On the Potential-difference produced by Contact. 367 



elasticity of steam destroys its special centripetal tendency, 

 relatively to the earth, without materially affecting the solar 

 influence on its orbital motion ; and its volume is increased 

 nearly in inverse ratio to the Sun's local attraction. 



Ganot* gives 1698 : 1 as the volume-ratio of steam at 100° 

 C. to water at 0° C, and he estimates the expansion of water 

 between 0° and 100°f at -0466 + '002584 = -049184. A nu- 

 cleal expansion of 1*049184 corresponds to a nebular expan- 

 sion of 1-049184^ = 1-066 • and 1698-r- 1*066 = 1593, or very 

 nearly n 3 ( = 1584). If Sun's mean distance is 93,000,000 

 miles, the mean ratio of its attractive force upon Earth to 

 Earth's equatorial gravitation is 1665. 



LI. On the Difference of Potential produced by the Contact of 

 different Substances. By Professor B. B. Clifton. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



IN the March Number of the Philosophical Magazine is 

 published a letter from Professors Ayrton and Perry, in 

 which they refer to my paper " On the Difference of Potential 

 produced by the Contact of different Substances " (Proceedings 

 of the Boyal Society, vol. xxvi.). To this letter I have 

 hitherto been prevented from making a reply ; but I now ven- 

 ture to ask for the insertion in your Journal of a few remarks 

 on some of the matters therein referred to. 



At the time my paper was written I was quite unaware of 

 the investigation of the same subject which had been under- 

 taken by Professors Ayrton and Perry ; and, indeed, the first 

 information I obtained relative to their work was derived from 

 the letter above mentioned. As soon as possible after the ap- 

 pearance of their communication I endeavoured to find their 

 paper, by searching through the various scientific periodicals 

 and the publications of scientific societies, but without success. 

 The title of the paper appeared in the ' Beport of the British 

 Association,' but the title only. 



As I was not present at the Meeting of the British Associa- 

 tion at Glasgow, at which some account of the work of Pro- 

 fessors Ayrton and Perry appears to have been given, but not 

 reported, my ignorance of their work is, I hope, excusable, 

 though I very much regret it. The paper in question has 

 recently been read before the Boyal Society, and it will, I 

 believe, shortly be published. 



* Fifth Engl, ed., § 353. f Ibid. §§ 294-5. 



