Electromotive Forces in the Voltaic Cell. 363 
from his numbers, as given pictorially in Wiedemann’s 
Lilektricitét, I make the tin-mercury Peltier force 1°75 
millivolt at 10°*. 
Taking one of these numbers (15,000 or 175,000 in C.G.S. 
units), or a better one when determined, we can calculate how 
near the given mass of tin must be brought to the mercury in 
order to generate the actual heat of combination, provided 
one knows the specific inductive capacity of absolute vacuum f. 
But I do not know it. Thus the supply of data for this case 
is distinctly unsatisfactory. 
28. Let us try whether we cannot do better with a single 
metal exposed to air, not troubling about the contact of two 
metals, which is unnecessary, but simply considering one metal 
in contact with air. 
* Since this was in type, a paper by C. L. Weber has appeared in 
Wiedemann’s Annalen for November 1884, on the thermoelectric pro- 
perties of amalgams, in which mercury itself was examined ; and from the 
data there recorded, together with Tait’s value for copper, I reckon the 
thermoelectric value of mercury at ¢° C. as 
431-+-‘d¢ absolute electromagnetic units. 
Whence the Peltier force at the same temperature is 
1181+5-G8¢+ 0052? microvolts. 
The Peltier force between tin and mercury at 10° is therefore 123,800 
absolute units, or 1:24 millivolt, which agrees well enough with the rough 
estimate above. : 
+ Taking this as 1, and assuming the estimate of molecular dimensions 
hereafter established, and working backwards, one can show that the 
Peltier force of tin and mercury at 10° is connected with the heat of com- 
bination of our 2'1 grammes of tin with the 502 grammes of mercury by 
the relation 
JII=3'6 x 10° H. 
The two rough estimates of JIT deduced from Matthiessen and Gaugain 
respectively (15,000 and 175,000) thus give H as about 4 and {4 
of a unit respectively. Hither of these is too small a quantity to be 
observed in the process of dissolving tin in mercury; so neglecting it we 
get, from that experiment, the latent heat of molten tin at 10° C. as 20-4. 
Another experiment made in a similar way gave 19-6. 
If the above reasoning be regarded as legitimate, a combination of 
thermoelectric measurements with observed heats of solution in mercury 
may furnish a means of estimating latent heats of fusion at various low 
temperatures in general. - 
Working back similarly to the heat of combination of 1 gramme of 
copper with 1 gramme of zinc, we calculate ‘077 unit as the heat developed 
at ordinary temperatures; only enough to raise the mass of brass formed 
through three eighths of a degree Centigrade. Ata higher temperature, 
such as 400° C., the Peltier force for these metals is greater, being 4600 
microyvolts, and the calculated heat of combination is then 4 of a unit per 
_ gramme of each, sufficient to raise the whole mass of metal through nearly 
2 degrees Centigrade. This, then, is the sort of elevation of temperature 
one may expect in making brass at a temperature of 400°. 
