transition points of many of these. Condensed systems are characterised by having a surface of separation between the substances on the two sides of the equilibrium sign (^) in the equation, and are thus necessarily heterogeneous. Thus, in the example mentioned, the Glauber salt crystals and the Epsom salts crystals are separated from the astrakanite crystals and the water by well-defined surfaces. For the liquid part of the system formed by the water and the salts dissolved therein^ the law of condensed systems " that the components are only stable in presence of each other at the transition point, 21*5°/' does not hold.
In so far as the quantitative respect is concerned, the case is somewhat different for those physical and chemical changes in which a gas is produced. As an example of this kind we may conveniently take the system: water ^ steam (or water vapour). At 20° and 17"4 mm. pressure this system possesses a transition point, for at this pressure water is only stable under 20° and steam only above 20"*, but at 20° the two forms can "co-exist." Here it is evidently necessary, in distinction to condensed systems, to give the (vapour) pressure for the transition point, since this has now as much influence as the temperature.
Formerly it was supposed that all chemical reactions took place completely. It was conceived that the stronger affinity caused the reaction to be complete at the expense of the weaker affinity. This view of chemical reactions was first systematised by Torbem Bergman, and it prevailed until quite recent times. Thermochemists, more particularly Berthelot, have striven to uphold this conception, which has no strict scientific foimdation.
The Phase Rule of Gibbs. — A state of equilibrium between substances in a homogeneous system is usually termed a homogeneous equilibrium. The corresponding name for equilibrium in a heterogeneous system is heterogeneous equU^mum. W. Gibbs (i?) calls the homogeneous parts of a heterogeneous system the "phases" of the system. In the equilibrium between ice, water, and water vapour (at 0"
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