114 MESSES. WALTER ROSENHAIN AND P. A. TUCKER. 



secondary tin is much larger, while it has segregated into masses which compare in 

 size with the primary tin in the same section ; it would seem, in fact, that in this 

 case a considerable proportion of the secondary tin had formed around the primary 

 tin present, or had coalesced with it. In this alloy, as in the 16 per cent, alloy, it 

 thus appears that the transformation which takes place at 149 C. involves the 

 rejection of tin from the solid solution. 



In view of these facts, and of the data supplied by the cooling-curves, the nature of 

 the transformation in question may be considered. Three alternative explanations 

 suggest themselves, viz. : 



(a) That the recalescence is due to the formation on cooling of a compound 



which only gradually dissociates on heating ; 

 (6) That the recalescence arises from the decomposition of a compound which exists 



at higher temperatures, but which is only slowly formed on heating ; and 

 (c) That the transformation is a change in the solid solution from a /8 to an a 



modification, the latter possessing a smaller solubility for tin than the 



former. 



Hypothesis () may be rejected at once on the grounds that the maximum heat 

 effect of the transformation does not occur at a concentration of tin which corresponds . 

 to any simple atomic formula, while the formation of a compound at the critical point 

 in these alloys would lead us to anticipate that the slowly-cooled alloys in which 

 the reaction had been permitted to take place would be more homogeneous than 

 the quenched alloys in which the reaction had been inhibited, whereas the reverse is 

 the case. 



Hypothesis (b) would agree with the microscopic evidence, but the difficulty still 

 remains that the maximum does not coincide with any simple atomic ratio. The 

 nearest simple formula would be Pb 3 Sn, and this would place the maximum at a 

 concentration close to 1G per cent, of tin. This hypothesis has, however, to face the 

 difficulty that there is no change of shape in the liquidus curve to correspond to the 

 existence of such a compound ; the liquidus, as determined in the present experiments 

 and in those of ROBERTS- AUSTEN already cited, shows no signs of either a maximum 

 or a break of continuity, while there is also no sign of any absorption of heat at any 

 temperature between the liquidus and the solidus which might account for the 

 formation of such a compound in that range of temperature. On this ground, 

 hypothesis (fc) must also be rejected. 



The remaining explanation is the one which appears best to fit the facts. On this 

 view the solid solution of tin in lead, when it contains more than 8 per cent, of tin, is 

 capable of existing in two forms which may be called a and ft respectively. The 

 saturated (/8) form of this solution, containing 16 per cent, of tin (approximately), 

 passes into the a form on cooling to 149 C., and at that point rejects a certain 

 amount of the dissolved tin in a finely divided " secondary " state. It is probable 



