C_{2}H_{2}O_{4}, gives a crystallo-hydrate with 2H_{2}O; but it may be referred to the type CH_{4}, or rather to the type of ethane, C_{2}H_{6}, in which all the atoms of hydrogen are replaced by hydroxyl, C_{2}H_{2}O_{4}2H_{2}O = C_{2}(OH)_{6} (_see_ Chapter XXII., Note 35). The four lower types RX, RX_{2}, RX_{3}, and RX_{4} are met with in compounds of the elements R with chlorine and oxygen, and also in their compounds with hydrogen, whilst the four higher types only appear for such acid compounds as are formed by chlorine, oxygen, and similar elements. Among the oxygen compounds the _saline oxides_ which are capable of forming salts either through the function of a base or through the function of an acid anhydride attract the greatest interest in every respect. Certain elements, like calcium and magnesium, only give one saline oxide--for example, MgO, corresponding with the type MgX_{2}. But the majority of the elements appear in several such forms. Thus copper gives CuX and CuX_{2}, or Cu_{2}O and CuO. If an element R gives a higher type RX_{_n_}, then there often also exist, as if by symmetry, lower types, RX_{_n_-2}, RX_{_n_-4}, and in general such types as differ from RX_{_n_} by an even number of X. Thus in the case of sulphur the types SX_{2}, SX_{4}, and SX_{6} are known--for example SH_{2}, SO_{2}, and SO_{3}. The last type is the highest, SX_{6}. The types SX_{5} and SX_{3} do not exist. But even and uneven types sometimes appear for one and the same element. Thus the types RX and RX_{2} are known for copper and mercury. Among the _saline_ oxides only the _eight types_ enumerated below are known to exist. They determine the possible formulæ of the compounds of the elements, if it be taken into consideration that an element which gives a certain type of combination may also give lower types. For this reason the rare type of the _suboxides_ or quaternary oxides R_{4}O (for instance, Ag_{4}O, Ag_{2}Cl) is not characteristic; it is always accompanied by one of the higher grades of oxidation, and the compounds of this type are distinguished by their great chemical instability, and split up into an element and the higher compound (for instance, Ag_{4}O = 2Ag + Ag_{2}O). Many elements, moreover, form transition oxides whose composition is intermediate, which are able, like N_{2}O_{4}, to split up into the lower and higher oxides. Thus iron gives magnetic oxide, Fe_{3}O_{4}, which is in all respects (by its reactions) a compound of the suboxide FeO with the oxide Fe_{2}O_{3}. The independent and more or less stable saline compounds correspond with the following eight types:-- R_{2}O; salts RX, hydroxides ROH. Generally basic like K_{2}O, Na_{2}O, Hg_{2}O, Ag_{2}O, Cu_{2}O; if there are acid oxides of this composition they are very rare, are only formed by distinctly acid elements, and even then have only feeble acid properties; for example, Cl_{2}O and N_{2}O.