A word may be added here with regard to the somewhat complicated equations such as those given. It is convenient to assume the existence of the anhydride of the acid as a constituent of the salt; thus potassium bichromate may for this purpose be regarded as consisting of K2O in union with 2CrO3. On acting on it with sulphuric acid in presence of an oxidisable compound, the K0O may be supposed to react with the acid thus: K2O + H2SO4 = K2SO4 + H2O. The chromium salt formed may be regarded (and this was formerly the point of view) as a compound of 3SOg, the anhydride of sulphuric acid, with Cr2O3, viz., Cr2Or3SO3, or Cr2(SO4)3. The formation of Cr2O3 from 2CrO3 involves the loss of 3O ; hence the equation given above. Similarly, the oxidising action of potassium permanganate may be formulated thus: K2O.Mn2O7 = K2O + 2MnO2 + 3O ; and K2O.Mn2O7 = K2O + 2MnO + 5O. With water present in the former action, the K2O becomes KOH, and the manganese dioxide becomes hydrated ; with sulphuric acid present in the latter, the K2O 1S converted into K2SO4, and the MnO into MnSO4. This old method of representing chemical changes had much to recommend it on the score of simplicity ; and it often is found convenient, although it is only a partial expression of the truth.