In 1876, Kohlrausch compared the conductivity of the chlorides, bromides, and iodides of potassium, sodium, and ammonium respectively. He found that altering the cation did not affect the _differences_ of conductivity between the three salts, thus showing that these differences of conductivity were dependent on the nature of the anion only, and not on the particular base with which it was combined. The difference of conductivity between an iodide and a bromide, for example, is the same whether potassium, sodium, or ammonium salts are compared. A similar experiment has been made with a series of cations combined with various anions. The difference of conductivity of the salts in the series is the same whichever anion is used, _i.e._ the difference of conductivity between potassium chloride and sodium chloride is the same as that between {31} potassium bromide and sodium bromide. Hence we may conclude that the conductivity of any salt is an ionic property. Kohlrausch's law may be expressed by the formula c = d(u + v), where c is the conductivity of the salt, d the degree of dissociation, _i.e._ the fraction of the electrolyte broken up into ions, and u and v the velocity of the anions and cations respectively. When all the molecules of the electrolyte are dissociated, d = 1, and the formula becomes c_{[infinity]} = u + v. As we have already seen, a salt is formed by the union of a metal M with an acid radical R. Potassium sulphate, K_2SO_4, consists of the metal K_2 and the acid radical SO_4. Ammonium chloride, NH_4Cl, consists of the basic radical NH_4 and the acid radical Cl. The various acids may be considered as salts of the metal hydrogen. Thus sulphuric acid, H_2SO_4, is the sulphate of hydrogen. Bases may be considered as salts with the hydroxyl group, OH, replacing the acid radical. Thus potash, KOH, is the hydroxyl of potassium. The various electrolytic combinations may be represented by the following symbols:-- Salts = MR. Acids = HR. Bases = MOH. The various chemical reactions of an electrolyte are all ionic reactions, the chemical activity of an electrolytic solution being proportional to its electric conductivity, _i.e._ the degree of dissociation of its ions. The acidity of an electrolytic solution is due to the presence of the dissociated ion H^+, and its strength is determined by the concentration of these free hydrogen ions. Hence the greater the degree of dissociation the stronger the acid. The basic character of a solution is determined by the presence of the hydroxyl radical OH^-. The greater the concentration of the hydroxyl ions, _i.e._ the greater the dissociation, the stronger is the base.