OK THE THENOMENA AND THEORIES OF SOLUTION. 457 



the increase in the specific heat of water at this higher temperature. 

 (For numerical values see Naumann, ' Thermochemie,' pp. 269 — 271.) 



It is also well known that the vapour pressure of water holding in 

 solution almost any dissolved solid is less than the vapour pressure of 

 pure water, and that the boiling point of a liquid is raised by the addition 

 to it of any soluble non- volatile substance. 



The well-known researches of Wiillner (Pogg. ' Ann.' ciii. p. 529, and 

 ex. p. 564) led him to the conclusion that the reduction in the tension of 

 the vapour of water consequent upon the addition of soluble substances 

 is proportional to the amount of dissolved substance (see, however, 

 Nicol, * Phil. Mag.' Oct. 1885). 



The law connecting the amount of dissolved substance with the 

 diminution of pressure, or the amount of diminution with the tempera- 

 ture, is, however, a matter of small importance in connection with the 

 present inquiry. The fact that there is reduction of pressure is the 

 important point, and this can only be explained upon the hypothesis that 

 there is no free water present at all ; that is, that there is no water 

 present which is not more or less under the influence of the dissolved 

 substance. If any water were present in an uncombined state the rate 

 of evaporation from the surface would doubtless be slower than the rate 

 of evaporation of pure water at the same temperature, but the pressure 

 of the evolved vapour must in time attain to the same maximum.. 



Water of Crystallisation. 



What becomes of water of crystallisation forms a part of the same 

 question as to the relation of solvent to solvend. We know that when 

 white copper sulphate is dissolved in water evolution of heat results, and 

 a blue solution is formed of the same colour as the crystals of the 

 hydrated salt and the solution formed by dissolving them in water. 

 Similarly blue anhydrous cobalt chloride dissolves in water, forming a 

 red solution of the same tint as the red crystals which contain the salt 

 and water united together. We are, therefore, disposed to conclude that 

 these salts, and by analogy those which are colourless, retain their hold 

 upon the water of crystallisation when they are dissolved in water. 



Thomsen has also shown ' that of the thirty- five salts examined by him 

 twenty-nine exhibit the peculiarity that when the anhydrous salt dissolves 

 in water with evolution of heat, the addition of more water also causes 

 evolution of heat, and that when, on the contrary, the heat of solution is 

 negative, the heat of dilution is also negative. The former without 

 exception unite with water to form crystalhsable definite hydrates ; the 

 latter do not. 



Thomsen states his opinion on the point thus (iii. p. 31) : — ' There 

 is no doubt that the salts which dissolve in water with evolution of much 

 heat, and form crystallised hydrates, are present also in the solution as 

 tydrated compounds ; but a determination of the number of water mole- 

 cules contained in such compounds would be very difficult,' &c. Thomsen 

 oonsiders the hypothesis very probable that a salt dissolved in water cannot 

 retain chemically combined a larger number of water molecules than are 



' Thomsen's Two Groups. — I. Chlorides, Ca, Mg, Zn, Ni, Cu; nitrates, Mg, 

 3In, Zn, Cu ; acetates, K, Na, Am, Zn ; sulphates, Mg, Mn, Zn, Cu, and NaHSO,. 

 II. Chlorides, Na, Am ; bromide, K ; cyanide, K ; sulphate, Am ; nitrates, Na, 

 Am, Sr, Pb; tartrate, Am; and bicarbonate, Am. Exceptions: KHSOj, AmHSO^, 

 :Sa,I, KjCO,, Na^COj, Na^SO,. 



