OF THE ALLOYS OF COPPER AND ZINC. 
545 
Molecular Dimensiojis of Copper and Zinc. 
e 
In Lord Kelvin’s pajier on “ Contact Electricity of Metals ” (Royal Institution, 
May 21, 1897), the question of the determination of the limits of the molecular 
dimensions of copper and zinc, by his method described in ‘ Nature’ (p. 551, 1870), is 
re-opened. 
On the assumption that “ the work done by the attraction of two metallically con¬ 
nected sheets of copper and zinc when allowed to approach through any distance 
towards contact is the dynamical equivalent of the portion of their heat of combina¬ 
tion due to the approach towards complete chemical combination constituted by the 
diminution of distance between them,” Lord Kelvin showed that the molecular 
dimensions were less than 10“® and greater than 10“^ centim. ; for the union of 
plates of copper and zinc 10~® centim. thick and 10“® centim. apart would result in 
the production of 7‘4 calories per gramme of brass formed. 
If the distance apart were reduced to 10“*^ centim. and the plates were each 
10“® centim. thick, then their union would produce 740 calories per gramme of brass 
formed. 
Since the author finds a maximum heat of formation per gramme of brass of 
52’5 calories, the limits of the molecular dimensions of copper and zinc above given 
would be confirmed. 
The foregoing assumption that the Volta effect is capable of explanation as the 
result of the chemical affinity existing lietween copper and zinc has been subjected to 
much criticism (Lodge, ‘ Phil. Mag.,’ vol. 49, 1900, p. 372). 
There seems, however, to have been a tendency to under-estimate the heat of com¬ 
bination of these metals, for the value obtained by the author, viz., 52‘5 calories per 
gramme of CuZuo, is equivalent to 10,143 calories per gramme-molecule—a quantity 
quite conqjarahle with the difterence between the heats of oxidation of copper and of 
zinc (about 49,000 calories) on which Principal Lodge bases his calculation of the 
Volta effect. 
Electro-deposition of Brass. 
Another matter of interest lies in the bearing of these results on the electro-depo¬ 
sition of brass from a solution such as that of the mixed potassium-cuprous and 
potassium-zinc cyanides. 
Here the metals unite on the cathode, and energy must be liberated. 
It would probably be difficult to recognise the production of heat from this source 
owing to the slow rate of deposition of the alloy and to the complication introduced 
by the generation of heat due to the resistance of the electrolyte. 
It certainly appears that a complete theory of the electro-deposition of brass must 
recognise the phenomenon. 
VOL. CXCVI.-A. 
4 A 
