“Salt” was the name which was given in the first place to the residue left by the evaporation of sea-water, but the designation was subsequently employed to include the other substances held in solution in the sea, and, at a still later period, the name was still further extended by chemists to cover all the combinations of a base and an acid which are now classed as “salts.” Sodium, or sodic chloride Na Cl, which is now distinguished as “common salt,” is an example of the simplest type of chemical salt, its molecule consisting of one atom of the metal sodium combined with one atom of the gas chlorine, both sodium and chlorine being mono-valent elements, i.e., one atom of each being able to unite with, or displace from a compound, one atom of hydrogen.
Rock-salt is rarely found in an absolutely pure anhydrous state, in which it is colourless and perfectly transparent. In most rock-salt mines such specimens are regarded as curiosities, but in the deposits of Nevada and of Wieliezka, in Hungary (where the salt, containing 100 per cent. NaCl, is the purest in the world), large masses of quite transparent salt are encountered. The white opaque mass which the ordinary person is accustomed to think of as rock-salt, is the purified product of commerce. The colour of sea-water is affected by its percentage of salt, the colour changing from blue to green as the quantity of salt decreases; but sea-salt is generally white, although not transparent owing to the presence of minute particles of water, air, etc., in its intercrystalline spaces. But rock-salt is never more than whitish inclining to grey, and, as a general rule, it is coloured by earth or mineral impurities. The Salt Range in the Panjab yields a substance that varies from pink to red, according to the different quantities of iron present as impurities. That found at Marston, in Cheshire, varies from yellow to red and brownish-red in colour. Small blocks of transparent salt of a deep sapphire blue are occasionally found in the Wieliezka mines. The colour disappears on heating, and when the salt is ground to powder. It is attributed by some chemists to the presence of subchloride of sodium, by others to the presence of thin cavities having parallel surfaces with gas inclusions.
Common salt, which is classed as “sweet” to distinguish it from the bitter-tasting salts of magnesium, has a peculiar saline taste which gains in pungency with refinement, and in its pure state is odourless. In solution, the smallest quantity perceptible to the taste is about 15 grains to the litre, roughly, 68 grains to the gallon.
Common salt is highly soluble in cold water, and rather more so in hot water, but while it dissolves slightly in alcohol, neither ether nor oil has any effect upon it. One hundred parts of distilled water at 60° F. (15·5° C.) will dissolve 35·9 parts of chemically pure NaCl. A saturated solution of common salt, therefore, contains 26·42 per cent. NaCl. The increase of solubility of NaCl in proportion to the rise in temperature, calculated by Gay Lussac and Poggiale, is particularly marked between 100 deg. and 110 deg., when boiling point is passed, the increase amounting to ·74 parts of 10 deg., as compared with an increase of one 1·09 parts between freezing and boiling points. In a double solution of NaCl and some other more soluble salt, as sodium or magnesium sulphate or magnesium chloride, the solubility of sodium chloride is very greatly reduced.
The evaporation of brine is slightly less rapid than that of ordinary pure water, and the boiling point of brine varies with the amount of NaCl present in solution, from 100·21 deg. when only 1 per cent. NaCl is present, to 108·99 deg. when the solution contains 29·4 per cent. of NaCl. A saturated solution of refined table-salt (i.e., a solution containing 26·4 per cent. NaCl) has, at normal temperatures, specific gravity 1·2. Salt crystals have specific gravity 2·167 at a temperature of 17°. The salt which separates at high temperature contains no water of crystallization. But when the thermometer falls much below -15° C. the crystals have the composition NaCl.2H₂O and are prismatic in shape. When heated, these give up their water of crystallization and take the simple composition NaCl.
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.