[22] It is, however, easy to imagine, and experience confirms the supposition, that in a complex siliceous compound containing for instance sodium and calcium, the whole of the sodium may be replaced by potassium, and _at the same time_ the whole of the calcium by magnesium, because then the substitution of potassium for the sodium will produce a change in the nature of the substance contrary to that which will occur from the calcium being replaced by magnesium. That increase in weight, decrease in density, increase of chemical energy, which accompanies the exchange of sodium for potassium will, so to speak, be compensated by the exchange of calcium for magnesium, because both in weight and in properties the sum of Na + Ca is very near to the sum of K + Mg. _Pyroxene_ or _augite_ can be taken as an example; its composition may be expressed by the formula CaMgSi_{2}O_{6}; that is, it corresponds with the acid H_{2}SiO_{3}; it is a bisilicate. In many respects it closely resembles another mineral called '_spodumene_' (they are both monoclinic). This latter has the composition Li_{6}Al_{8}Si_{15}O_{45}. On reducing both formulæ to an equal contents of silica the following distinction will be observed between them: spodumene (Li_{2}O)_{6}(Al_{2}O_{3})_{8}30SiO_{2}; augite (CaO)_{15}(MgO)_{15}30SiO_{2}. That is, the difference between them consists in the sum of the magnesia and lime (MgO)_{15} + (CaO)_{15} replacing the sum of the lithium oxide and alumina (Li_{2}O)_{6} + (Al_{2}O_{3})_{8}; and in the chemical relation these sums are near to one another, because magnesium and calcium, both in forms of oxidation and in energy (as bases), in all respects occupy a position intermediate between lithium and aluminium, and therefore the sum of the first may be replaced by the sum of the second. If we take the composition of spodumene, as it is often represented to be, Li_{2}O,Al_{2}O_{3},4SiO_{2}, the corresponding formula of augite will be (CaO)_{2},(MgO)_{2},4SiO_{2}, and also the amount of oxygen in the sum of Li_{2}OAl_{2}O_{3} will be the same as in (CaO)_{2}(MgO)_{2}. I may remark, for the sake of clearness, that lithium belongs to the first, aluminium to the third group, and calcium and magnesium to the intermediate second group; lithium, like calcium, belongs to the even series, and magnesium and aluminium to the uneven.