1897.] 



on Contact Electricity of MetaU. 



543 



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 beau- 

 tiful experiment, demonstrating a large amount of heat of combination 

 between aluminium and copper, in illustration of his mode of experi- 

 menting 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 deflection of spot of light reflected from 

 mirror of galvanometer.] 



§ 31. Another method of investigating the heat of combination 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 constituents in mixed powders. The 

 former quantity must be less than the latter by exactly the amount 

 of the heat of combination. 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 trustworthy results can be 

 obtained. 



[Added Feb. 1898. — By dissolving a gramme of a powdered 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 combination for four diflferent 

 alloys, as shown in the following table : — 



No. 



II. 

 III. 



IV. 



Alloy. 



("48 per cent, zinc \ 

 \52 „ copper/ ** 

 (Approximately chemical combining 

 proportions.)* 



1 30 per cent, zinc 1 

 \70 „ copper/ 



/76*7 per cent, silver \ 



" \23-3 „ copper/ •' 



(Approximately chemical combining 



proportions.)* 



/51 -6 per cent, silver "I 

 \48-4: „ copper/ 



Heat of combination 



per gramme of alloy 



in gramme-water- 



Cent. thermal units. 



77 



34-6 



18 



The combining proportions are — 



(i) 50 • 8 zinc with 49 • 2 copper, 

 and (ii) 77-4 silver „ 22*6 „ 



