452 



Dr. W. C. Roberts-Austen. 



at n probably indicates that the lead in the entectic may surfuse 

 slightly even though the crystallisation of tin has begun. 



By taking a series of lead-tin alloys which do not contain more than 

 2 or 3 per cent, of tin, in addition to the amount required to constitute 

 the eutectic (62 per cent, of tin), the temperatures at which the 

 lead begins to crystallise below 183, can be measured. The results 

 are shown in the diagram, fig. 5. In it the co-ordinates are tem- 



FIG. 5. Freezing-point curves of lead-tin alloys. 



185- 

 X 



Le&d 



EutecC 



c. 



Percentage of\ Tin. 



59 



60 



62 



66 



67 



68 



peratures and percentages of tin. Each alloy examined has, it will 

 be seen, 'at least two freezing points, and some appear to have three, 

 but in all the alloys one of the freezing points (the eutectic) is at 

 the constant temperature 183. In the eutectic alloy (62 per cent, of 

 tin), theoretically these points, d, e, should coincide. In fig. 5 they 

 are not quite coincident, and this is due to the fact that the 

 lines AB, CD, drawn through the observed freezing points, are of 

 necessity somewhat lower than the ideal solubility curves. The 

 freezing points which lie below the horizontal or eutectic line were 

 recorded while surfusion occurred. For instance, take the alloy 

 containing 64 per cent, of tin, its initial freezing point would be at 

 a (185), that of its eutectic at 6 (183), while the third freezing 

 point, c (176), is due, as has already been explained, to the falling 

 out of lead while the fluid mass was in the surf used condition. In 

 order to observe the uppermost of the three points (185% it is 

 necessary to stir the mass to prevent the possibility of snrfusion. 

 On the other hand, a record of the lowest freezing point can only be 

 obtained in a fluid mass which is perfectly tranquil. When lines 

 are drawn through all the points recorded it will be seen that the 

 line AB, representing the solidification of lead, extends far below the 

 horizontal line XX. It should be remembered that if the fluid mass 



