642 Lord Kelvin [May 21, 



§ 29. Consider, now, a large number of discs of zinc and copper, 

 each of 1 square centimetre area, and thickness D, and polished on 

 both sides. On one side of each disc attach three very small columns, 

 of length D, of glass or other insulating material, and place one disc 

 on top of the insulators of another, zinc and copper alternately, so as 

 to make a dry insulated pile of the metal discs, separated by air 

 spaces each equal to the thickness D. If in the building of this pile 

 each disc is kept metallically connected with the one over which it is 

 placed, while it is being brought into its position, work will be done 

 upon it by electric attraction to the amount shown in column 3, and 

 the total work of electric attraction during the building of the pile 

 will be the amount shown in column 3, multiplied by one less than 

 the number of discs. 



But if each disc, after being metallically connected with the one 

 on which it is to be placed, till it comes within some considerable 

 distance — say 300 D, for example, from the disc over which it is to 

 rest — is then disconnected and kept insulated while carried to its 

 position in the pile, no work will be done on it by electric attraction. 

 And if now, lastly, metallic connection is made between all the discs 

 of the pile, currents pass from each copper to each zinc disc, and 

 heat is generated to an amount equal to that shown in column ,4, 

 multiplied by one less than the number of discs ; and if this heat 

 is allowed to become uniformly diffused through the metals, they rise 

 in temperature to the extent shown in column 6. 



All these statements assume that the electric attraction increases 

 according to the inverse square of the distance between opposed faces 

 of zinc and copper. We have already (§ 28) seen that this assump- 

 tion cannot be extended to such small distances as 10"^ of a centi- 

 metre. We have now further proof of this conclusion beyond the 

 possibility of doubt, because the large numbers in columns 5 and 6 

 for 10"^ are enormously greater than any rational estimate we can 

 conceive for the heat of combination of equal parts of zinc and copper 

 per gramme of the brass formed. (See § 32 below.) 



§ 30. When, on a Friday evening in February 1883 — fourteen 

 years ago — quoting from an article which had been published in 

 Nature "j" in 1879, I first brought these views before the Royal Insti- 

 tution, we had no knowledge of the amount of heat of combination 

 of zinc and 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 even- 

 ing, with an investigation on the heat of combination of copper and 

 zinc, by which he has found that the melting together of 30 per cent. 



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



