1895. 



on the Barer 3Ietals and their Alloys. 



511 



or surfusion, had occurred, then there would be, as is often the case, a 

 dip where the freezing begins, and then the temperature curve rises 

 suddenly. 



If the metal is alloyed with large quantities of other metals, then 

 there may be several of these freezing points, as successive groups of 

 alloys fall out of solution. The rough diagrammatic method is not 

 sufficiently delicate to enable me to trace the subordinate points, but 

 they are of vital importance to the strength of the metal or alloy, and 

 photography enables us to detect them readily. 



Take the case of the tin-copper series ; you will see that as a mass 

 of tin-copper alloy cools, there are at least two distinct freezing points. 

 At the upper one the main mass of the fluid alloy became solid ; at 

 the lower, some definite group of tin and copper atoms fall out, the 



Fig. G. 



position of the lower point depending upon the composition of the 

 mass. 



Now turn to more complex curves taken on one plate by making 

 the sensitised photographic plate seize the critical part of the curve, 

 the range of the swing of the mirror from hot to cold being some 

 sixty feet. The upper curve (Fig. 6) gives the freezing point of 

 bismuth, and you see that surfusion, a, is clearly marked, the tem- 

 perature at which bismuth freezes being 268° C. The lower curve, 

 marked " tin," represents the freezing point of that metal, which we 

 know is 231° C, and in it surfusion, 6, is also clearly marked. The 

 curve marked standard gold contains a subordinate point, c, which 

 you will observe is lower than the freezing point of tin, and it is 

 caused by the solidification of a small portion of bismuth, which 

 alloyed itself with some gold atoms, and remained fluid below the 

 freezing point not only of bismuth itself but of tin. Now gold with 

 a low freeeing point in it like this is found to be very brittle, and we 



