[65] According to Lesc[oe]ur (1883), at 100° a concentrated solution of barium hydroxide, BaH_{2}O_{2}, on first depositing crystals (with + H_{2}O) 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, BaH_{2}O_{2} + H_{2}O, which are formed, but they also lose water (dissociate, effloresce at 100°), leaving the hydroxide, BaH_{2}O_{2}, 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 33H_{2}O, on crystallising has a tension of 230 mm.; the crystals, BaH_{2}O_{2} + 8H_{2}O, which separate out, have a tension of 160 mm.; on losing water they give BaH_{2}O_{2} + H_{2}O. 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 Lesc[oe]ur (1891) showed. To this category of compounds belong B_{2}O_{3}(3 + _x_)H_{2}O, C_{2}O_{4}H_{2}(2 + _x_)H_{2}O, BaO(9 + _x_)H_{2}O, and SrO(9 + _x_)H_{2}O. And a still greater tension is possessed by Na_{2}SO_{4}10H_{2}O, Na_{2}CO_{3}10H_{2}O, and MgSO_{4}(7 + _x_)H_{2}O. 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, Na_{2}HPO_{4} + 12H_{2}O, present a tension of 0·7 compared with water, until they lose 5H_{2}O, then 0·4 until they lose 5H_{2}O 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 CuSO_{4} to form three hydrates, CuSO_{4}5H_{2}O, CuSO_{4}3H_{2}O, and CuSO_{4}H_{2}O. 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