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



examination. When these alloys are polished, the lead, or lead-rich constituent, 

 assumes a dark tint, no doubt owing to slight surface oxidation, while any free 

 tin present remains bright. When necessary, the contrast thus produced can be 

 heightened by etching. These alloys are best etched by the aid of a weak electric 

 current passed through a saturated solution of lead nitrate. Nitric acid can also be 

 used as an etching agent, but it has the disadvantage that at times it is apt to 

 darken the tin as well as the lead. The opening up of the alloys rich in lead to 

 microscopic examination, which has thus become possible, has, it is believed, consider- 

 ably increased the certainty of the results indicated by the pyrometric study of 

 the alloys. 



Experimental Results. 



The Cooling-curves of the Alloys. Five series of cooling-curves were taken, many 

 of the individual curves being, however, repeated several times under varying 

 conditions. Three of these series are inverse- rate curves, while two are differential. 

 The inverse-rate curves of Series A were taken primarily for the determination of the 

 " liquidus " curve, i.e., for the determination of the points of initial freezing of the 

 alloys ; the observations were, however, carried to temperatures well below that of 

 the final solidification of the metal, and on the majority of the curves, therefore, the 

 arrest due to the freezing of the eutectic as well as that due to a transformation 

 occurring in the solid alloys are shown. In taking these curves uniform quantities of 

 200 grammes of each alloy were used and the rates of cooling were kept as constant 

 as possible. Typical examples of the curves of this series (A) representing the cooling 

 of the alloys containing 10, 20, 30, 50, 63, 75, and 85 per cent, of tin respectively are 

 shown in fig. 1. The points of initial freezing derived from these curves are shown by 

 the small circles on the line AEB in the equilibrium diagram of fig. 30 (p. 117). The 

 curve thus obtained is in good agreement with that given by RoBERTS-AtrSTEN.* 

 With regard to the eutectic, it will be seen that in the alloy containing 10 per cent, 

 of tin the curve shows a very small eutectic peak, and this peak is much smaller than 

 would be anticipated if the eutectic extended to within a fraction of one per cent, of 

 the lead end of the series. The curves of this series (A), however, are not regarded 

 as giving very reliable data for temperatures below the liquidus for alloys near the 

 lead end of the series, and they have accordingly been used only for the determination 

 of the liquidus for that range of alloys. In the proved absence of complications at the 

 other end of the series, however, the points obtained from these curves have been 

 utilised for both liquidus and solidus in the range of alloys containing more than 

 65 per cent, of tin. 



Series B and C consist of differential curves, and typical examples of both series are 



* Paper referred to above. 



