4 Mc BAIN 



four substances by Noyes and by Noyes and Sammet, and a long 

 series has appeared from Jahn's laboratory, which are espe- 

 cially interesting as they are all made according to the same 

 method ; the analytical work is wonderful. Noyes has more 

 confidence in the absolute value of his results than Jahn has in 

 his, although the experiments of Noyes and Sammet hardly 

 surpass the best work of Jahn's pupils. However I have 

 copied the results of Noyes to four decimal places, while Jahn 

 calculates his to three. 



Luther ^ estimates the reliability of the best measurements as 

 amounting to one or two units in the third decimal place; and 

 E. Wolff, ^ one of Jahn's pupils, speaks of the " hitherto accepted 

 limit of accuracy of one per cent." It seems much easier to 

 obtain constant results than absolutely reliable values ; the dif- 

 ference between the final values of two investigators is often 

 several times as great as the maximum difference between the 

 individual results of either. 



ERRORS INTRODUCED BY THE USE OF DIAPHRAGMS. 



Even in the earliest experiments the difficulty of preventing 

 the mixing of the solutions through convection currents was met 

 by the use of diaphragms. Faraday used a porous asbestos 

 wick, Daniell and Miller porous earthenware, while Hittorf 

 employed intestinal membrane in most of his measurements. 



Recently Bein^ and then Hittorf * investigated the influence 

 of various membranes upon the migration values, and found 

 that while some of them, such as earthenware, had no influence 

 apart from cataphoresis, others changed the migration values of 

 many substances very greatly. Nernst attempted ° to deal with 

 these cases by his theory of the phenomenon at the meniscus 

 between two immiscible solvents during electrolysis, and Hit- 

 torf' replied; it seems to me that the objections raised by the 



^ Winkelmann's Ilandbuch der Phjsik, 2 Aufl., 1905, Bd. IV, S68. This ex- 

 cellent review of the subject contains also a selection of the most accurate work 

 up to 1903, including experimental details. 



* Dissertation, Berlin, 1903. 



'Zeit. phjs. Chcm., 2S, 439 (1S99). 



*Arch. neer. (3), 6, 671 (1901); Zeit. phjs. Chem.,j9. 613 (1902); 4J, 239 

 (1903) ; Zeit. f. Elcktrochcm., S, 4S1 (1902). 



*Drude's Ann., S, 600, 609 (1902) ; Zeit. f. Elektrochem., 7, 644 (1902). 



^Drude's Ann., 9, 243 (1902). 



