1897-98.] Dr A. Galt on Heat of Combination of Metals. 143 
Then an approximation to the mean absolute amount of heat 
evolved in the solution of one gramme of the mixed metals, or of 
one gramme of an alloy of the same metals in the same propor- 
tions, is ascertained by the following expression 
H = ^[{(wp) +w?}s + c] . 
For the mixture 
H = 2 x 8*792[{(60 x 1*355) + *5}*634 + 3*5] 
= 2 x 8*792[55*36] .... =973*45 
For the alloy 
H = 2x 8*1 x 55*36 .... =896*83 
Difference, . . . .76*6 
This difference approximately represents, in (gramme-water) heat 
units Centigrade, the heat of combination of *52 gramme of copper 
with *48 gramme of zinc. It will be noticed that *5 has been used 
for w instead of 1, this being done to make some allowance for 
the lower specific heat of the metals when compared with that of 
the acid. An allowance, however, is almost unnecessary, as it 
has scarcely any influence on the value of the difference between 
the absolute amounts of heat obtained for the mixture and the 
alloy. 
Andrews* obtained 1420 thermal units as the heat of solution 
of one gramme of zinc, and 650 thermal units as the heat of solu- 
tion of one gramme of copper, both in diluted nitric acid. Taking 
48 per cent, of the former result, and 52 per cent, of the latter, 
to enable a comparison to be made with my results, the amounts 
are, respectively, 681*6 and 338, or a total of 1019*6, which is 
fully four per cent, greater than my value, 973*4, for the mixture. 
The difference is, doubtless, largely accounted for by the presence 
of impurities in the metals used, and by the radiation of heat 
from the apparatus during the time of solution. 
In chemical reactions in which copper and zinc take part these 
metals are usually bivalent elements, and in all such cases their 
relative combining weights are, therefore, as 63 to 65, or about 49*2 
per cent, copper to 50*8 per cent, zinc, proportions nearly the same 
Scientific Papers, p. 215. 
