96 Mr. A. P. Laurie on the Existence of a 



Having thus dealt with alloys occurring in Matthiessen's 

 first two groups, it seemed of interest to examine next an 

 alloy in which he believed chemical combination to have taken 

 place, and I have therefore selected the gold-tin alloys as 

 most suitable for this purpose. 



Matthiessen's conductivity-curve for these alloys resembles 

 a VV i n shape, and he suggests that there are three compounds 

 formed by these metals, one corresponding to the maximum 

 point, and two to the minimum points of the curve. To these 

 compounds he gives the formulae Au 4 Sn, x4.uSn, and Au 2 Sn 5 

 respectively*. It remains to be seen what fresh light the 

 new method throws upon these conclusions. 



With a view to these experiments I obtained some pre- 

 cipitated gold and some of their finest grain tin from Messrs. 

 Johnson and Matthey, and made up with these a series of 

 gold-tin alloys by melting the metals in a clay tobacco-pipe, 

 and after thorough mixing drawing the melted metal into the 

 stem. The alloys so obtained were white and brittle, the only 

 one having a slightly yellow colour being the last of the series, 

 containing 10 per cent, of tin. The percentage composition 

 was calculated from the weights of each metal taken. This 

 was found sufficiently accurate, and was checked in the case 

 of one alloy, as will be presently explained, by an assay which 

 Prof. Roberts-Austen kindly had made for me at the Mint. 

 The result of the assay only differed by "2 per cent, from the 

 composition calculated from the* weights of metal taken. 



The alloys having been prepared, a small voltaic cell was 

 arranged, consisting of a solution of stannic chloride in the 

 outer cell, prepared by passing chlorine into stannous chloride, 

 and a solution of gold chloride in the inner cell, in which was 

 immersed a plate of pure gold. A brass clip tipped with pla- 

 tinum and connected to mercury poles in a paraffin block was 

 used to hold the little rods of alloys. Each rod of alloy was 

 carefully cleaned with a fresh piece of glass-paper just before 

 being immersed in the cell. 



The measurements were made wdth a Thomson's quadrant- 

 electrometer and checked against a standard Latimer-Clarke 

 cell. 



Two or three peculiarities showed themselves with this cell 

 which I have not observed in the case of my experiments 

 w T ith other alloys. In the first place, a considerable deflexion 

 was obtained between two plates of gold, one in the outer and 

 one in the inner cell, apparently owing to an E.M.F. between 



* Dr. A. Matthiessen on "The Electric Conducting-power of Alloys," 

 Philosophical Transactions, 1860, vol. cl., Part I., p. 167, Tin-Gold Series. 



