  The simplest formula representing the composition of acetic acid is CH2O using modern atomic weights. This formula cannot accommodate the fact that, in the laboratory, the hydrogen in acetic acid can be replaced by chlorine in four distinct ways yielding four distinct chemical compounds. Three of those compounds are acids that have properties very similar to acetic acid, and in which the relative weights of chlorine vary as 1:2:3. The fourth compound has the properties of a salt rather than an acid. These experimental facts can be captured in a formula by doubling the numbers and rearranging the symbols, so that we have C2H4O2, rearranged to read C2H3O2H. The experimental facts can now readily be understood in terms of the substitution of one or more of the hydrogens by chlorine, with the three chloro-acetic acids represented as C2H2ClO2, C2HCl2O2H and C2Cl3O2H and the salt, acetyl chloride, as C2H3O2Cl. Such formulae came to be known as ‘rational formulae’ as distinct from the ‘empirical formula’ CH2O. Representing the replacement of one element in a compound by another in the laboratory by the replacement of one symbol by another in a chemical formula became a standard and productive device that was to eventually yield the concept of valency in the 1860s. (Oxygen has a valency of two because two hydrogens need to be substituted for each oxygen.)