On the Determination of Chemical Affinity. 169 



in the shining rectangle, just above the theoretical limit of 

 the prismatic spectrum, given by many phosphorescent sub- 

 stances. 



University of New York, 

 Dec. 1, 1880. 



XXIV. On the Determination of Chemical Affinity in terms of 

 Electromotive Force. — Part III.* By C. R. Alder Wright, 

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

 Mary's Ho spited Medical School^. 



On the Absolute Values of the Ohm and of the Mechanical Equi- 

 valent of Heat, and on the Nature of Polarization and Elec- 

 trolysis, and the amount of work done when the latter occurs. 



55. aiNCE the publication of Parts I. and II. of these 

 ^ researches, it has been obligingly pointed out by 

 Mr. L. B. Fletcher, of Baltimore (Phil. Mag. [5] x. p. 436), 

 that, by a slip, it is stated in § 35 that the values of the B. A. 

 unit of resistance deducible from the 1867 experiments of 

 Joule and from those detailed in Part II., are from 1*5 to 2'0 

 per cent, above one earth-quadrant per second, instead of that 

 amount below this theoretical value, it being assumed that the 

 mean water-friction value of Joule (41*555 megalergs, § 34) 

 is correct; and Mr. Fletcher also states that the first results of 

 an experimental redetermination of the absolute value of the 

 ohm, on which he is now engaged, are similar in character. 

 Besides the valuations of Lorenz and Kohlrausch, briefly 

 referred to in Part I. as illustrations of the want of absolute 

 certainty as to the true value of the B.A. unit, two other sets 

 of observations on this point have been made within the last 

 two or three years — one by Prof. H. A. Rowland (Silliman's 

 Journal [3] xv. pp. 281, 325, and 430, 1878), and the other 

 by H. F. Weber (Phil. Mag. [5] v. pp. 30, 127, and 189, 

 1878). By making observations of the value of the current 

 induced in one coil by the establishment of a measured cur- 

 rent in another one, Rowland obtained values of the resistance 

 of the former such that, by comparison with four copies of the 

 B.A. unit (or a multiple thereof), the following numbers were 

 obtained as the resistance of the latter: — 



* Part I., Proceedings of the Physical Society, 1880, vol. iii. p. 213 ; 

 also Phil. Mag-. April 1880. Part II., Proc. Phys. Soc. 1880, vol. iii. 

 p. 246; also Phil. Mag. May 1880. 



f Communicated by the Physical Society. 



