86 SCIENCE PROGRESS. 



Alder Wright has traced the critical curve for the 

 following cases, the first named metal of each triad being 



Sn, Pb, Zn 

 Sn, Bi, Zn 

 Sn, Pb, Al 

 Cd, Bi, Zn 

 Cd, Pb, Zn 

 Sb, Pb, Al 

 Sb, Bi, Al 

 Ag, Bi, Al 

 Ag, Bi, Zn 



the solvent metal. 



A critical curve having been determined 

 we can at once predict from it whether a 

 proposed mixture will remain homogeneous 

 or will separate into two layers. If the 

 point corresponding to the mixture be 

 plotted, and lies on or outside the critical 

 curve, the alloy will be homogeneous ; if 

 it lies inside, the alloy will split into conju- 

 gates. A critical curve must of course be constructed as the 

 result of a series of experiments, carried out at a constant 

 temperature. If the temperature rises, the curve shifts, 

 generally in the sense that the higher the temperature, the 

 smaller is the area inside the curve ; that is, the less solvent 

 is needed to make a homogeneous or real alloy. The Bi 

 Zn Cd curve given in figure 2 is very sensitive to change 

 of temperature. 



If the critical curves of the triads of metals, habitually 

 used together in the arts, were determined at the lowest 

 temperatures compatible with perfect fluidity, we might 

 perhaps find in them a rational basis for predicting the 

 proportions of the metals to be employed in the most 

 useful alloys. 



Alloys which were real, in the above sense, would 

 probably prove to be mechanically homogeneous when 

 solid ; although even in these, a microscopical examination 

 would in most cases show structure. 



THE SOLIDIFICATION OF ALLOYS. 

 In studying the process by which an alloy solidifies 

 observations with the thermometer are of great value. 

 For, so long as the mass of cooling substance remains 

 completely liquid, and abrupt chemical changes do not 

 take place, a thermometer immersed in it will indicate 

 a uniform rate of cooling, but the separation of solid matter 

 will be marked by the evolution of heat, of fusion or solu- 

 tion. If the formation of solid matter is gradual and con- 



