Note 65 — Chapter I · ON WATER AND ITS COMPOUNDS

According to Lescœur (1883), at 100° a concentrated solution of barium hydroxide, BaH2O2, on first depositing crystals (with + H2O) has a tension of about 630 mm. (instead of 760 mm., the tension of water), which decreases (because the solution evaporates) to 45 mm., when all the water is expelled from the crystals, BaH2O2 + H2O, which are formed, but they also lose water (dissociate, effloresce at 100°), leaving the hydroxide, BaH2O2, which is perfectly undecomposable at 100°—that is, does not part with water. At 73° (the tension of water is then 265 mm.) a solution, containing 33H2O, on crystallising has a tension of 230 mm.; the crystals, BaH2O2 + 8H2O, which separate out, have a tension of 160 mm.; on losing water they give BaH2O2 + H2O. This substance does not decompose at 73°, and therefore its tension = 0. In those crystallohydrates which effloresce at the ordinary temperature, the tension of dissociation nearly approximates to that of the aqueous vapour, as Lescœur (1891) showed. To this category of compounds belong B2O3(3 + x)H2O, C2O4H2(2 + x)H2O, BaO(9 + x)H2O, and SrO(9 + x)H2O. And a still greater tension is possessed by Na2SO410H2O, Na2CO310H2O, and MgSO4(7 + x)H2O. Müller-Erzbach (1884) determines the tension (with reference to liquid water) by placing tubes of the same length with water and the substances experimented with in a desiccator, the rate of loss of water giving the relative tension. Thus, at the ordinary temperature, crystals of sodium phosphate, Na2HPO4 + 12H2O, present a tension of 0·7 compared with water, until they lose 5H2O, then 0·4 until they lose 5H2O more, and on losing the last equivalent of water the tension falls to 0·04 compared with water. It is clear that the different molecules of water are held by an unequal force. Out of the five molecules of water in copper sulphate the two first are comparatively easily separated even at the ordinary temperature (but only after several days in a desiccator, according to Latchinoff); the next two are more difficultly separated, and the last equivalent is retained even at 100°. This is another indication of the capacity of CuSO4 to form three hydrates, CuSO45H2O, CuSO43H2O, and CuSO4H2O. The researches of Andreae on the tension of dissociation of hydrated sulphate of copper showed (1891) the existence of three provinces, characterised at a given temperature by a constant tension: (1) between 3–5, (2) between 1–3, and lastly (3) between 0–1 molecule of water, which again confirms the existence of three hydrates of the above composition for this salt.