94 " MESSES. WALTER EOSENHAIN AND P. A. TUCKER. 



shown in figs. 2 and 3 respectively in the form of "derived differential" curves.* 

 The curves, as reproduced in the figures, are very much reduced from the original 

 curves as plotted from the observations ; in order to illustrate the smoothness of the 

 curves, as originally obtained, the observed figures of the curves of Series B are given 

 in Table II., p. 97, exactly as given in the Laboratory notebook. These figures apply 

 to the ordinary, or " direct," differential curves ; the abscissae of the derived curves 

 are obtained by subtracting successive readings of the differential galvanometer from 

 one another. 



The blank body used in taking these curves was a piece of pure lead of the same 

 size as the cylinders of the alloys. In Series B these cylinders were of the size 

 generally used in taking the recalescence curves of steel, viz., -| inch long by -f inch 

 diameter, the cylinders weighing approximately 35 grammes. In Series C, with the 

 object of obtaining greater sensitiveness, the weight of the cylinders was increased to 

 175 grammes. In both cases the rate of cooling was very slow, the fall of temperature 

 from 180 (J. to 100 C. taking from 30 to 40 minutes. It was at first expected that 

 tliis rate of cooling would be sufficiently slow to allow of the attainment of complete 

 equilibrium, but subsequent observations showed that this was not quite the case. 



The curves of these two series (B and C) were not taken for alloys containing more 

 than 40 per cent, of tin. With higher percentages of tin, the amount of eutectic 

 present is so great that the entire cylinder "runs down" when heated above 180 C., 

 the metal thus eluding observation by the ordinary differential method during the 

 subsequent cooling. Even in the case of alloys containing above 25 per cent, of tin a 

 certain amount of the liquid eutectic exudes and runs away from the cylinder, thus 

 failing to impart its heat of solidification to the thermocouple on subsequent cooling. 

 This fact must be borne in mind in connexion with the approximate quantitative 

 interpretation of these curves. 



On examining the curves of Series B it is found that the eutectic peak first appears 



^ The well-known "differential cooling-curves" are obtained by means of the apparatus of EOBERTS- 

 AUSTEX, in which a specimen of the alloy under observation is allowed to cool in the same furnace, and, 

 therefore, at approximately the same average rate, as a standard neutral or comparison body. Similar 

 thermojunctions are inserted in these two cooling bodies and are so connected to the "differential" 

 galvanometer as to oppose one another ; any evolution or absorption of heat in the experimental body then 

 produces a deflection of the differential galvanometer, as it sets up a difference of temperature between the 

 two cooling bodies. In the usual form of "differential cooling-curve" ihe position of the differential 

 galvanometer is plotted (or recorded) as abscissa, with the actual temperature of the cooling body- 

 indicated by a third thermocouple as ordinate. The "derived differential curves" of the present paper 

 simply represent the differential coefficient of the ordinary differential curve, i.e., abscissae represent the 

 changes of position of the differential galvanometer which have occurred during successive equal decrements 

 of temperature. The nature and use of these curves has been fully described by the author in a paper on 

 "Observations on Eecalescence Curves" read before the Physical Society of London, January 24, 1908. 

 In accordance with the conclusions of that paper all differential observations in the present paper are 

 plotted and interpreted in the form of derived differential curves, while the approximate quantitative 

 interpretation of the cooling-curves has also been carried out on the lines laid down in that paper. 



