378 Richards — Application of the Phase Rule, etc. 



single component under atmospheric pressure. Obviously the 

 highest observed temperature is the most probable, since oxy- 

 gen, or other dissolved substance, will lower the melting point 

 of the metal. 



The chief interest of this note centers in the double melting 

 point of copper. Contrary to superticial expectation, the 

 lower melting point of 1065°, obtained by melting copper in 

 the air, with full opportunity for oxidation, was found by 

 Holborn and Day to be far more constant and trustworthy 

 than the higher one, 1084°, where every precaution was taken 

 to exclude the air and to reduce the cuprous oxide present. 



The Phase Rule at once explains this observation. When 

 air is admitted to the surface of melting copper, the metal 

 oxidizes, and the oxide is at first dissolved by the copper. The 

 tendency of the copper is thus to remove molecular oxygen by 

 converting it into cuprous oxide. Until the copper is saturated 

 with cuprous oxide, the melting point will continue to sink as 

 the metal oxidizes. When, however, the saturation point is 

 reached, any additional cuprous oxide will appear as a new 

 liquid phase, and will have no effect upon the melting point. 

 The components are two, copper and oxygen ; and the phases 

 are four, solid copper, a solution of cuprous oxide in liquid 

 copper, liquid cuprous oxide, and vapor.* Hence the condi- 

 tions for a fixed point demanded by the Phase Rule are present. 



Since the solubility of cuprous oxide in copper undoubtedly 

 varies with the temperature, it is clear that the adjustment 

 of the equilibrium should be so deliberate as to allow the 

 relieving of possible supersaturation. 



The higher melting point, 1084°, of pure copper is clearly 

 to be obtained only by the complete removal of the second 

 component, oxygen — a process which is somewhat difficult. 

 When this component has been removed, three phases are 

 enough to fix the triple-point of fusion of the pure substance, 

 as usual. 



Thus in this case, as in so many others, the Phase Pule 

 serves to classify the facts in such a manner as to make them 

 more clearly comprehensible and more readily retained in the 

 memory. 



Chemical Laboratory of Harvard College, 

 Cambridge, Mass., March 1st, 1902. 



* Since the nitrogen of the air does not enter into the reaction, it may be 

 neglected. Its only effect is to cause a slight additional pressure on the 

 system too small to produce perceptible result. 



