1902.] Properties of the Alloys of the Gold-Silver Series. 161 



of the powder, a small quantity of the hydrochloride of the last- 

 mentioned substance separated. 



[The investigation of these intramolecular changes is being continued. 

 It has been found that a similar interchange of halogen for hydroxyl 

 takes places very readily in solutions of chloro- and bromo-naphthalene- 

 diazoniuni salts, even in the presence of excess of acid. The author is 

 of opinion that the observation of Gaess and Ammelburg,* that an 

 aqueous solution of l-nitro-2-naphthalenediazonium sulphate yields 

 the 1 : 2-naphthalenediazo-oxide, is an example of the type of trans- 

 formation here considered.] 



" On certain Properties of the Alloys of the Gold-Silver Series." 

 By the late Sir W. C. Roberts-Austen, K.C.B., D.C.L., 

 F.E.S., and T. Kirke Rose, D.Sc. Received October 22 — 

 Read December 11, 1902. 



[Plate 3.] 



In a former communication to the Society! the curve of the initial 

 freezing points of the alloys of gold and copper and some micrographic 

 evidence as to their structure were given, and it was shown that 

 according to the theory of solutions the alloys rich in gold should not 

 be homogeneous after they have solidified. The fact that they are 

 not uniform was confirmed by analysis. The subject has, however, 

 more than theoretical interest, and the inference was drawn that 

 standard gold, which consists of eleven parts by weight of gold to one 

 of copper, is unsuitable as a material for the preparation of the trial 

 plates by which the standard of the coinage is tested. These trial 

 plates according to law must contain 916*6 parts of gold and 83'3 of 

 " alloy," that is of some other metal, and it remained to be determined 

 what the other metal should be. 



It will be at once apparent that the alloy or mixture of the two 

 metals must, if the cold mass is to be uniform, solidify as a whole, 

 that is to say, that the crystals first formed should be of the same 

 composition as the mother liquor, and this condition can be fulfilled 

 by isomorphous mixtures only. It has long been recognised that the 

 gold-silver alloys are cases of isomorphism, and Gautier, in 1896, stated^ 

 that the freezing-point curve of the series followed a straight line if 

 the percentages by weight of the constituents were taken as abscissae. 



This curve was re-determined by experiment, a number of alloys 

 being made up and autographic records taken of their cooling curves 

 by the Roberts- Austen recording pyrometer. The results obtained 



* ' Berichte,' 1894, vol. 27, p. 221] . 



t ' Roy. Soc. Proc.,' vol. 67 (1900), p. 105. 



X ' Bull, de la Soc. d' Encouragement,' Oct., 1898. 



