22 OUTLINES OF PHYSICAL CHEMISTRY
2 atoms of carbon : C — C
8 „ „ C — C — C
4 „ „ C— C— C— C or C— C— C
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��5 „ „ C-C-O— C— C, C— 0— C— C
i
G
��or C
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��and so on. When the carbon atoms become numerous the number of possible structures becomes very considerable.
If, instead of hydrogen only, other elements of different kinds are united to the carbon nucleus, then the number of possible arrangements increases very rapidly. Thus, for a compound of the type
III
we can foresee 280 modifications.
b. The second part of the problem to be solved is of a chemical and experimental nature. A large number of molecules, such as CH 4 , CC1 4 , CHC1 3 , CH 3 .OH, C0 2 , COCl 2 , C 2 H 6 , C 2 H 5 C1, and so on, admit of only one inter- pretation.
But frequently the molecule admits of several interpre- tations, and the true constitution, deduced from chemical and physical investigation of the substance, is based on the following two rules :
1. When a reaction takes place between molecules, the atoms or groups of atoms transferred exchange places without the structure of the displaced group of atoms being
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