In the preceding we see not only the regularity and simplicity which govern the formation and properties of the oxides and of all the compounds of the elements, but also a fresh and exact means for recognising the analogy of elements. Analogous elements give compounds of analogous types, both higher and lower. If CO_{2} and SO_{2} are two gases which closely resemble each other both in their physical and chemical properties, the reason of this must be looked for not in an analogy of sulphur and carbon, but in that identity of the type of combination, RX_{4}, which both oxides assume, and in that influence which a large mass of oxygen always exerts on the properties of its compounds. In fact, there is little resemblance between carbon and sulphur, as is seen not only from the fact that CO_{2} is the _higher form_ of oxidation, whilst SO_{2} is able to further oxidise into SO_{3}, but also from the fact that all the other compounds--for example, SH_{2} and CH_{4}, SCl_{2} and CCl_{4}, &c.--are entirely unlike both in type and in chemical properties. This absence of analogy in carbon and sulphur is especially clearly seen in the fact that the highest saline oxides are of different composition, CO_{2} for carbon, and SO_{3} for sulphur. In Chapter VIII. we considered the limit to which carbon tends in its compounds, and in a similar manner there is for every element in its compounds a tendency to attain a certain highest limit RX_{_n_}. This view was particularly developed in the middle of the present century by Frankland in studying the metallo-organic compounds, _i.e._ those in which X is wholly or partially a hydrocarbon radicle; for instance, X = CH_{3} or C_{2}H_{5} &c. Thus, for example, antimony, Sb (Chapter XIX.) gives, with chlorine, compounds SbCl_{3} and SbCl_{5} and corresponding oxygen compounds Sb_{2}O_{3} and Sb_{2}O_{5}, whilst under the action of CH_{3}I, C_{2}H_{5}I, or in general EI (where E is a hydrocarbon radicle of the paraffin series), upon antimony or its alloy with sodium there are formed SbE_{3} (for example, Sb(CH_{3})_{3}, boiling at about 81°), which, corresponding to the lower form of combination SbX_{3}, are able to combine further with EI, or Cl_{2}, or O, and to form compounds of the limiting type SbX_{5}; for example, SbE_{4}Cl corresponding to NH_{4}Cl with the substitution of nitrogen by antimony, and of hydrogen by the hydrocarbon radicle. The elements which are most chemically analogous are characterised by the fact of their giving compounds of similar form RX_{_n_}. The halogens which are analogous give both higher and lower compounds. So also do the metals of the alkalis and of the alkaline earths. And we saw that this analogy extends to the composition and properties of the nitrogen and hydrogen compounds of these metals, which is best seen in the salts. Many such groups of analogous elements have long been known. Thus there are analogues of oxygen, nitrogen, and