On Surfusion in Metals and Alloys. 451 



Fm. 4. 



eutectic alloy of the lead-tin series. This alloy freezes at a constant 

 temperature (183). The eutectic constitutes a fluid residual " liquor " 

 which is left after the excess of either lead or tin has fallen out as 

 the mass cools down. When, however, the tin is present in slight 

 excess of the amount required to constitute the entectic, the whole 

 mass of the alloy may remain fluid at temperatures below that at 

 which the eutectic would naturally freeze. It may even cool to a 

 temperature at which it can no longer maintain all its lead in solu- 

 tion, and some lead will, therefore, fall oat while surfusion is actually 

 taking place. This explains the existence of the point d, which marks 

 an arrest in the fall of temperature, in Plate 9, fig. 6 (on the line ab, 

 representing the fall of temperature during surfusion). Conversely, 

 when the surfusion is ended, the latent heat is released, and the 

 line bee represents the rise of temperature due to the release of latent 

 heat. There should also be on this line an indication of a retarda- 

 tion or arrest in the rise of temperature, because the lead which fell 

 out of solution (at the point d) has to be re-melted. This point of 

 arrest will be found at e. 



In the same way, in fig. 7. Plate 9, which represents a fine case of 

 surfusion in the lead-tin alloy containing 64 per cent, of tin, there is 

 also a point at /, on the line gh, at which point the lead fell out of 

 solution during the surfusion. Fig. 8, Plate 9, represents the sur- 

 fusion of a lead-tin alloy containing 68 per cent, of tin. In this case 

 the tin is in large excess, and freezes first. In becoming solid, the 

 tin would have been represented by a horizontal line somewhere 

 about the point s, on the line ij, had not surfusion occurred, and 

 been terminated by the solidification of tin at &. This is followed 

 by the solidification of the eutectic mother liquor represented by 

 the line Im. This eutectic does not surfuse because the tin (the 

 metal in this alloy which is prone to surfuse) is already present in 

 the crystalline form. 



In fig. 9, Plate 9, a similar case is represented, but a small, though 

 distinct, surfusion at the point n has been recorded. This surfusion 



