108 MESSES. WALTER EOSENHAIN AND P. A. TUCKER. 



as to reach a zero value at or about concentrations of 6 and 60 per cent, of tin 

 respectively. 



Other peculiarities connected with this evolution of heat are shown by. cooling- 

 curves taken under various circumstances. Thus alloys which have undergone 

 prolonged heating at 175 C. show this evolution of heat much more markedly, and 

 over a shorter range of temperature than alloys which are cooled in the ordinary way 

 from the molten state. This indicates that the change which produces this evolution 

 of heat is the inversion of a change which is itself gradual, and thus liable to be 

 incomplete unless ample time be allowed. This question was tested by taking two 

 successive cooling-curves of the same alloy, the first when the alloy had been rapidly 

 heated to about 180 C. and allowed to cool at once, the second when the alloy had 

 been kept hot for an hour and a half before the cooling was allowed to begin. The 

 two curves are shown in fig. 12, and it will be seen that while the recalescence is 

 observed in both cases, it is very feeble in the first case and quite well-marked in the 

 second. The resulting conclusion that the change on heating is gradual is further 

 borne out by the appearance of the heating-curves of these alloys. These are not 

 reproduced, since they show nothing well defined ; apparently the change, on heating 

 is very gradual and is therefore spread out over a wide range of temperature, merely 

 producing slight slopes in the various curves. 



Another fact which appears to be connected with the same properties of the 

 substances involved is found in the alloys lying between 8 and 16 per cent, of tin. 

 In these alloys the temperature at which the evolution of heat occurs on cooling 

 depends upon the time for which they have been exposed to a temperature of 175 C., 

 i.e., the alloys which have been kept at that temperature for six weeks show the 

 arrest-point at a lower temperature than those which have only been kept hot for 

 four weeks, while these in turn show the arrest-points at lower temperatures than 

 those which have been simply slowly cooled from the molten state ; still more rapidly- 

 cooled alloys show the point at still higher temperatures. The results of a number 

 of experiments on this point are plotted in the diagram of fig. 13. The explanation 

 for these observations is probably to be found in the formation Of concentric layers 

 of solid solutions of different concentrations in more rapidly-cooled alloys. The 

 temperature at which the recalescence occurs depends upon the concentration of tin 

 in the alloy (in alloys with less than 18 per cent, of tin), and consequently the layers 

 richest in tin will undergo the change as soon as the temperature appropriate to their 

 concentration is reached and in the absence of homogeneity this will be higher than 

 the temperature of reaction corresponding to the average composition of the whole 

 of the alloy. It is this beginning of the reaction which is sharply indicated on the 

 curves. The shape of these curves, however, strongly suggests that in these cases the 

 change which has commenced in the layers of highest tin content is transmitted to 

 the other layers, although these had not reached the temperature at which they 

 would have spontaneously undergone the change in question. Other observations 



