Contact Electricity of Metal*. 107 



before the Royal Institution, we had no knowledge of the 

 amount of heat of combination of zinc aud copper, nor indeed 

 of any other two metals. It appeared probable to us, from 

 Volta's discovery of contact electricity between dry metals, 

 that there must be some heat of combination ; but I could 

 then only express keenly-felt discontent with our ignorance 

 of its amount. Now, however, after twenty-seven years' 

 endurance, I am happily relieved since yesterday by Professor 

 Roberts- Austen, who most kindly undertook to help me in 

 my preparations for this evening, with an investigation on 

 the heat of combination of copper and zinc, by which he has 

 found that the melting together of 30 par cent, of zinc with 

 70 per cent, of copper generates about 36 heat-units (gramme- 

 water-Cent.) per gramme of the brass formed. I am sure 

 you will all join with me in hearty thanks to him, both for 

 this result and for his further great kindness in letting us now 

 see a very beautiful experiment, demonstrating a large amount 

 of heat of combination between aluminium and copper, in 

 illustration of his mode of experimenting with zinc and copper, 

 which could not be so conveniently put before you because 

 of the dense white fumes inevitable when zinc is melted in the 

 open air. 



[Experiment : A piece of solid aluminium dropped into 

 melted copper : large rise of temperature proved by thermo- 

 electric test. Result seen by all in large deflexion of spot 

 of light reflected from mirror of galvanometer.] 



§ 31. Another method of investigating the heat of combi- 

 nation of metals, which I have long had in my mind, is to 

 compare the heat evolved by the solution of an alloy in an 

 acid with the sum of the heats of combination of its two con- 

 stituents in mixed powders. The former quantity must be 

 less than the latter by exactly the amount of the heat of com- 

 bination. This investigation was undertaken a month ago by 

 Mr. Gait in the Physical Laboratory of the University of 

 Glasgow, and he has already obtained promising results ; but 

 many experimental difficulties, as was to be expected, have 

 presented themselves, and must be overcome before trust- 

 worthy results can be obtained. 



\_Added Feb. 1898. — By dissolving a gramme of a pow- 

 dered alloy, and again a gramme of mixed powders of the two 

 metals in the same proportion, in dilute nitric acid, Mr. Gait 

 has now obtained approximate determinations of heats of 



* ' Nature,' vol. i. p. 551, « On the Size of Atoms." 



