108 Sir \V. l:.il,ms-Au>t-n ami Dr. T. Kirkc Hose. 



cent, of gold, is really the eutectic. The reason, apart from micro- 

 graphic evidence, is not only that the freezing point of the 82 per cent, 

 gold alloy is lower than that of any other member of the series, but 

 the autographic record reproduced in fig. 2 shows that the angles at A 



Fio. 2. 



and B, where the solidification begins and ends, are quite sharp, while 

 the portion between A and B, which represents the actual solidification 

 of the alloy, is horizontal. Neither of these conditions are met with in 

 the autographic records of the other alloys of the series. Moreover, the 

 fracture of the 82 per cent, alloy is conchoidal, as in the case of a great 

 number of other eutectics, owing to the extremely fine state of division 

 of the constituents, which makes these alloys appear to be homo- 

 geneous. The exact composition of the eutectic is, however, difficult to 

 determine. 



A comparison of the freezing-point curve of the gold-copper with 

 that of silver-copper alloys shows that there are striking similarities 

 when the number of atoms in the alloys are taken as abscissae. It was 

 shown by Levol* as long ago as 1852 that the only homogeneous alloy 

 of silver and copper corresponded in composition with the formula 

 Ag 3 Cuo, and Heycock and Nevillet confirmed the anticipation of one 

 of us,} which was not verified at the time, that it would prove to be 

 the eutectic of the series. In the gold-copper series the alloy contain- 

 ing 59 - 49 atoms of gold and 40'41 atoms of copper has a lower 

 freezing point than any other alloy examined, although it is hardly to 

 be distingu shed from the alloys containing a little more copper. The 

 curve of fusibility of the series is much more rounded near this point 

 than that of most binary alloys, and bears a superficial resemblance to 

 that of two substances forming a continuous series of mixed crystals, 

 but micrographic study of the series conclusively shows that it possesses 

 a eutectic. 



Levol, ' Annales de Chim. et de Phys.,' rol. 36 (1852), p. 193 ; vol. 39 (1853), 

 p. 163. 



t ' Phil. Trans.,' A, vol. 189 (1897), p. 25. 

 J Roberts-Austen, loc. cit. 



