88 



Mr. J. Brown. 



[June 16, 



While the results in Table III are naturally not so regular as those 

 in Table I, most of them are nearer the theoretical values than those 

 of the same combinations in aqueous solutions. In this respect the 

 results for cells in which magnesium or aluminium is concerned are 

 particularly striking. For comparison I take the following numbers 

 (in volts) for metals in aqueous solutions of their chlorides, from 

 Wright and Thompson;* Braun,t has found a similar discrepancy 

 for magnesium in aqueous cells, and attributes it to hydrogen de- 

 posited on the magnesium. 



A still closer agreement between the observed and theoretical 

 numbers for the metals in Table III is produced by the application of 

 empirical corrections, as in Table IV ; and these are of a similar order 

 of magnitude in the two tables, except in the case of zinc. 



On testing iron, aluminium, and magnesium for electromotive force 

 due to difference of temperature, iron was found to behave like copper, 

 the hot metal being the negative pole. Aluminium in its double 

 chloride with sodium gave, at first, a strong similar effect ; but after 

 some twenty minutes when a dark deposit, which appeared to con- 

 tain lead, carbon and iron reduced by the immersed metal, had ceased 

 to separate, and the liquid had become colourless and clear the effect 

 ceased. It may have been due to impurity in the chloride. With 

 magnesium I could not detect any electromotive force due to difference 

 of temperature. Therefore, considering the uncertainty introduced 

 by these irregularities, and the complication introduced by the use of 

 double salts, cadmium has been allowed to remain as the zero of the 

 scale of corrections, as in Table II. 



My experiments were carried out in the laboratory of Queen's 

 College, Belfast, and I am much indebted to Professor Everett for 

 permission to work there and for the use of his electrometer and 

 other apparatus. I am also indebted to Professors Everett and 



* ' Phil. Mag.,' ser. 5, vol. 19, pp ; 211, 213, 1885. 

 t ' Wied. Ann.,' vol. 16, p. 578, 1882. 



