ON GOLD-ALUMINIUM ALLOYS. 205 



We will now briefly discuss the singularities in the curve.* From A, the freezing 

 point of pure gold, to B, the curve is nearly a straight line. The freezing points of 

 the very dilute solutions are, as usual, very steady temperatures, but as we approach 

 B the upper freezing point is indicated by a slight pause only in the cooling, and lower 

 second freezing points become well marked. These are all at about 545, and are very 

 steady temperatures. They evidently occur when, after the crystallisation of some 

 gold, the residual liquid has reached a composition of 19 or 20 atoms of aluminium. 



From B onwards a new branch of the curve starts which ends in a eutectic 

 point at C, when the alloy contains about 21 '5 atoms of aluminium. An alloy of 

 this composition has the low melting point of 525. This is lower than that of any 

 other mixture of gold and aluminium. We have here the true eutectic of these two 

 metals. The second freezing point at 21 atoms probably belongs to the horizontal 

 line of eutectics through C, which is better marked by other points. There was 

 surfusion at this second freezing point, and therefore it is certain that a new solid 

 now began to form in the crucible. It was noticed that the alloy with 22'3 atoms of 

 aluminium solidified at an extremely constant temperature. This is in harmony with 

 the fact that we are now close to the eutectic angle. 



If we pass from C towards D by adding aluminium, the upper freezing point is at 

 first very transient, while the lower one, belonging to the same alloy, is a very 

 steady temperature. But as we approach D, near 28 atoms, the reverse is true. 

 For example, above 26 atoms of aluminium the lower freezing point is lost, and the 

 upper one becomes an extremely steady temperature. Surfusion was noticed at most 

 of the freezing points, both upper and lower, between C and D. 



At D there is a singularity in the curve ; the freezing points have become very 

 steady temperatures, and, as usual with very steady freezing points, the curve is 

 flat. At a point close to 28 atoms of aluminium a new and rapidly rising branch 

 of the curve begins, the earlier freezing points on it being fugitive. Moreover, 

 through the point of bifurcation the original line of freezing points is continued in a 

 line of second freezing points, which is horizontal for some distance and then 

 descends a little. The points on this line are marked by very steady temperatures 

 like a line of eutectics. From the experimental data for the curve it is difficult to 

 decide whether the summit of the branch CD is to the left or right of the point of 

 bifurcation, or whether there is a short flat to the left of D. But the microscope 

 supplies some reason for thinking that the branch DE cuts the branch CD so that the 

 summit of the latter is a very little to the right of the intersection D, and therefore 

 corresponds to a body that could only be obtained quite pure by surfusion. If we 

 assume that the summit has the formula Au s Al 2 , it should be at 28 '6 atoms of 

 aluminium. The curve of second freezing points starting from D, at first horizon- 

 tally and gradually sinking, simulates a continuation of the branch CD, but as they 



* The rapid depression in the freezing point of gold, due to the presence of small quantities of 

 aluminium, and the great rise in the freezing point as the composition corresponding to the compound 

 AuAl, is approached, have been already discovered by Sir William Roberts-Austen. 



