Let X be chlorine or hydrogen. Then as examples of the first type we have: H_{2}, Cl_{2}, HCl, KCl, NaCl, &c. The compounds of oxygen or calcium may serve as examples of the type RX_{2}: OH_{2}, OCl_{2}, OHCl, CaO, Ca(OH)_{2}, CaCl_{2}, &c. For the third type RX_{3} we know the representative NH_{3} and the corresponding compounds N_{2}O_{3}, NO(OH), NO(OK), PCl_{3}, P_{2}O_{3}, PH_{3}, SbH_{3}, Sb_{2}O_{3}, B_{2}O_{3}, BCl_{3}, Al_{2}O_{3}, &c. The type RX_{4} is known among the hydrogen compounds. Marsh gas, CH_{4}, and its corresponding saturated hydrocarbons, C_{_n_}H_{2_n_ + 2}, are the best representatives. Also CH_{3}Cl, CCl_{4}, SiCl_{4}, SnCl_{4}, SnO_{2}, CO_{2}, SiO_{2}, and a whole series of other compounds come under this class. The type RX_{5} is also already familiar to us, but there are no purely hydrogen compounds among its representatives. Sal-ammoniac, NH_{4}Cl, and the corresponding NH_{4}(OH), NO_{2}(OH), ClO_{2}(OK), as well as PCl_{5}, POCl_{3}, &c., are representatives of this type. In the higher types also there are no hydrogen compounds, but in the type RX_{6} there is the chlorine compound WCl_{6}. However, there are many oxygen compounds, and among them SO_{3} is the best known representative. To this class also belong SO_{2}(OH)_{2}, SO_{2}Cl_{2}, SO_{2}(OH)Cl, CrO_{3}, &c., all of an acid character. Of the higher types there are in general only oxygen and acid representatives. The type RX_{7} we know in perchloric acid, ClO_{3}(OH), and potassium permanganate, MnO_{3}(OK), is also a member. The type RX_{8} in a free state is very rare; osmic anhydride, OsO_{4}, is the best known representative of it.[6]