By its magnetic properties iron thus reveals itself as a substance capable of assuming the condition of solid matter to a degree surpassing ordinary solidity. As an exceptional kind of metal it forms the counter-pole to mercury, in which the solid-fluid condition characteristic of all metallic matter is as much shifted towards the fluid as in iron it is to the solid. (Note in this respect the peculiar resistance of iron to the liquefying effect which mercury has on the other metals.) This picture of magnetism enables us to understand at once why it must occur together with heat at the place where an electric polarity has been cancelled by the presence of a conductor. We have seen that electricity is levity coupled in a peculiar way with gravity; it is polarized levity (accompanied by a corresponding polarization of gravity). An electric field, therefore, always has both qualities, those of levity and of gravity. We saw a symptom of this in electrical attraction and repulsion, so called; the attraction, we found, was due to negative density, the repulsion to positive density, imparted to space by the electrical fields present there. Now we see that when, through the presence of a conductor, the electrical field round the two opposing poles vanishes, in its place two other fields, a thermal and a magnetic, appear. Clearly, one of them represents the levity-part, the other the gravity-part, of the vanished electric field. The whole process reminds one of combustion through which the ponderable and imponderable parts, combined in the combustible substance, fall apart and appear on the one hand as heat, and on the other as oxidized substance ('ash'). Yet, between these two manifestations of heat there is an essential qualitative difference. Although, from our view-point, magnetism represents only one 'half of a phenomenon, the other half of which is heat, we must not forget that it is itself a bipolar force. Thus, despite its apparent relation to gravity it does not represent, as gravity does, one pole of a primary polarity, with heat as the other pole. Rather must it carry certain qualities of levity which, together with those of gravity, appear in a polarically opposite manner at its two poles. (Details of this will be shown later when we come to investigate the individual qualities of the two poles of magnetism and electricity.) Hence the heat that forms the counterpart to magnetism cannot be pure levity either. As the result of a certain coupling with gravity, it too has somehow remained polarically split.