That their particles are brought nearer by cold and pressure, or removed farther from one another by heat, might be expected; but it could not have been anticipated that their relative positions could be so entirely changed as to alter their mode of aggregation. It follows, from the low temperature at which these changes are effected, that there is probably no portion of inorganic matter that is not in a state of relative motion. Professor Mitscherlich’s discoveries with regard to the forms of crystallised substances, as connected with their chemical character, have thrown additional light on the constitution of material bodies. There is a certain set of crystalline forms which are not susceptible of variation, as the die or cube (N. 166), which may be small or large, but is invariably a solid bounded by six square surfaces or planes. Such also is the tetrahedron (N. 167) or four-sided solid contained by four equal-sided triangles. Several other solids belong to this class, which is called the Tessular system of crystallisation. There are other crystals which, though bounded by the same number of sides, and having the same form, are yet susceptible of variation; for instance, the eight-sided figure with a square base, called an octahedron (N. 168), which is sometimes flat and low, and sometimes acute and high. It was formerly believed that identity of form in all crystals not belonging to the Tessular system indicated identity of chemical composition. Professor Mitscherlich, however, has shown that substances differing to a certain degree in chemical composition have the property of assuming the same crystalline form. For example, the neutral phosphate of soda and the arseniate of soda crystallise in the very same form, contain the same quantities of acid, alkali, and water of crystallisation; yet they differ so far, that one contains arsenic and the other an equivalent quantity of phosphorus. Substances having such properties are said to be isomorphous, that is, equal in form. Of these there are many groups, each group having the same form, and similarity though not identity of chemical composition. For instance, one of the isomorphous groups is that consisting of certain chemical substances called the protoxides of iron, copper, zinc, nickel, and manganese, all of which are identical in form and contain the same quantity of oxygen, but differ in the respective metals they contain, which are, however, nearly in the same proportion in each. All these circumstances tend to prove that substances having the same crystalline form must consist of ultimate atoms having the same figure and arranged in the very same order; so that the form of crystals is dependent on their atomic constitution.