I will here consider hydrochloric acid somewhat in detail, because it is very important in the digestion of food, being the principal fluid composing the gastric juice of the stomach. Hydrochloric acid is always made by treating common salt (one afflicted with acid fermentation should omit the use of salt and soda), under high temperature, with sulfuric acid. This product is given off as a gas, which dissolved in water forms hydrochloric acid, sodium sulfate remaining behind as a result of this process. The chemist describes the action that takes place by writing what is called a chemical equation, as follows: 2NaCl + H₂SO₄ = Na₂SO₄ + 2HCl Sodium chlorid + Sulphuric = Sodium + Hydrochloric (common salt) acid Sulfate acid The reader will observe that there are as many parts of each element on the right as on the left-hand side of the = mark. Two parts of common salt yield two parts each of sodium (Na) and chlorin (Cl). The sodium appears as _Na_ in the sodium sulfate, and the chlorin as _Cl_ in the two parts of hydrochloric acid. This method of expressing chemical action by these equations may be somewhat confusing at first to those who have not studied chemistry, but it is best to have all such become familiar with them that they may have the further benefit of understanding the general terms of chemistry. Hydrochloric acid gives up its hydrogen when brought into contact with certain metals like iron, zinc, etc., and takes up these metallic elements in place of the hydrogen. Thus zinc and hydrochloric acid give zinc chlorid and hydrogen. Zn + 2HCL = ZnCl₂ + H₂ Zinc + Hydrochloric acid = Zinc chlorid + Hydrogen ACIDS, BASES, NEUTRALIZATION, SALTS [Sidenote: Relation of acids to bases] We have already discussed a number of substances called acids. It is necessary to inquire why chemists call them acids. What is there in common, for example, between the heavy, oily liquid sulfuric acid and the colorless gas, hydrochloric acid? It is not possible to understand the nature of their common properties without examining a class of substances called alkalis or bases. Acids and bases have the power to destroy the characteristic properties of each other. When an acid is brought into contact with a base, in proper proportions, the characteristic properties of both the acid and the base are destroyed. They are said to neutralize each other. [Sidenote: Common acids and bases and tests therefor]