Both the diminishing of spatial extension and the breaking up of a whole into parts entail an increase in the quality 'dry'. This applies not only in the sense that the parts which have become independent units are 'dry' in relation to each other - formerly coherent matter being turned into dust - but also in the other sense, and one valid in both cases, that levity and gravity are losing part of their previous inter-connexion. If this twofold process of 'becoming dry' reaches a certain intensity, the substances concerned, provided they are inflammable, begin to burn, with the result that dry heat escapes and dry ash is formed. We note that in each case we are dealing with a change in the relationship between the poles of a polarity of the first order. We will now apply this picture of the process of friction to the instance when, as a result of this action, electricity appears. Originally the evoking of the electric condition was ascribed solely to the nature of amber, the only substance known to possess this property. To-day we know that not the amber alone, but its coming together with another substance of different nature, in this instance an animal substance of the nature of hair or silk, is required. Whatever substances we use for friction, they must always be different in nature, so as to allow both kinds of electricity to appear at once. Which of the two kinds imposes its presence the more strongly upon the observer depends on purely extraneous conditions which have nothing to do with the process itself. Obviously, if we wish to understand the qualitative difference between the two kinds of electricity, we must investigate the qualitative difference in the material substances, which give rise to electricity when they are rubbed together. We shall again follow the historical line by examining the two substances which first taught man the polar nature of electricity. They are glass and resin, after which, as we mentioned, the two electricities were even named in the beginning. Our functional conception of matter, developed earlier (Chapter XI), allows us to recognize in these two substances representatives of the Salt-Sulphur polarity. Indeed, glass as a mineral substance, which actually owes its specific character to the presence of silicon in it, clearly stands on the phosphoric-crystalline side, while resin, being itself a sort of 'gum', on the sulphurous-volcanic side. In fact, sulphur itself was soon found to be a particularly suitable substance for producing 'resin'-electricity.