FORM′ULA

[L.] In pharmacy and medicine , a short form of prescription; a recipe. By chemists the term is applied to a grouping of symbols, expre ing the composition of a body; thus, HCl (standing for 1 atom of hydrogen united to 1 atom of chlorine) is the formula for hydrochloric acid. A chemical formula is termed empirical when it merely gives the simplest po ible expre ion of the composition of the substance to which it refers. A rational formula, on the contrary, aims at describing the exact composition of molecule, or combining weight of the substance, but stating the absolute number of atoms of such of the elements e ential to that object, as well as the mere relations existing between them. The empirical formula is at once deduced from the analysis of the substance, reckoned to 100 parts; the rational formula requires, in addition, a knowledge of its combining quantity, which can only be obtained by direct experiment, by synthesis, or by the careful examination of one or more of its most definite compounds. Thus, the composition of acetic acid is expre ed by the formula CH 2 O, which exhibits the simplest relations of the three elements; if we want to expre the quantities of these, in atoms required to make up one molecule of acetic acid, we have to adopt the formula C 2 H 4 O 2 or HC 2 H 3 O 2 .