Dii. Thomson on the Melting Points of AUoya of Lead, ^c. 81 



antimony. But the melting point being above 650", I could not 

 determine it by a common thermometer. According to Guyton de 

 Morveau, the molting point of antimony is 953° ; but his points are 

 all a great deal too high. Dr. Cromwell Mortimer had long ago 

 determined it to be 810°, which I believe to be near the truth. Now, 

 the mean melting point of an alloy of 1 atom lead, and 1 antimony, is 

 708°, and I believe that the true melting point is very little inferior; 

 it is above G50°. This alloy is a fine white; it is britUe, and expands 

 when the two metals unite. 



Sp. gr. Mean sp. gr. 1000 become 



1 atom lead, \ , , , 9-222 9482 1028 



1 atom antimony, } 



As tin has the property of lowering the melting points of lead and 

 zinc, I thought it worth while to try whether it had the same cfifect on 

 antimony. Accordingly, 800 parts of antimony were melted with 725 

 parts of tin, constituting the equivalent of an atom of each. The 

 alloy was very brittle ; its fusing point was considerably above 600°, 

 so that I was unable to determine it by a thermometer ; but I con- 

 sider it as nearly the mean between that of antimony and tin. Its 

 specific gravity was 6*927 ; it ought to be 6'839. It is therefore heavier 

 than the mean ; 1000 parts when alloyed become 989, or diminish in 



bulk T(Wo Pai-ts. 



I shall terminate this paper with a remark, which I consider as of 

 some interest. 



If you plunge a thermometer into melted lead, and observe it with 

 attention, you will find that it will sink constantly till it reaches 607°, 

 but here it will remain stationary for at least two minutes; (pro- 

 vided the quantity of lead be considerable, half a pound for example ;) 

 and when it begins to fall again, it will be found that the lead has 

 become solid. Hence, the stationary temperature marks the melting 

 point of the metal. 



If we try the same process with tin, the thermometer will sink to 

 438°, and, after remaining for a little while there, it will start suddenly 

 to 442°, at which point it remains stationary much longer than in the 

 lead ; and when it begins to fall again, the tin will be found solid. 

 Hence, 442° is the melting point of tin. 



When we dip a thermometer into a melted alloy of lead and tin, 

 and observe the rate at which it sinks, we shall find that it wiU show 

 two stationary points, — the first a good deal shorter than the second. 

 I call these, by way of distinction, the short and the l(mg stationary 

 points. The first stationary point continues for about a minute : the 

 second is seldom shorter than three minutes, and sometimes much 

 longer. The greater the proportion of lead, the higher is the first, or 

 short stationary point, and the more tin the lower it is ; so that, in an 

 alloy of 1 atom lead and 4 atoms tin, the two stationary points nearly 

 coincide. 



