SALT (FR., Sel ; GER.; Salz, Chlornatrium).—Formula, NaC1 ; hardness, 2 .5 ; sp. gr., 2 .1-2.57.
If we consider the natural products of the earth in their relative economic importance, salt, one of the most abundant and universally diffused, commends itself pre-eminently to our notice. Yet comparatively little has been written concerning it, and the industry, one of the most important we possess, may be said, with the exception of a few trifling innovations, to be practically in the same position as it was 50 or 100 years ago.
Common salt (sodium chloride) may be directly produced by the combination of chlorine with sodium. It has been stated that sodium takes fire when immersed in chlorine gas ; but Wanklyn has shown that, unless some moisture he present, such is not the case, and it is certain that metallic sodium remains bright for some time, even when immersed in liquefied chlorine anhydride. The composition of salt is :— Equivalents. Percentages.
Chlorine .. .. 35 . 5 60 . 7 Sodium .. .. 23 . 0 39.3 A blue sodium subchloride is (probably erroneously) stated to be produced by passing hydrogen over sodium chloride at high temperatures. Salt is isometric ; it crystallizes in anhydrous cubes, and other congeneric forms ; its cleavage is perfect ; taste, cooling and agreeably saline ; when pure, it is white, and is ofteu found in nature in pellucid and perfectly colourless crystals as rock-salt, but more frequently rock-salt is grey, rose, brick-red, yellow, violet, blue, or green, being stained by iron, bitumen, or other impurities. When crystals of salt form by evaporation on the surface of still brine, as frequently occurs in the manufacture, the cubes have a tendency to agglomerate themselves by their angular edges, so as to build hollow four-sided cups, called " hopper-crystals " (Fr., tremis). Fishery-, bay-, and dessert- salts illustrate this peculiar form of crystallization. Although salt crystals are anhydrous, they are liable to contain water mechanically intercalated between their crystalline plates, causing them to decrepitate when somewhat suddenly and strongly heated. This decrepitation rarely occurs with rock-salt, and only in a small degree with the heavier larger crystals of salt produced during the slow spontaneous evaporation of sea or other salt water. Salt fuses at 776° (1428° F.), and volatilizes, but not in covered vessels at a temperature approaching its point of fusion, sustaining thereby considerable loss of weight. When a saturated
solution is cooled to — 10° (14° F.) or a few degrees lower, it crystallizes in hexagonal tables of hydrated sodium chloride (NaC1+2H20); and a further reduction of the temperature to about — 22° (— 5.80°F.) causes the separation of bundles of fibrous or needle-shaped crystals, having the composition of a. cryobydrate, and containing ten equivalents of water (NaC1-1-10 11,0) ; both these forms deliquesce with the mere heat of the hand, and may be seen to resolve themselves rapidly with a species of decrepitation into sodium chloride solution and numerous small cubes of common salt.
Sodium chloride is decomposed slowly at a red heat in presence of aqueous vapour into caustic soda and hydrochloric acid, according to the formula 2NaC1 ÷E120 = 2EICH-Na20. This fact has been utilized in an attempt to manufacture soda from common salt, by mixing salt with sili ceous sand, placing the mixture in a retort, heating to redness, and passing steam ; but the experiment gave no hopes of commercial success.
Salt is nearly as soluble in water at ordinary temperatures as at the boiling-point ; and when a saturated solution is heated in a vessel admitting of evaporation, it crystallizes out, forming hopper crystals at the surface if the liquid bc maintained tmnquil, or sinking to the bottom as a fine crystalline powder (butter-salt) if the liquor be kept in a state of agitation. Salt is one of the most highly diathermanous bodies, and at the same time one of the most perfect in its absence of ther mochroie priperties, permitting the passage alike of dark and of visible heat rays, s.nd of heat rays of ull degrees of refrangibility. Specimens of colourless pellucid roek-salt are therefore highly prized in researches on radiant beat. Transparent rock-salt transmits no less than 92.3 per cent. of radiant heat from every source, whether the radiating surface be highly incandescent, or the mys be invisible; while the best specimens of flint-glass transmit only 28 per cent. of the heat radiated by red-hot platinum, and still less of dark heat rays ; and ice cuts off all radiant heat from either of these sources. Melloni regards clear rock-salt as being completely diatherrnanous, attri buting the 7.7 per cent. by which the intensity of the incident rays is diminished to an effect of reflection at the surfaces of ingress and egress, not to interior absorption.