Melting-points of Aluminium, Silver, Gold, Copper, fyc. 45 



case where one junction of the couple is kept at 0° C, and the 

 other is at any other temperature t C, or r = t + 273° absolute; 

 m and n are constants, different for the two expressions; 

 %le denotes the resultant thermal E.M.F. of the circuit, viz. 

 that which is the object of direct measurement. The first, 

 called the exponential equation, is 



2o£ = mT ra -/3 (where /3= mr" =m x 273 ra ). 



The second expression, called the logarithmic equation, is 



% t e = mt n , or \og?L t e = nlogt-\-\ogm. 



Both formulas have been applied to the data of the present 

 investigation given in Table I., with results shown below. 

 The Avenarius formula has also been applied for purposes of 

 comparison. 



To evaluate the constants m and n of the exponential 

 equation (for method, consult the paper referred to) it is 

 necessary to have values of l^e at three known temperatures. 

 Of these, however, one may be 2 £ = 0, at r=273°, i, e. with 

 both junctions in ice. It therefore remains to fix upon two 

 other temperatures between which to interpolate, or, in other 

 words, two other temperatures which shall be assumed as 

 known. In looking over the ground, it seemed that the 

 boiling-point of sulphur, being so high and so accurately 

 determined by Callendar and Griffiths*, 



444-53 + 0-082(H- 760), 



was preeminently one of these points. The other must be 

 much higher, and the melting-point of pure gold seemed to 

 be almost, if not quite, the only one upon which reliance could 

 be placed. 



Apart from freedom from oxidation and its conveniently 

 high point of fusion, gold seemed the more suitable because its 

 melting-point had recently been so carefully measured by 

 Holborn and Wien, and because the metal could be obtained 

 of the necessary purity. Add to these considerations the fact 

 that its melting-point in a state of at least fairly high purity 

 has been measured by more experimenters than that of any 

 other high melting metal, so that it serves as an excellent 

 connecting link between their work, and we have claims which 

 no other substances can at present offer. The fusion-point of 

 gold was therefore chosen as the second reference or calibration 

 temperature. As to the figure to be assumed as the melting- 



• * Phil. Trans, clxxxii. pp. 119, 157 (1891). 



