3 '24 DR. WALTER ROSENHAIN AND MR. SYDNEY L. ARCHBUTT ON THE 



indication of the commencement of an evolution of heat, no sharp indication of the 

 , ,/ of such an evolution can be obtained.* In the present experiments mgots 

 used each weighed 150 gr, and all of them had been annealed together at . 

 temperature of 430" C. for about 100 hours, experiment having shown that 

 was sufficient to secure complete equilibrium. 



Fig. 5. 



These ingots were cooled in an electrically heated furnace placed inside a water- 

 jacket, and the initial temperature as well as the rate of cooling were kept as nearly 

 uniform as possible throughout the series. The resulting curves were plotted on a 

 large scale and the peaks of the inverse rate curves were measured with a planimeter, 

 or, in some cases, the areas were determined by plotting on thin card, cutting out the 

 peaks and weighing the pieces of card on a sensitive balance. The observed areas of 

 the peaks depend upon the choice of a point on the cooling-curve to represent the 

 end of the heat evolution, and it has already been pointed out that this choice cannot 

 be made with any degree of certainty. In the present series of curves these points 

 were inserted by estimation and the endeavour was made to place them on corre- 

 Bponding parts of the curves for all the alloys. The areas of the peaks thus measured 

 represent approximately the quantities of heat evolved by each of the alloys and, 

 within the limits of accuracy of the method, these may be taken to be proportional in 

 the present case to the quantity of eutectic undergoing solidification in each alloy. 

 These quantities as ordinates are plotted on abscissae representing the concentration 

 of the alloys in the upper curve of fig. 6. 



It will be seen that the points thus obtained lie with considerable accuracy upon 

 the straight line which cuts the zero line at a concentration of 78 '8 per cent, of zinc, 

 this concentration representing the limit of the eutectiferous alloys as determined by 

 this method of extrapolation. It will be shown later that the termination of the 



* See "Observations on Recalescence Curves," by W. ROSENHAIN, 'Proc. Phys. Soc.,' 1908. 



