CHAPTER X SODIUM CHLORIDE—BERTHOLLET'S LAWS—HYDROCHLORIC ACID

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; 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,2H 2 O. At the ordinary temperature these crystals split up into sodium chloride and its solution. Unsaturated solutions of table salt when cooled below 0° give crystals of ice, but when the solution has a composition NaCl,10H 2 O 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