space, then no work is done. As Begnault's formula refers to constant pressure, the heat of vaporisation fi^ at constant volume must be smaller by 27 cal. (see p. 91), i.e. —
/I, = (14,332 - 14-51 T) cal.
According to this formula the heat of vaporisation should be equal to at temperature 7= y -= 715° C. As a
matter of fact, the heat of vaporisation becomes equal to at a much lower temperature, namely, at the critical tempe- rature, which is about 365° C.
Sometimes the sign of the value of ja becomes changed at a particular temperature. In such cases the system possesses either a maximum or a minimum. According to the investiga- tions of Etai*d and Engel (14), this behaviour is shown by the solubility of the sulphates, and at high temperatures by salts in general Thus, ferrous sulphate has a maximum solubility at 63°, zinc sulphate at 82°, nickel sulphate at 122°, and copper sulphate at 130°. At the temperature at which this occurs it is frequently found that a change takes place in the solid whereby water of crystallisation is lost, and two diflferent salts are present, the solubility curves of which cut each other. Thus, at 34° sodium sulphate passes from the modi- fication NaaS04 + IOH2O (the ordinary crystallised Glauber salt) into the anhydrous salt NaaS04. The former dis- solves in water with absorp- tion of heat (18,760 cal. according to Thomson), the latter with a slight evolu- tion of heat (4600 caL). As the temperature rises, the solubility of the former salt increases, as the diagram (Fig. 20) shows, and that of
the latter salt diminishes; this gives rise to an (apparent) maximum solubility of Glauber salt at 34°. If we may
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