176 Dr. Wright and Messrs. Thompson and Leon. [Feb. 



any form, but is most conveniently chosen eqnilateral ; and, to repre- 

 sent the composition of any mixture of the three, imagine weight - 

 equal to those of the substances A, B, C placed at the points A, B, C, 

 and find their centre of gravity, P. To each different set of propor- 

 tions A : B : C (the letters here denoting weights) will correspond a 

 different position of P, which point will serve to represent to the ej 

 the composition of an actual or ideal alloy (supposing the substances 

 to be metals) formed of the three metals in the given proportions. 

 If the quantity of the solvent be sufficient, P will represent on tht 

 diagram the composition of an actual alloy. If it be insufficient, tl 

 alloy represented as to composition by P will be ideal only ; and 

 attempting to form it the mass will separate into two layers. If w( 

 suppose the agitation to have been sufficient, there will be equilibriui 

 of solution at the surface of junction, and the mass will have reacht 

 its final state. Supposing this condition to have been attained, 

 the two portions be analysed, and the points Q, R representing tl 

 compositions be laid down on the diagram, and joined by a straight 

 line. From the construction, this line must pass through the poii 

 P if there has been no loss by volatilisation or oxidation. Let 

 same thing be done for several other proportions of the ingredient 

 Then the points Q, R will lie in a curve aQLR6, cutting AB in tv 

 points a, b, which represent, the first, a saturated solution of B in 

 the second, a saturated solution of A in B. Call this curve tl 

 critical curve, and the lines such as QR tie-lines, or simply ties. Tl 

 the critical curve and the system of ties will represent the compl 

 result of the experiments, supposing them to have been exactly 

 Alloys of a pair may conveniently be called conjugate. Intermedij 

 tie-lines may be interpolated by eye ; or if we prefer we may sul 

 tute for the system of ties their envelope, on which plan the resnl 

 of the experiments would be completely represented by two curve 

 the critical curve and the envelope. 



The critical curve separates mixtures of which alloys can actually 

 be formed from those on attempting to form an alloy of which the 

 mass separates into two layers. In the latter case, if through P 

 draw a tangent to the envelope, cutting the critical curve in Q, 

 the points Q, R will represent the compositions of the portions int 

 which the mass separates, while their weights will be as PR to PQ. 



If L be the limiting position of the chord QR, or, in other words, 

 the point of contact with the critical curve of a common tangent to it 

 and the envelope, as P tends to coincide with L, the two strata into 

 which the mass separates tend to become identical in nature. If we 

 take a mixture of A and B, represented by a point c in aft, and con- 

 tinually increase the quantity of C from 0, the point P will ascend 

 from c towards C until it reaches the critical curve. At this stage 

 the quantity of the second alloy has just dwindled away to nothing, 





