The researches of Scheffer (1888) on the rate of diffusion (in water) of solutions of hydrochloric acid show that the coefficient of diffusion k decreases with the amount of water n, if the composition of the solution is HCl,nH2O at 0°:—
| n | = | 5 | 6·9 | 9·8 | 14 | 27·1 | 129·5 |
| k | = | 2·31 | 2·08 | 1·86 | 1·67 | 1·52 | 1·39 |
It also appears that strong solutions diffuse more rapidly into dilute solutions than into water.
[39] If it be admitted that the maximum of the differential corresponds with HCl,6H2O, then it might be thought that the specific gravity is expressed by a parabola of the third order; but such an admission does not give expressions in accordance with fact. This is all more fully considered in my work mentioned in Chapter I., Note [19].
[40] As in water, the coefficient of expansion (or the quantity k in the expression St = S0 - kS0t, or Vt = 1/(1 - kt)) attains a magnitude 0·000447 at about 48°, it might be thought that at 48° all solutions of hydrochloric acid would have the same coefficient of expansion, but in reality this is not the case. At low and at the ordinary temperatures the coefficient of expansion of aqueous solutions is greater than that of water, and increases with the amount of substance dissolved.
[41] The figures cited above may serve for the direct determination of that variation of the specific gravity of solutions of hydrochloric acid with the temperature. Thus, knowing that at 15° the specific gravity of a 10 p.c. solution of hydrochloric acid = 10,492, we find that at t° it = 10,530 - t(2·13 + 0·027t). Whence also may be found the coefficient of expansion (Note [40]).
[42] Thus, for instance, with feeble bases they evolve in dilute solutions (Chapter III., Note 53) almost equal amounts of heat; their relation to sulphuric acid is quite identical. They both form fuming solutions as well as hydrates; they both form solutions of constant boiling point.
[42 bis] Pybalkin (1891) found that copper begins to disengage hydrogen at 100°, and that chloride of copper begins to give up its chlorine to hydrogen gas at 230°; for silver these temperatures are 117° and 260°—that is, there is less difference between them.
[43] When an unsaturated hydrocarbon, or, in general, an unsaturated compound, assimilates to itself the molecules Cl2, HCl, SO3, H2SO4, &c., the cause of the reaction is most simple. As nitrogen, besides the type NX3 to which NH3, belongs, gives compounds of the type NX5—for example, NO2(OH)—the formation of the salts of ammonium should be understood in this way. NH3 gives NH4Cl because NX3 is capable of giving NX5. But as saturated compounds—for instance, SO3,H2O, NaCl, &c.—are also capable of combination even between themselves, it is impossible to deny the capacity of HCl also for combination. SO3 combines with H2O, and also with HCl and the unsaturated hydrocarbons. It is impossible to recognise the distinction formerly sought to be established between atomic and molecular compounds, and regarding, for instance, PCl3 as an atomic compound and PCl5 as a molecular one, only because it easily splits up into molecules PCl3 and Cl2.
[44] Sal-ammoniac is prepared from ammonium carbonate, obtained in the dry distillation of nitrogenous substances (Chapter [VI].), by saturating the resultant solution with hydrochloric acid. A solution of sal-ammoniac is thus produced, which is evaporated, and in the residue a mass is obtained containing a mixture of various other, especially tarry, products of dry distillation. The sal-ammoniac is generally purified by sublimation. For this purpose iron vessels covered with hemispherical metallic covers are employed, or else simply clay crucibles covered by other crucibles. The upper portion, or head, of the apparatus of this kind will have a lower temperature than the lower portion, which is under the direct action of the flame. The sal-ammoniac volatilises when heated, and settles on the cooler portion of the apparatus. It is thus freed from many impurities, and is obtained as a crystalline crust, generally several centimetres thick, in which form it is commonly sold. The solubility of sal-ammoniac rises rapidly with the temperature: at 0°, 100 parts of water dissolve about 28 parts of NH4Cl, at 50° about 50 parts, and at the ordinary temperature about 35 parts. This is sometimes taken advantage of for separating NH4Cl from solutions of other salts.