Pure sodium chloride, in well-defined crystals (slowly deposited at the bottom of the liquid) or in compact masses (in which form rock salt is sometimes met with), is a colourless and transparent substance resembling, but more brittle and less hard than, glass.12 Common salt always crystallises in the cubic system, most frequently in cubes, and more rarely in octahedra. Large transparent cubes of common salt, having edges up to 10 centimetres long, are sometimes found in masses of rock salt.13 When evaporated in the open the salt often separates out on the surface14 as cubes, which grow on to each other in the form of pyramidal square funnels. In still weather, these clusters are able to support themselves on the surface of the water for a long time, and sometimes go on increasing to a considerable extent, but they sink directly the water penetrates inside them. Salt fuses to a colourless liquid (sp. gr. 1·602, according to Quincke) at 851° (V. Meyer); if pure it solidifies to a non-crystalline mass, and if impure to an opaque mass whose surface is not smooth. In fusing, sodium chloride commences to volatilise (its weight decreases) and at a white heat it volatilises with great ease and completely; but at the ordinary temperature it may, like all ordinary salts, be considered as non-volatile, although as yet no exact experiments have been made in this direction.
A saturated15 solution of table salt (containing 26·4 p.c.) has at the ordinary temperature a specific gravity of about 1·2. The specific gravity of the crystals is 2·167 (17°). The salt which separates out at the ordinary and higher temperatures contains no water of crystallisation;16 but if the crystals are formed at a low temperature, especially from a saturated solution cooled to -12°, then they present a prismatic form, and contain two equivalents of water, NaCl,2H2O. At the ordinary temperature these crystals split up into sodium chloride and its solution.17 Unsaturated solutions of table salt when cooled below 0° give18 crystals of ice, but when the solution has a composition NaCl,10H2O it solidifies completely at a temperature of -23°. A solution of table salt saturated at its boiling point boils at about 109°, and contains about 42 parts of salt per 100 parts of water.
Of all its physical properties the specific gravity of solutions of sodium chloride is the one which has been the most fully investigated. A comparison of all the existing determinations of the specific gravity of solutions of NaCl19 at 15° (in vacuo, taking water at 4° as 10,000), with regard to p (the percentage amount of the salt in solution), show that it is expressed by the equation S15 = 9991·6 + 71·17p + 0·2140p2. For instance, for a solution 200H2O + NaCl, in which case p = 1·6, S15 = 1·0106. It is seen from the formula that the addition of water produces a contraction.20 The specific gravity21 at certain temperatures and concentrations in vacuo referred to water at 4° = 10,00022 is here given for
| 0° | 15° | 30° | 110° | |
|---|---|---|---|---|
| p = 5 | 10372 | 10353 | 10307 | 9922 |
| 10 | 10768 | 10728 | 10669 | 10278 |
| 15 | 11164 | 11107 | 11043 | 10652 |
| 20 | 11568 | 11501 | 11429 | 11043 |
It should be remarked that Baumé's hydrometer is graduated by taking a 10 p.c. solution of sodium chloride as 10° on the scale, and therefore it gives approximately the percentage amount of the salt in a solution. Common salt is somewhat soluble in alcohol,23 but it is insoluble in ether and in oils.
Common salt gives very few compounds24 (double salts) and these are very readily decomposed: it is also decomposed with great difficulty and its dissociation is unknown.25 But it is easily decomposed, both when fused and in solution, by the action of a galvanic current. If the dry salt be fused in a crucible and an electric