Electromotive Forces in the Voltaic Cell. 361 
Next take the same weights of zinc and copper as before 
and alloy them, getting heat H; then dissolve the brass in the 
same acid as before, getting heat H;. This is another plan of 
passing from separate zinc and copper to a solution of a salt 
of brass. 
Now, unless external work and secondary products are 
different in the two cases, we are justified in writing the heats 
evolved in the two cases equal :— 
H,+H,+h=H+ Hs. 
H is the unknown quantity to be determined, and its determi- 
nation involves four separate measurements, H,, H., Hs, h. 
The only one of these at all easy to observe is h, and this 
my assistant, Mr. Butler, has done. Proportions of zinc and 
copper sulphate, containing equal weights of zine and copper, 
are dissolved in as little water as will keep in solution any 
double salt that may be formed on mixing. The zinc sulphate 
is enclosed in a thin bulb or tube inside the other solution, 
and left, sereened from stray heat, for some hours. The bulb 
is then broken, or the liquid otherwise blown out of it, and 
the liquids mixed. No certain change of temperature so great 
as a hundredth of a degree has been observed. 
27. In thinking over what metals were more suitable, it struck 
me that the heat of formation of amalgams was a subject easy 
of direct attack. I therefore, as a preliminary, have dissolved 
a little granulated tin in mercury. Of course the latent heat 
of liquefaction of tin has to be allowed for, and the actually 
observed result is a cooling; but I hoped that the cooling 
observed would be less than what the latent heat would 
account for, and that I might then calculate the real evolution 
of heat due to combination. Unfortunately the only data I 
know of with reference to the latent heat of tin relate to its 
ordinary melting-point, at which point it is given by Rudberg 
as 13:3, and by Person as 14°25. We have no ground what- 
ever for believing latent heat to be constant, and Iam therefore 
utterly in the dark as to what the latent heat of tin at ordi- 
nary temperatures may be. That liquid tin could be super- 
cooled to ordinary temperatures without solidification is 
unlikely. I give, however, the data of my experiment (which 
was carefully performed) in case better latent-heat data are 
known to some one else:—2°10 grammes of thin granulated 
tin at 12°-4 were dropped into 502-00 grammes of mercury at 
a steady temperature of 10°85, contained in a large thin pro- 
tected test-tube, of which the part sharing the temperature of 
the mercury weighed 8 grammes. After solution, which took 
ten minutes, the resulting temperature was found to be 8°82. 
