9. _Connexion of Chemical and Crystalline Polarities._--The close connexion between the Chemical Affinity and the Crystalline Attraction of elements cannot be overlooked. Bodies never crystallize but when their elements combine chemically; and solid bodies which {380} combine, when they do it most completely and exactly, also crystallize. The forces which _hold together_ the elements of a crystal of alum are the same forces which make it a _crystal_. There is no distinguishing between the two sets of forces. Both _chemical_ and _crystalline_ forces are _polar_, as we stated in the last chapter; but the polarity in the two cases is of a different kind. The polarity of chemical forces is then put in the most distinct form, when it is identified with electrical polarity; the polarity of the particles of crystals has reference to their geometrical form. And it is clear that these two kinds of polarity must be connected. Accordingly, Berzelius expressly asserts[20\5] the necessary identity of these two polarities. 'The regular forms of bodies suppose a polarity which _can be_ no other than an electric or magnetic polarity.' This being so seemingly inevitable, we might expect to find the electric forces manifesting some relation to the definite directions of crystalline forms. Mr. Faraday tried, but in vain, to detect some such relation. He attempted to ascertain[21\5] whether a cube of rock crystal transmitted the electrical force of tension with different intensity along and across the axis of the crystal. In the first specimen there seemed to be some difference; but in other experiments, made both with rock crystal and with calc spar, this difference disappeared. Although therefore we may venture to assert that there must be some very close connexion between electrical and crystalline forces, we are, as yet, quite ignorant what the nature of the connexion is, and in what kind of phenomena it will manifest itself. [Note 20\5: _Essay on Chemical Prop._ 113.] [Note 21\5: _Researches_. Art. 1689.]