Note 6 — PRINCIPLES OF CHEMISTRY CHAPTER XV THE GROUPING OF THE ELEMENTS AND THE PERIODIC LAW

The still more complex combinations—which are so clearly expressed in the crystallo-hydrates, double salts, and similar compounds—although they may be regarded as independent, are, however, most easily understood with our present knowledge as aggregations of whole molecules to which there are no corresponding double compounds, containing one atom of an element R and many atoms of other elements RXn. The above types embrace all cases of direct combinations of atoms, and the formula MgSO4,7H2O cannot, without violating known facts, be directly deduced from the types MgXn or SXn, whilst the formula MgSO4 corresponds both with the type of the magnesium compounds MgX2 and with the type of the sulphur compounds SO2X2, or in general SX6, where X2 is replaced by (OH)2, with the substitution in this case of H2 by the atom Mg, which always replaces H2. However, it must be remarked that the sodium crystallo-hydrates often contain 10H2O, the magnesium crystallo-hydrates 6 and 7H2O, and that the type PtM2X6 is proper to the double salts of platinum, &c. With the further development of our knowledge concerning crystallo-hydrates, double salts, alloys, solutions, &c., in the chemical sense of feeble compounds (that is, such as are easily destroyed by feeble chemical influences) it will probably be possible to arrive at a perfect generalisation for them. For a long time these subjects were only studied by the way or by chance; our knowledge of them is accidental and destitute of system, and therefore it is impossible to expect as yet any generalisation as to their nature. The days of Gerhardt are not long past when only three types were recognised: RX, RX2, and RX3; the type RX4 was afterwards added (by Cooper, Kekulé, Butleroff, and others), mainly for the purpose of generalising the data respecting the carbon compounds. And indeed many are still satisfied with these types, and derive the higher types from them; for instance, RX5 from RX3—as, for example, POCl3 from PCl3, considering the oxygen to be bound both to the chlorine (as in HClO) and to the phosphorus. But the time has now arrived when it is clearly seen that the forms RX, RX2, RX3, and RX4 do not exhaust the whole variety of phenomena. The revolution became evident when Würtz showed that PCl5 is not a compound of PCl3 + Cl2 (although it may decompose into them), but a whole molecule capable of passing into vapour, PCl5 like PF5 and SiF4. The time for the recognition of types even higher than RX8 is in my opinion in the future; that it will come, we can already see in the fact that oxalic acid, C2H2O4, gives a crystallo-hydrate with 2H2O; but it may be referred to the type CH4, or rather to the type of ethane, C2H6, in which all the atoms of hydrogen are replaced by hydroxyl, C2H2O42H2O = C2(OH)6 (see Chapter XXII., Note 35).