i IS PROCEEDINGS OP THE A iDEMT. 



Thus 96,610 coulombs correspond to one grain equivalent ol 

 itrolyte. 



Th( se numbers are of interest from a theoretical standpoint, but their 

 practical value as means of determining current Btrength must depend 

 upon thi of their application. The value for copper is a mean 



tween two limits, and is not easy to reproduce with exactness; but for 

 most purposes the weight deposited by current densities between 0.008 

 and 0.012 amperes per square centimeter, from ten percent solutions of 

 cupric sulphate cooled below zero and protected by an inert atmosph 

 is sufficiently close to the real value. Neverthel mpirical ta 



like those of Gray will probably remain the most convenient method of 

 computing current strength from the deposition of copper. 



On the other band, the intricacy added by the porous cup in the silver 

 determination is not Berious, and the results obtained seem trustworthy. 

 Obviously any method capable ofbeing repeated with constant result • 

 standardized with accuracy by mean- of known currents, would Berve all 

 practical purposes; and a method which i< constant because the sou 

 of error have been at least partially removed is certainly worthy of 

 attention. 



This point Buggests a discussion which has recently arisen concern- 

 ing the electrical determination of -Ionic's equivalent, which depend- upon 

 measuring the heat corresponding to a known amount of electrical 

 energy. Griffiths, in an appendix* to an elaborate paperf upon this 

 Bubject, calls attention to the fact that an error in the electrochemical 

 equivalent of silver of 0.1 per cent would explain the difference between 

 his value of the mechanical equivalent of heat and Rowland's. The 

 equation of Griffiths for the calculation of his value is 



./ 



/■:- f<f\ 



RM\8df 



in which E signifies the constant potential difference, R— the resistance 

 converting electricity into heat. M the heat-capacity which is heated, 

 the interval of time, and 80 the change of temperature. The electro* 

 chemical equivalent of silver enters into this expression in the determina- 

 tion of the value of E in reference to R. Griffiths's value for J is largi t 

 than Rowland's, hence his value for E is greater than that which Row- 

 land would have observed il he had used an electrical method; or, in 

 other woids, too small an amount of silver may have been taken as the 



» Nature, 56, 268 (1897). Proc. Roy. Soc, 53, 6 (lb'.':)). 



