ELECTROMOTIVE FORCES IN THE VOLTAIC CELL. 527 



amalgam at a temperature just below the melting point of tin, and obtain 

 (if possible) the nett evolution of heat then. 



Suppose the heat of combination of the 21 grammes of tin with 

 mercury to be somehow or other determined, we have next to suppose tho 

 amalgam made otherwise, bringing the molecules together in a reasoned 

 ■wav. Let the same quantity of tin be brought to within molecular 

 distance of the mercury in successive pieces of very thin foil, first made 

 to touch at one corner and then laid down. 



It is quite true that each flake would be charged with a Volta E.M.F. 

 of, say, - 6 volt, and so would attract the mercury and do a certain amount 

 of work in laying itself down. But it is not fair to compare an operation 

 thus conducted in air with the dropping of a solid mass of tin into 

 mercury ; to be able to compare the two operations one must perform the 

 foil experiment in absolute vacuum. This being done, the contact E.M.F. 

 is no longer - 6 volt, but only about -00015 volt according to the experi- 

 ments of Matthiessen. Good data for this quantity are however wanting ; 

 mercury is not one of the metals included in Professor Tait's series. It 

 was observed by Gangain ; and by rather hypothetical deduction from his 

 numbers, as given pictorially in Wiedemann's ' Elektricitiit,' I make the 

 tin-mercury Peltier force 1"75 millivolts at 10°. l 



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. 2 But I do not know it. Thus the supply of data for 

 this case is distinctly unsatisfactory. 



1 Since this was in type, a paper by C. L. Weber lias appeared in Wiedemann's 

 Annalen for November 1884, on the thermoelectric properties 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 t° Centi- 

 grade as 



431 + -ot absolute electro-magnetic units. 



AYhence the Peltier force at the same temperature is 



1181 + 5-68£ + -005£ 2 microvolts. 

 The Peltier force between tin and mercury at 10° is therefore 123,800 absolute units, 

 or 1*24 millivolts, 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 combination of our 2'1 grammes of tin 

 with the 502 grammes of mercury by the relation, 



J n = 36 x 10° a/H. 



The two rough estimates of J n deduced from Matthiessen and Gaugain respectively 

 (15,000 and 175,000) thus give H as about i and i of a unit respectively. Either i A 

 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. 



AVorking 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 tempera- 

 tures ; only enough to raise the mass of brass formed through three-eighths of a degree 

 Centigrade. At a higher temperature such as 400° C. the Peltier force for these 

 metals is greater, being 4,600 microvolts, and the calculated heat of combination 

 is then ^ 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 tempera- 

 ture one may expect in making brass at a temperature of 400°. 



