54 Dr. J. II. Gladstone on the Specific Refraction 



to two or more liquids, tolls us that the electromotive force of 

 any arrangement is equal to the algebraical sum of nil the 



separate differences of potential at the various surfaces of con- 

 tact of dissimilar substances, each difference of potential being 

 measure^ by an induction method such as we have described 

 and used. The chemical theory tells us that the energy given 

 out in any electric circuit must be equal to the consumption 

 of chemical energy in the circuit. The electromotive force, 

 then, in any circuit can either be calculated on the contact 

 theory, if we know each of all the differences of potential at 

 the various surfaces of separation of dissimilar substances 

 (solid, liquid, or gaseous) in the circuit, or it may be calcu- 

 lated on the chemical theory, if we know exactly what are 

 all the physical and chemical changes taking place and the 

 heat-equivalent of every one of them. And the amount of the 

 electromotive force determined in either of these two ways 

 must be identical. 



The question of the relative delicacy of the two methods is 

 a totally different question. Any balance accurately made 

 will weigh accurately; but one form of chemical balance is 

 more delicate than another : so, in the same way, there is 

 every reason for believing that an electric test is a far more 

 delicate test of chemical action than the analytical methods 

 employed by the chemist. For example, the action of even 

 small quantities of paraffin-wax on metal, which would quite 

 escape the test of a chemical analysis, we have not only de- 

 tected, but even measured, with an electrometer *. 



VII. Specific Refraction and Dispersion of Isomeric Bodies. 

 By J. H. Gladstone, Ph.D., F.R.S.} 



AMONG the properties of a body which are least liable to 

 change, and which are the most capable of throwing 

 light on its molecular constitution, is its specific refraction. 

 This is the refractive index minus unity divided by the 



density, or ^ 



In early papers on the subject^, this specific refraction (or 

 " specific refractive energy," as it was then called) was 

 shown to be a constant, unaffected, or nearly so, by change 



* " Contact Theory of Voltaic Action," Part II., by Profs. W. E. Ayrton 

 and John Perry, Proc. Roy. Soc. No. 186, pp. 20-84, 1878. 



t Communicated by the Physical Society, having been read at the Meet- 

 ing on November 27/1880. 



| Phil. Trans. 1803, p. 323, and 1809, p. 9 ; Journ. Chem. Soc. 1805, 

 p. 108; Landolt, Pogg-. Ann. exxii. p. 545; Wiillner, ibid, exxxiii. p. 1. 



