A DREAM OF FAIR HYDRONE 487 



linking the ends of open simple chains together. For example, 

 if trimethylene bromide, BrCH 2 . CFL. CHLBr (which is to be 

 regarded as propane, CH 3 .CH 2 .CH n , in which an atom of 

 hydrogen in each of the CH 3 groups is displaced by an atom 

 of bromine) be subjected to the action of sodium, it is not only 

 deprived of the two bromine atoms, but the two ends of the chain 

 become linked together : 



CH, 



BrCH 2 .CH 2 .CH a Br- 2Br= /\ 



B. 2 C- ^CH, 



Even two carbon atoms may be united in a closed ring, as in 

 ethene or ethylene, which is formed by mixing alcohol with 

 concentrated sulphuric acid and heating the mixture : 



CH,.OH CH, 



i = + OHj 



CH..H CH 



But apparently it is not possible to convert every open chain 

 hydrocarbon into the corresponding ring system — the most 

 complex compounds of this kind known at present contain only 

 seven atoms of carbon. 



The recognition of such limitations necessitates the recogni- 

 tion of structural peculiarities in the carbon atoms, the effect of 

 which is to limit their activity, so that they can only be arranged 

 in a certain limited number of ways. It is important that these 

 peculiarities should be noticed. 



A model or symbol in remarkable accord with the general 

 chemical behaviour of carbon is afforded by the regular 

 tetrahedron. Assuming such a tetrahedron to be inscribed 

 inside a sphere, the centre of influence of the carbon atom may 

 be supposed to be at the centre of the sphere and the four 

 affinities of the atom may be thought of as directed outwards 

 towards the four solid angles of the tetrahedron ; in this arrange- 

 ment, the angle included between two affinities is I09°28\ 



Such a model is easily constructed by joining together at 

 their edges four equilateral triangles cut out of cardboard. But 

 for the purpose of studying the manner in which carbon atoms 

 can be united together, it is better to bend two pieces of wire at 

 an angle of io9°28' and then to solder these together at the bend 



