100 MESSRS. WALTER ROSENHAIN AND P. A. TUCKER. 



alloys were maintained at 175 C. for a period of six weeks, specimens being removed 

 and experimented upon in various ways at intervals. The most important evidence 

 derived from these heated specimens is that obtained from a study of their micro- 

 structure, but this will be better understood when the complications arising from the 

 transformations which occur at lower temperatures in the solid alloys have been 

 discussed ; the account of the micro-structure of these specimens will therefore be 

 given later. The heat-treated specimens were, however, also used for obtaining 

 cooling-curves of these alloys in a state closely approaching complete equilibrium. 

 In some cases the cooling-curve was taken without allowing the specimen to undergo 

 any intermediate cooling and re-heating, while in other cases specimens were allowed 

 either to cool slowly in the ordinary way, or were cooled rapidly (quenched) and 

 subsequently re-heated for the observation of the cooling-curve. The initial tempera- 

 ture for all these curves was taken slightly above the freezing-point of the eutectic, 

 so that the eutectic arrest if any should appear on the curves. The inverse-rate 

 curves derived from specimens which had been heated at 175 C. for four and six 

 weeks respectively are given in figs. 5, G, and 7. It will be seen that the eutectic 

 arrest appears for the first time in these curves in the alloy containing 18 per cent. 

 of tin. Microscopic evidence, however, leads to the view that the first appearance 

 of eutectic occurs in alloys slightly richer in lead, viz., at a concentration of about 

 1G per cent, of tin. 



The cooling-curves of all five series referred to above, in .so far as they relate to the 

 alloys rich in lead, show a further recalescence occurring in the solid alloys. In one 

 direction this recalescence has been traced down to the alloy containing 8 per cent, 

 of tin, while in the other it can just be detected by delicate means in the alloy 

 containing 60 per cent, of tin (just below the eiitectic concentration). The tempera- 

 ture at which this arrest-point occurs in the cooling of these alloys is lowest in the 

 8 per cent, alloy, and attains a maximum temperature at a concentration of about 

 18 per cent, of tin, occurring at this highest temperature in all alloys richer in tin up 

 to, or nearly up to, the eutectic alloy, where the arrest ceases to be observable. As 

 will be seen from the diagram of fig. 30, this maximum temperature is 149 C., while 

 the lowest temperature found in the 8 per cent, alloy is 72 C. As the temperature 

 at which this recalescence occurs falls to this low point at a tin content of 8 per cent., 

 it was at first supposed that the -non-observation of the arrest in alloys of lower tin 

 content might be due to the fact that the temperature at which the recalescence 

 occurs had fallen down to or even below the ordinary temperature. To decide this 

 question, both heating- and cooling-curves of alloys containing 2 per cent, and 4 per 

 cent, of tin and also of pure lead were taken down to very low temperatures by means 

 of liquid air and a thermo-couple of Iron-Constantan. 



For the purpose of obtaining a cooling-curve over this low range of temperatures 

 the specimen of metal into which the thermoj unction had first been inserted was 

 wrapped in thick asbestos cloth and first gently warmed to a temperature just above 



