produced by the Contact of different Substances. 369 



My apparatus enabled me not only to observe the sign of the 

 difference of potential due to metal-liquid contacts, but also to 

 obtain quantitative determinations ; as, however, some of these 

 differences of potential are very small, and a slight error in 

 the construction of the condenser, which I believed to exist, 

 would probably produce considerable errors in the values 

 obtained for the potentials, I thought it best not to introduce 

 quantitative results in which I felt little confidence into my 

 paper, which I only regarded as a preliminary notice. 



Between most of the results obtained by Professors Ayrton 

 and Perry and those published by myself, there is a fairly 

 satisfactory agreement from a qualitative point of view ; but 

 between such quantitative measures as admit of comparison 

 there seem to be considerable descrepancies : these I trust 

 will disappear when we introduce into our respective methods 

 the improvements which we each admit to be necessary. 



Several of the criticisms on my paper, which have been 

 introduced into the letter of Professors Ayrton and Perry, do 

 not appear to me well founded ; but I must for want of time 

 postpone the discussion of them ; and possibly the more 

 thorough investigation of the subject, which we each con- 

 template, may render this discussion unnecessary. 



As the value of any results we may obtain in the future 

 would be much increased by a ready means of comparison, I 

 venture to suggest that we should adopt the same difference 

 of potential as the unit. If I am right as to the variation in 

 the difference of potential exhibited by the terminals of Clark's 

 standard cell, it will clearly be undesirable to employ this cell; 

 and I should suggest the use of the standard Daniell's cell 

 referred to in my paper, viz. copper in a saturated solution of 

 copper sulphate, and amalgamated zinc in a mixture of one 

 part by weight of pure sulphuric acid and four parts by weight 

 of distilled water. This cell seems to me to give a remarkably 

 constant difference of potential when prepared on different 

 occasions ; and the accurate expression of this difference, in 

 volts, when obtained, would at once allow all our results to be 

 translated into terms of the latter unit. 



I am, Gentlemen, 



Yours faithfully, 



P. B. Clifton. 



Oxford, April 22, 1878. 



Phil Mag. S. 5. Vol. 5. No. 32. May 1878, 2 B 



