The former is the molecule of potassic hydrate, which is the most caustic or strongest of alkalies; the latter, that of nitric acid, the most corrosive or powerful of acids. These are the extremes of the series, of which there are many intermediate members, all being more or less alkaline, that is caustic and turning litmus-paper blue, when the third element is a simple metallic atom; and acid, corrosive, and turning litmus-paper red, when it is a compound radical of a group of metallic and oxygen atoms. This shows to what an extent whole classes of substances may have a general resemblance in their constitution, and yet differ most widely in their qualities by the substitution of one element for another. These special qualities may be made to diminish and finally disappear by mixing the two opposite substances, or, as it is called, neutralising an acid by an alkali or an alkali by an acid. Thus, if hydrochloric acid, HCl, be poured into a solution of sodic-hydrate, Na—O—H, the alkaline qualities of the latter diminish and finally disappear, the result of the neutral solution being water, H—O—H, and sodic-chloride, or common salt, Na—Cl. It is evident that this result has been produced by the hydrogen atom in H—Cl and the sodium atom in Na—O—H changing places, the former preferring to unite with oxygen to form water, while the displaced sodium atom finds a refuge with chlorine. The oxygen atom has dropped sodium and taken hydrogen, just as the magnet dropped nickel and took iron.