102 PRINCIPLES OF CHK-MJSTKY 



crystallo-hydrates is naturally, as with solutions, less than the vapour 

 tension of water itself '"' at the same temperature, and therefore many 

 anhydrous salts which are capable of combining with water absorb 

 aqueous vapour from moist air ; that is, they act like a cold body on 

 which water is deposited from steam. It is on this that the desiccation 

 of gases is based, and it must farther be remarked in this respect that 

 certain substances for instance, potassium carbonate (Iv 3 CO 3 ) and 

 calcium chloride (CaCL>) not only absorb the water necessary for the 

 formation of a solid crystalline compound, but also give solutions, or 

 deliquesce, as it is termed, in moist air. Many crystals do not effloresce 

 in the least at the ordinary temperature ; for example, copper sulphate, 

 which may be preserved for an indefinite length of time without efflo- 

 rescing, but when placed under the receiver of an air pump, if efflores- 

 cence be once started, it goes on at the ordinary temperature. The 

 temperature at which the entire separation of water from crystals takes 

 place varies considerably, not only for different substances but also for 

 different portions of the contained water. Very often the temperature 

 at which dissociation begins is very much higher than the boiling point 

 of water. So, for example, copper sulphate, which contains 36 p.c. of 

 water, gives up 2 8 '8 p.c. at 100, and the remaining quantity, namely 

 7*2 p.c., only at 240. Alum, out of the 45'5 p.c. of water which it con- 

 tains, gives up 18-9 p.c. at 100, 17'7 p.c. at 120, 7-7 p.c. at 180, and 

 1 p.c. at 280; it only loses the last quantity (1 p.c.) at their temperature 

 of decomposition. These examples clearly show that the annexation of 

 water of crystallisation is accompanied by a rather profound, although, 

 in comparison with instances which we shall consider later, still incon- 



65 According to Lescoeur (1883), at 100 a thick solution of barium hydroxide, BaH 2 O 2r 

 on first depositing crystals (with + H.>O) has a tension of about 630 mm. (instead of 7(50 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 + HoO, which are formed, but they 

 also lose water (dissociate, effloresce at 100), leaving the hydroxide, BaH^O^, which is per- 

 fectly undecomposable at 100 that is, does not part with water. At 73 (the tension of 

 water is then 265 mm.) a solution, containing 33H.^>O, on crystallising has a tension of 

 280 mm. ; the crystals BaH 2 O + 8H 2 O, which separate out, have a tension of 1(10 mm. ; on 

 losing water they give BaH 2 O 2 --HoO. This substance does not decompose at 7:! . and 

 therefore its tension =0. Miiller-Erzbach (1884) determines the tension (with reference 

 to liquid water) by placing similar long tubes with water and tin- substances experi- 

 mented with in a desiccator, the rate of loss of water giving the relative tension. Thus, 

 at the ordinary temperature, crystals of sodium phosphate, Na.,HPO j -r 12H.->O, present 

 a tension of 0*7 compared with water, until they lose 5H 2 O, then 0'4 until they lose ">HoO 

 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 held firmly, even at 100. 



