Common salt is a crystalline substance which crystallizes in the Isometric, Monometric, or Tesseral system. That is to say, each crystal has three equal perpendicular planes of symmetry and six equal diagonal planes of symmetry. The crystals generally form cubes having six rectangular and equilateral faces. When these form on the surface of brine the sides often collapse, giving the distinctive “hopper-shaped” forms. More rarely the crystals form in octahedra, having eight equal, equilateral triangular faces, or in long needles under certain modifying conditions. The hollow quadrangular pyramidal form with an irregular inner surface arranged in steps, which manufactured salt generally takes, is the result of continuous depositions of crystals from a constantly saturated solution of brine during a considerable period, being superimposed layer after layer upon each other. In his exhaustive explanation of these phenomena, given in his _Principles of Chemistry_, Mendeléeff says: “If a solution of sodium chloride be slowly heated from above, where the evaporation takes place, the upper layer will become saturated before the lower and cooler layers, and therefore crystallization will begin on the surface, and the crystals first formed will float--having also dried from above--on the surface until they become quite soaked. Being heavier than the solution the crystals are partially immersed in it, and the following crystallization, also proceeding on the surface, will only form crystals by the side of the original crystals. A funnel is formed in this manner. It will be borne on the surface like a boat (if the liquid be quiescent) because it will grow more from the upper edges. We can thus understand this, at first sight, strange funnel-form of crystallized salt. To explain why the crystallization under the above conditions begins at the surface and not at the lower edges, it must be mentioned that the specific gravity of a crystal of sodium chloride is 2·16, and that a solution saturated at 25° contains 26·7 per cent. of salt and has a specific gravity 1·2004 at 25°; at 15° a saturated solution contains 26·5 per cent. of salt and has a specific gravity 1·203 at 15°. Hence, a solution saturated at a higher temperature is specifically lighter, notwithstanding the greater amount of salt it contains. With many substances, surface crystallization cannot take place, because their solubility increases more rapidly with the temperature than their specific gravity decreases. In this case the saturated solution will always be in the lower layers, where also the crystallization will take place.”