Richards — Application of the Phase Rule, etc. 377 



Aet. XXXI. — Note on the application of the Phase Pule 

 to the fusing points of copper, silver, and gold / by 

 Theodore William Richards. 



In an interesting and very thorough investigation by Hol- 

 born and Day,* the fusing points of certain metals were recently 

 studied with reference to their use as standards of temperature. 

 As a result of this investigation, the investigators found that 

 gold gave a very constant fusing point at 1064° ; that silver 

 was extraordinarily inconstant in its melting point ; and that 

 copper was capable of yielding two constant melting points of 

 1065° and 1084°, according to the presence or absence of air. 



Besides the practical importance of these results, a theoreti- 

 cal interest attaches to them : and since this side of the matter 

 was not emphasized in the paper in question, this brief note is 

 written to call attention to it. 



According to the Phase Rule of Willard Gibbs, a fixed 

 point in any system not affected by electrical or other extra- 

 ordinary stress is determined by the simultaneous existence of 

 a number of phases or other fixed conditions, two more than 

 the number of components. 



Let us apply this rule to the simple cases under review. 

 Clearly one should expect gold to yield a constant melting 

 point, whether in or out of the air, because its tendency to 

 dissolve either oxygen or nitrogen is very slight ; and practi- 

 cally only one component, the metal itself, is concerned. 

 The three fixed conditions are of course the solid, liquid and 

 vapor states of the gold, or perhaps more strictly speaking, 

 solid and liquid gold and the atmospheric pressure. 



On the other hand, the tendency of silver to dissolve oxygen 

 is well known. This tendency introduces another component 

 without introducing another phase ; unless the concentration of 

 the oxygen dissolved in the silver be determined, the number 

 of fixed conditions is too few to determine a fixed point. But 

 it is unusually difficult either to fix or determine the amount 

 of oxygen which the silver dissolves in any given case.f 

 Hence an uncertainty amounting to ten or fifteen degrees, 

 unless oxygen is wholly excluded, is unavoidable. In the 

 latter case Holborn and Day obtained a constant melting 

 point, 961*8, which corresponds to the two phases of the 



* Holborn and Day, this Journal (4), x, 171 (1900), also xi, 145 (1901). 



flf a constant pressure of oxygen were maintained, the other fixed 

 condition demanded by the Phase Eule would be theoretically supplied. But 

 the great variation of the solubility of oxygen in silver with the temperature 

 makes it very difficult to secure this constant pressure or to attain equi- 

 librium. 



