[15] By cooling a solution of table salt saturated at the ordinary temperature to -15°, I obtained first of all well-formed tabular (six-sided) crystals, which when warmed to the ordinary temperature disintegrated (with the separation of anhydrous sodium chloride), and then prismatic needles up to 20 mm. long were formed from the same solution. I have not yet investigated the reason of the difference in crystalline form. It is known (Mitscherlich) that NaI,2H2O also crystallises either in plates or prisms. Sodium bromide also crystallises with 2H2O at the ordinary temperature.
[16] Notwithstanding the great simplicity (Chapter I., Note [49]) of the observations on the formation of ice from solution, still even for sodium chloride they cannot yet be considered as sufficiently harmonious. According to Blagden and Raoult, the temperature of the formation of ice from a solution containing c grams of salt per 100 grams of water = -0·6c to c = 10, according to Rosetti = -0·649c to c = 8·7, according to De Coppet (to c = 10) = -0·55c - 0·006c2, according to Karsten (to c = 10) - 0·762c + 0·0084c2, and according to Guthrie a much lower figure. By taking Rosetti's figure and applying the rule given in Chapter I., Note [49] we obtain—
i = 0·649 × 58·5 / 18·5 = 2·05.
Pickering (1893) gives for c = 1 - 0·603, for c = 2 - 1·220; that is (c up to 2·7) about - (0·600 + 0·005c)c.
The data for strong solutions are not less contradictory. Thus with 20 p.c. of salt, ice is formed at -14·4° according to Karsten, -17° according to Guthrie, -17·6° according to De Coppet. Rüdorff states that for strong solutions the temperature of the formation of ice descends in proportion to the contents of the compound, NaCl,2H2O (per 100 grams of water) by 0°·342 per 1 gram of salt, and De Coppet shows that there is no proportionality, in a strict sense, for either a percentage of NaCl or of NaCl,2H2O.
[17] A collection of observations on the specific gravity of solutions of sodium chloride is given in my work cited in Chapter I., Note [50].
Solutions of common salt have also been frequently investigated as regards rate of diffusion (Chapter [I].), but as yet there are no complete data in this respect. It may be mentioned that Graham and De Vries demonstrated that diffusion in gelatinous masses (for instance, gelatin jelly, or gelatinous silica) proceeds in the same manner as in water, which may probably lead to a convenient and accurate method for the investigation of the phenomena of diffusion. N. Umoff (Odessa, 1888) investigated the diffusion of common salt by means of glass globules of definite density. Having poured water into a cylinder over a layer of a solution of sodium chloride, he observed during a period of several months the position (height) of the globules, which floated up higher and higher as the salt permeated upwards. Umoff found that at a constant temperature the distances of the globules (that is, the length of a column limited by layers of definite concentration) remain constant; that at a given moment of time the concentration, q, of different layers situated at a depth z is expressed by the equation B - Kz = log.(A - q), where A, B, and K are constants; that at a given moment the rate of diffusion of the different layers is proportional to their depth, &c.
[18] If S_0 be the specific gravity of water, and S the specific gravity of a solution containing p p.c. of salt, then by mixing equal weights of water and the solution, we shall obtain a solution containing ½p of the salt, and if it be formed without contraction, then its specific gravity x will be determined by the equation 2 / x = 1 / S0 + 1 / S , because the volume is equal to the weight divided by the density. In reality, the specific gravity is always found to be greater than that calculated on the supposition of an absence of contraction.
[19] Generally the specific gravity is observed by weighing in air and dividing the weight in grams by the volume in cubic centimetres, the latter being found from the weight of water displaced, divided by its density at the temperature at which the experiment is carried out. If we call this specific gravity S1, then as a cubic centimetre of air under the usual conditions weighs about 0·0012 gram, the sp. gr. in a vacuum S = S1 + 0·0012 (S1 - 1), if the density of water = 1.
[20] If the sp. gr. S2 be found directly by dividing the weight of a solution by the weight of water at the same temperature and in the same volume, then the true sp. gr. S referred to water at 4° is found by multiplying S2 by the sp. gr. of water at the temperature of observation.