242 



*/. M. Lewis : 



contact with solutions containing hydrogen ions, using this as 

 a criterion of their comparative acidity. 



In order to demonstrate this, advantage is taken of the pro- 

 perty of condensing gases possessed by " platinum black," and 

 the metal deposited in this form upon platinum or gold consti- 

 tutes the electrode. 



When such an electrode is placed in an atmosphere of hydro- 

 gen, the condensation of the gas on its surface is of such an 

 order that the arrangement may be regarded as a solid rod of 

 hydrogen, and, with certain technical restrictions, used as if it 

 were such. 



We may show the production of an e.m.f. with this electrode 

 by setting up an arrangement similar to that described as a " con- 

 centration battery," only in this case means must be provided 

 for keeping the platinum black saturated with hydrogen, while 

 the two electrodes dip into solutions of an acid which differ in 

 concentration from each other. A diagrammatic view of the 

 apparatus is shown in Fig. 4. 



Fig. 4. 



