The discontinuity in v , and other properties, e.g. internal energy, u , and entropy, s , of the substance, is called a first order phase transition. In order to specify the unique experimentally observed pressure, p s ( T ) , at which it occurs another thermodynamic condition is required, for from Fig.1 it could clearly occur for any pressure in the range p m i n ≤ ≤ p ≤ ≤ p m a x . Such a condition was first enunciated in a clever thermodynamic argument by Maxwell at a lecture he delivered to the British Chemical Society on Feb 18, 1875 (Fig.1, including the letters B C D E F, is the curve he described):