44 
.MESSRS. C. T. HEYCOCK AND E. H. NEVILLE ON 
From 50 to 60 the temperature, though quite constant at the F.P., did not remain 
constant for so long a time as before. 
From 60 to 85, the last point that could be observed, the halt in the fall of 
temperature at the F.P. became rapidly less marked, that at 80 and 85 being 
very transient. 
Pepeated efforts were made to determine the F.P. of mixtures containing more 
than 85 atomic percentages of lead, but the only stop in the cooling takes place at 
327°’5, the F.P. of pure lead, or more strictly the F.P. of a eutectic mixture of 
copper and lead. It will be seen that, at 60 atomic percentages of lead, the eutectic 
point was 327°. In this series of experiments we did not attempt to make a minute 
study of the eutectic alloy of copper and lead ; but on a previous occasion, using a 
sensitive mercury thermometer (‘Chem. Soc. Journ.,’ 1892, p. 906), we found that 
copper lowered the F.P. of lead by 1°‘17 C., and that the eutectic alloy contained 
rather less than '2 atomic percentage of copper. 
The difficulty of tracing the upper branch of the curve between 85 and 100 atomic 
percentages of lead is not due to the insolubility of copper in lead, for, at a red heat, 
copper dissolves freely; but rather to the fact that, between these limits, the solu¬ 
bility changes but little with change of temperature, so that at any given temperature 
the amount of solid that separates is small. It is evident that the curve must 
become almost vertical for dilute solutions. 
At the end of each series, after removing the pyrometer and stirrer, the furnace 
was closed as completely as possible, and the alloy allowed to cool slowly in a current 
of reducing gas. When cold, the alloys were weighed. They were in the form of 
cylinders, 3 centiins. wide and from 4 to 7 centims. long. Each ingot was completely 
covered with a thin layer of soft metal, which appeared to be almost pure lead, but, 
on cutting a plane face from end to end along the cylinder, the character of the alloy 
could be studied. 
At 24'76 atomic percentages of lead the alloy was found to weigh 413 grains, 
instead of 415, which is the sum of the weights of the metals used. During the 
earlier experiments with this alloy, a few particles had been splashed out of the 
crucible. The alloy appeared quite free from oxide. 
The structure revealed by the face cut on the cylinder was as follows :—two or 
three inillims. at the top were lead-coloured, with a few spots of copper that were 
invisible to the naked eye, but could be seen by the aid of a lens. Below this, for 
4'5 centims., the metal was copper-coloured, but the lens, or indeed a careful 
inspection with the naked eye, showed a great number of irregular roundish spots of 
lead-coloured metal embedded in the mother substance. The bottom centimetre 
consisted of grey metal which, to the naked eye, presented a well-marked tinge of 
red from the large number of spots of copper scattered through it. The lines 
separating the middle copper layer from the upper and lo^ver layers were perfectly 
