On the Determination of CI lemical Affinity. 261 



The length of the substance in which the ray is rotated a com- 

 plete revolution, or 360°, will then be 



mi n . 2 / . . di V 



where it is to be noted that \ is the length of a complete wave. 



Taking the wave of yqJ-^j centim. length and the index of re- 



di 

 fraction 4, we find, supposing — =0, 



D = 240 centims., nearly. 



We do not know the magnetic force used by Verdet; but it 

 was evidently of the same order of magnitude. He found D 

 to be about as follows — 300 for heavy glass, 700 for flint glass. 

 Hence the rotation calculated for gold is of the same order of 

 magnitude as the rotation observed in some common sub- 

 stances. 



Thus the new electromagnetic phenomenon explains in the 

 most perfect manner the magnetic rotation of the plane of 

 polarization of light; and we are almost in the position to 

 pronounce positively that the two phenomena are the same. 

 Should this preliminary theory of the subject stand the test of 

 time, it hardly seems to me that we can regard it in any other 

 light than a demonstration of the truth of Maxwell's theory of 

 light; for the rotation of the plane of polarization is thus a 

 necessary consequence of the laws of electromagnetism, and 

 this, added to the other facts of the case, raises Maxwell's theory 

 almost to the realm of fact. 



XXXVI. On the Determination of Chemical Affinity in terms 

 of Electromotive Force. — Part IV. By C. E. Alder Wright, 

 D.Sc. (Lond.), Lecturer on Chemistry and Physics in St. 

 Mary's Hospital Medical School. 

 [Plate III.] 



On the Validity of Faraday's Law as to the Proportionality be- 

 tween the Mass Electrolysed and the Quantity of Electricity 

 passing ; on the Numerical Relations] hips between the Counter 

 Electromotive Force setup by weak Currents and the Rate of 

 Flow of these Currents; and on the Conditions influencing 

 these Relationships and the consequent Dependence of the 

 E.M.F. of certain forms of Electromotors on the Currents 

 generated by them. 



77. ri^HE original experiments of Faraday and of many 

 -L subsequent observers have amply established, for 



moderately strong currents, the truth of the proposition 

 Phil. Mag. S. 5. Vol. 11. No. 68. April 1881. ' U 



