydrogen atoms so as to be nearer together 

 than the carbons, does not meet with objections from the 

 standpoint of the chemist. 



The molecule of C 3 H 6 , shown in fig. 9, may be formed by 

 adding a third molecule of CH 2 to the combination in fig. 8. 

 Here we have the beginning of the ring formation, the three 

 carbon atoms forming an equilateral triangle of larger 

 dimensions because they repel each other and tend to separate. 

 They are held in check by a double ring of hydrogen atoms 

 of smaller diameter, the one above and the other below the 

 plane of the carbon ring. 



By adding to this a fourth molecule of (JH 2 the ring- 

 molecule of 4 H S is formed, as in fig. 10, the carbon atoms 

 forming a still larger plane figure, this time a square, and 

 the hydrogens forming two squares of smaller dimensions, 

 the one above and the other below the plane of carbon atoms. 



