EUTECTIC EESEAECH: THE ALLOYS OF LEAD AND TIN. 115 



that the solubility of tin in the a body decreases still further as the temperature falls, 

 but this decrease is certainly slight its possible existence, however, makes it difficult 

 to give any estimate of the solubility of tin in the a body at its transition temperature. 

 On heating, the reverse change (from a to /3) only takes place gradually, a fact which 

 is partly accounted for by the slowness with which the ft Ixxly takes up its full amount 

 of tin. When the concentration of tin in the ft body is lower than 16 per cent., the 

 transformation on cooling is retarded until a lower temperature is reached ; at the 

 same time the intensity of the reaction falls off until, with a tin content less than 

 8 per cent., the ft form appears to remain stable down to the temperature of liquid 

 air. The fact that the intensity of the reaction falls oft' so markedly as the concen- 

 tration of tin falls below IG per cent, suggests that the actual separation of the tin 

 itself is to some extent the source of the heat evolved, but it is not at present possible 

 to discriminate between the various possible sources of heat in a complex reaction. 

 The fact, however, that botli the temperature and intensity of a change of this kind 

 varies with the concentration of the dissolved element finds it analogue in the higher 

 arrest-points of low-carbon steel. 



A point of some difficulty yet remains to be dealt with. The data of the cooling- 

 curves show that this heat-evolution diminishes to zero at or near the concentration 

 of the eutectic alloy, and it is therefore evident that the transformation is confined to 

 the structurally free lead constituent, the lead-rich constituent of the eutectic taking 

 no part in the reaction. This fact can only be reconciled with the Phase llule by 

 supposing that while the structurally free ft solution and the lead- rich constituent of 

 the eutectic must be the same phase when first formed, yet the lead constituent of 

 the eutectic retains the /3 state (in meta-stable equilibrium) when the transition 

 temperature is passed. This explanation is rendered probable by the observation 

 already described, that the crystallites of the lead constituent are generally completely 

 surrounded by a sheath of pure tin which separates them from the lead constituent of 

 the eutectic ; this envelope of tin no doubt serves to prevent the propagation of the 

 reaction from the structurally free ft body to the corresponding constituent of the 

 eutectic. 



The view that owing to the retention of the ft state in a meta-stable form the lead- 

 rich constituent of the eutectic is not identical with the stable a body is further borne 

 out by an examination of the densities of the alloys, which have been carefully 

 determined for this purpose. The density of the a body containing 16 per cent, of 

 tin (part of which is present as free "secondary" tin) is found to be 10 '31, while the 

 density of pure tin is 7 '30. Taking the percentage composition of the pure eutectic 

 at 37 '07 per cent, of lead and 62'93 per cent, of tin, and supposing the lead-rich 

 constituent of the eutectic to hold 16 per cent, of tin in solution, the percentage 

 composition of the eutectic in terms of its actual constituent phases becomes 44 - 13 per 

 cent, of lead-rich ft constituent and 55 '87 per cent, of tin. From the observed density 

 of the eutectic, an obvious calculation shows that the density of the lead-rich 



Q 2 



