FORMULA. — The term is used in chemistry to denote a collection of symbols representing a compound. The formula of a compound is written by placing side by side the symbols of the elements which it contains, and also the small figures indicating the number of combining proportions of each element which are present in the compound repre sented, or, in other words, the chemical formula of a compound is intended to represent the quantitative composition of the compound, and to tell what atoms and how many are combined to make the smallest particles of the compound itself. The method by which formula are determined is this :—First the substance must be analyzed, and secondly the molecular weight must be determined. We will take, for example, water. By analyzing this compound called water we find it consists of hydrogen and oxygen, r part of the former and 8 parts of the latter, both by weight. We have next to determine the specific gravity of water vapor, and find that, as compared with air, its specific gravity is 0.623. We have now the relative weight of the molecule of water, and by adopting the usual standard it shows the molecular weight to be 18. The atomic weight of oxygen is 16, and that of hydrogen 1. So we now have all we require for writing the formula. A molecule, consisting of hydrogen and oxygen in the proportions given, and the weight of which is 18 in terms of accepted unit weight, must contain two atoms of hydro gen and one atom of oxygen. The formula is therefore written H,0 or OH,.
Mercurous chloride contains one combining portion of mercury and one of chlorine; its formula is therefore written HgCl. Mercuric chloride, however, contains one combining pro portion of mercury and two of chlorine • its formula is In the table of elements is given their respective symbols, by which it will be found easier to understand the formula given.
Chemical formula have a quantitative besides a qualitative meaning ; for instance, HgCI, not only signifies that mercurous chloride is composed of mercury and chlorine, but it also means 235.4 of mercurous chloride composed of 200 parts of mercury and 35.4 of chlorine. These numbers will be found opposite to the elements. Formula of compounds have still another meaning, which it is neccssary to understand. They always represent one molecule, and the numbers indicate the number of atoms of the different elements in that molecule. For instance, K,Cr,O, serves to represent one molecule of potassium dichromate, consisting of two atoms of potassium weighing 39.1 each, two atoms of chromium each weighing 52.3, and seven atoms of oxygen weighing 16 each. By adding the weights of all the atoms we get the molecular weight. (See also Chemistry.) The chemical formula of each substance is given under the respective headings.
The word formula is also applied to recipes for making compounds expressed in ordinary words and figures.