Pure sodium chloride is not deliquescent (i.e., it does not dissolve and become liquid by absorbing moisture from the air), but, owing to the presence of minute quantities of magnesium chloride (one of the most deliquescent substances known), all except the most refined table-salt appears to be so to a slight extent. Even the finest table-salt is slightly hygroscopic, its crystals absorbing as much as ·6 per cent. moisture from a damp atmosphere. In some of the mines of Cheshire and Austria the very fine saline dust that is diffused through the atmosphere is found by the miners to be extremely irritating to the eyes and lungs, but all the more usual kinds of salt are sufficiently hygroscopic to indicate plainly the condition of the atmosphere.
Sodic chloride melts at a very high temperature, and at a still higher temperature it evaporates, while at white heat it forms thick clouds.
It would be supposed that in the same ocean areas, the proportion of the salt contents, except where marked differences in temperature occur, would be fairly constant, but it has been demonstrated that, even where masses of water of varying densities are superimposed upon each other, no very complete process of diffusion takes place between them, and practical salt-makers are familiar with differences in density which occur in different parts of the same salt pan.
The hardness of a mineral depends upon the degree of cohesion of its particles; but although no unit of hardness has been determined upon, and therefore no accurate method of measuring hardness has been arrived at, minerals have been approximately classed in a comparative table of ten substances, of which talc is placed at one end and diamond at the other. In this table, rock salt appears in the second place, and its hardness is estimated at 2·5. Its cohesion or power of supporting pressure is, therefore, about twice as great as that of bricks, and the practical advantage of this property is fully employed in rock-salt mines, where galleries and roofs are supported upon pillars of salt.
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.”
The acoustic properties of common salt render it an excellent medium for the transmission of sound, and as it possesses in a high degree the power of staying decomposition in dead organisms, it is, perhaps, the commonest of all preservatives. It is largely owing to its preservative property that common salt is an absolute necessity to the life of man and the higher animals, from a quarter to half an ounce a day being sufficient to prevent the putrefaction of food in the digestive tract in the case of an adult. In agriculture, salt is not only valuable as a destroyer of weeds and insect life, but used sparingly and with knowledge, it forms an excellent manure; while its more strictly chemical value in the manufacture of soda, chlorine, etc., causes it to play an important part in many branches of industry.
Even at the highest temperatures, heat cannot effect the decomposition of common salt. At a red heat, pure sodic chloride melts and becomes liquid, and if cooled again, a solid crystalline mass is formed. Ordinary salt fuses at a lower temperature and volatilizes when heated in an open vessel. But even in a closed vessel the purest salt will volatilize at a white heat. When gases or fluids are present in the crystalline cavities, heat causes decrepitation.
On the subject of the composition of brine, it is only necessary to add that it is so extremely variable that no two districts produce brine springs of the same strength and density, while the composition of ocean brine varies not only from ocean to ocean, but also for different parts and different depths in the same plane of water, and with the different distances from the mouths of large rivers. In the Cheshire district, the Brine test or Salinometer is graduated to show ounces in the gallon; but the gallon is the old Winchester Gallon of 231 cub. in. and not the Imperial Gallon of 277·274 cub. in. These are related to each other in the proportion of 10 to 12, therefore the Imperial Gallon will contain ⅕ more than the old gallon. Fully saturated brine by the Salinometer contains 42 oz. (2 lb. 10 oz.), therefore, in the Imperial Gallon 50·4 oz. As brines vary from 2 lb. 8 oz., or 40 oz. old measure, or 3 lb. or 48 oz. Imperial to 2 lb. 10 oz., or 3 lb. 2 oz. Imperial, so 1,000 gallons, which has been chosen as the measure for assessing brine-pumpers—under the Brine Pumping Compensation for Subsidence Act of 1891—will contain under the old measurement 2,625 lb. and under the Imperial 3,125 lb. of salt.