620 Proceedings of Royal Society of Edinburgh. [sess. 
I may here note, however, that the copper-silver alloys show 
little or no heat of combination, that many of the copper-zinc alloys 
do indicate heat of combination, the highest value, 52 gramme- 
water heat units centigrade per gramme of alloy, being obtained 
from those alloys whose composition is at or near the ordinary 
chemical combining proportions of the metals considered as bi-valent 
elements, that is 49T per cent, copper and 50*9 per cent. zinc. 
With lower percentages of copper the heat of combination rapidly 
falls until, at about 30 per cent, copper, the alloy shows no heat of 
combination. 
But a still more surprising result — and this is the chief cause of 
my present communication — was obtained when alloys containing, 
respectively, 20’5, 16*0, and 10*5 per cent, of copper were tested. 
Each of these gave a greater heat of solution than the correspond- 
ing mixture, proving that in all of them the heat of combination 
had a negative value. The amount of this negative value was 
fourteen gramme-water heat units centigrade per gramme for the 
alloy containing 205 per cent, copper and also for the alloy con- 
taining 16’0 per cent, of copper; while it was eight gramme-water 
heat units centigrade for the alloy containing 10-5 per cent, of 
copper. This result was then verified in another way. 
Half a gramme of an alloy numbered “5,” and containing 26’5 
per cent, copper, and which had already shown no heat of combina- 
tion positive or negative, was dissolved in the usual manner, and the 
result obtained compared with that given under similar conditions, 
by a mixture of - 23 gramme of free zinc with ‘27 gramme of an 
alloy, numbered “C,” and which contained the metals in the chemical 
combining proportions (49T per cent, copper and 50 '9 per cent, zinc) 
previously referred to. As *27 gramme of alloy N o. C contained 
T325 gramme of copper, the quantitative composition of the two 
powders tested was the same. A mean of five tests gave 8*64° C. 
as the rise of temperature in the experiment with alloy No. 5, and 
8’42° C. with the mixture. In one powder the quantity (-1325 
gramme) of copper present was alloyed with its chemical equivalent 
of zinc ; and in the other the same quantity of copper was also 
present, but it was alloyed with an excess of zinc above the chemi- 
cal equivalent. We might have anticipated that the large heat 
of combination previously noticed for alloy No. C would also have 
