80 Dr. Thomson on the Melting Points of Alloys of Lead, ^c. 



I maj mention here that pewter consists cliieflj of an alloy of zinc 

 and tin, with tho addition of sorao bismuth and copper. Common 

 pewter is an alloy of 1 part by weight of zinc and 17 of tin; while 

 plate pewter is said to be 



100 zinc, 8 tin, 2 bismuth, and 2 copper : 



while another kind of pewter is mentioned composed of 4 zinc, and 1 

 lead, called ley pewter. 



When tin and bismuth aro melted together they expand ; for the 

 specific gravity is less than the mean. The amount of expansion will 

 appear from the following table ; — 



Sp. gr. Mean sp. gr. 1000 become 



1, . . . 8-709 8-942 1027 



2, . . . 8-418 8-5135 1011 



Tin and zinc constituting common pewter, I thought it worth while 

 to determine the melting point of these two metals. Tho melting 

 point of zinc is above the range of a common theremometer, being, 

 according to Danicll, 773°. But I thought it probable, as tin lowers 

 the melting point of lead, that it might have the same effect on zinc. 

 And I found it so. I made two alloys, the first composed of 1 atom 

 tin, and 1 atom zinc ; and the second of two atoms tin, and 1 atom 

 zinc. The melting points — 



1, . . . . 384« I 2, . . . . 386" 



obviously approaching very near each other, and probably identical. 

 The mean melting point of an alloy of tin and zinc is 607^°, which 

 is 212° higher than that at which the alloy really melts. 



The alloy of tin and zinc is malleable. It is much more beautiful 

 than zinc, and in fact, has a close resemblance to pewter. When tin 

 and zinc are melted together they expand, and the expansion for 1 

 atom tin, and 1 zinc, is greater than in the case of lead and tin. 

 This will appear by the following table : — 



Sp. gr. Mean sp. gr. 1000 become 



Jnt*^^!^";.) • • • 6-426 7-1815 1117 



1 atom zinc, J 



2 atoms tin, j 

 1 atom zmc, / 



There was some diflSculty in uniting these two metals. The zinc does 

 not melt till it becomes nearly red hot. The consequence was, that 

 the tallow which I threw into the crucible to prevent oxydizement, 

 caught fire. There was always a little dross floating on the top, which 

 was probably zinc. This might occasion some error, and might be 

 the cause why the melting points of the two alloys did not agree 

 exactly. 



* To try whether antimony would have any effect in lowering the 

 melting point of lead, I made an alloy of 1 atom lead, and 1 atom 



