402 PROCEEDINGS OF THE AMERICAN ACADEMY. 



was too small to produce a bubble, which indeed appeared to form quan- 

 titatively according to Faraday's law. A current as low as 10 -8 amperes, 

 resulting from a potential of approximately .2 volt, gave a bubble visible 

 to the naked eye within a minute, although the mass of this bubble must 

 have been less than 10 -11 grams. The apparatus, therefore, as a current 

 detector, is almost as sensitive as a good galvanometer. On account of 

 the formation of these bubbles it was found impracticable to experiment 

 with the mercury below the end of the tube D, and in all our experi- 

 ments the top of the mercury meniscus was kept at a level with the end 

 of the tube. 



The various tubes leading into the cell passed tightly through a rubber 

 stopper. The use of rubber, although possibly objectionable, was de- 

 cided upon in order to facilitate such alterations in the apparatus as the 

 course of the experiments might make desirable. All possible precautions 

 were taken to prevent the rubber from contaminating in any way the 

 contents of the cell. 



For preparing pure hydrogen a large generator with electrodes of zinc 

 amalgam and copper, of the type first used by Cooke and Richards,* was 

 employed. It was so arranged that the liquid could be withdrawn and 

 new zinc and acid added without the entrance of a trace of air. The 

 new acid, before it was introduced, was always treated lor several days 

 beforehand with a stream of hydrogen. 



Before entering the cell the hydrogen passed through a wash-bottle 

 A, containing sulphuric acid of the same strength as that in the cell, and 

 on emerging it passed through a wash-bottle of mercury F, so that the 

 pressure in the apparatus was always several centimeters of mercury above 

 that of the atmosphere, thus preventing the leaking in of air. 



The figure shows also the essential electrical connections. M repre- 

 sents a storage battery whose current passes through a resistance box of 

 the form invented by Ostwald for measuring potentials. By moving the 

 pegs L and K of this box a current of any desired strength could be 

 tapped off to the cell, in circuit with which a resistance of 100,000 

 ohms was placed. 



The two quantities to be measured were the potential between the 

 electrodes and the current. The latter was obtained by measuring 

 the potential fall in the 100,000 ohms' resistance. In other words the 

 measurements consisted in finding the potentials between G and I, and 

 between H and G. These were obtained by means of a potentiometer 



* Amer. Chem. Jour , 10, 81 (1888). 



