128 



PROCEEDINGS OP THE AMERICAN ACADEMY. 



The only possible method of determining the amounts of these various 

 modifying effects se< ms to be to vary the Bize of the exposed surface of 



cathode; for apon the extent of this exposed Burface depend the t 



Berious causes of error. This i le of experimentation was adopted by 



Shaw, and especially l>y Gray in the paper just cited. Gray's resul is 

 are so interesting thai it Beems worth while to repeat a part of the plate 

 which depicts his observations. The abscissas register the numbei i 



BOUare centimeters area per ampere, while the onlinates give the values 



proportional to the amount of copper deposited by a given current in a 

 given time. The solution of cuprie sulphate had a Bpecific gravity vary- 

 ing from 1.15 to 1.18, or contained perhaps 250 grams <>t' crystallized 



salt to the litre. 



Area per ampere 











.3 



'1.996 





Figure 2. — Diagram hbawn proh thb Ni hbstcai Table given bt Gray. 



(Phil. Mag., [6], 25, 182 [1888].) 



It is evident that with large areas of cathode (small current densities) 

 the fairly straight lines tend to converge at a point somewhat above the 

 top of the table. On the other band, when the current density becomi - 

 great, the curves fail of their mark and bend sharply to the left. Gray 

 dismisses this remarkable tendency with the assumption of the "pro- 

 tective action" of strong currents already mentioned, and takes as the 

 true value the results obtained at "2° Centigrade with current densities 

 higher than 0.01 ampere per square centimeter. 



