ABSTRACT
“Constitutional” formulae based on notions of valency began to be used soon after 1860, but these formulae had no pretension to representing the relative positions of atoms in space. They served merely to show in what order the atoms were supposed to be linked one to another in a molecule, and thus served to some extent to epitomise the chief chemical changes to which the compound would be liable. By valency must be understood the habit of some elements, of which hydrogen is the most important, to enter into combination with no more than one other element at the same time, while others may combine with two or more. 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.
