52 Mr. W. C. Roberts on the Experimental Illustration 



On completion of the circuit the following facts will be ob- 

 served : from the positive wire, gas (oxygen) is freely evolved, 

 while the negative wire is perfectly quiescent, the hydrogen 

 being for some time entirely absorbed by the metal. When the 

 hydrogen makes its appearance it rises from the end nearest to 

 the positive electrode. 



On reversing the direction of the current, evolution of gas 

 ceases from both wires, the hydrogen being occluded by the one, 

 and the oxygen being consumed by the previously absorbed hy- 

 drogen in the other*. Attention should also be directed to the 

 flexure produced by the unequal absorption of gas on different 

 sides of the wire. 



To obtain a direct demonstration of the expansion, the writer 

 availed himself of the deportment of a compound riband of pal- 

 ladium and platinum when made to form the negative electrode 

 of a battery decomposing acidulated water. The riband con- 

 sists of two strips, one of palladium, the other of platinum-foil, 

 300 millims. long, 3 millims. wide ; these are soldered together 

 and coiled into a circle, the palladium being inside. If, in the 

 first instance, the coil be connected with the zinc end of the 

 battery, hydrogen will be thrown on the surface of the palla- 

 dium, which absorbs the gas, and, by the consequent expansion 

 of that metal only, opens the coil, the motion being rendered vi- 

 sible by a light moving index. 



On reversing the direction of the current, oxygen will be 

 thrown on the compound riband, and by its combination with 

 the previously absorbed hydrogen, will relax the spiral and 

 cause the index to move rapidly back to zero. 



But the employment of an index to magnify the motion is 

 scarcely necessary with so rapid an angular velocity at command. 

 The simplest form, and at the same time the most efficient, consists 

 in placing as the electrodes two strips of palladium-foil varnished 

 on one side and coiled into spirals (each 300 millims. by 5 to 7 

 millims.) as indicated in the figuref. When one of the strips is 



* This experiment was shown at the Meeting of the British Association 

 at Norwich, August 1868. 



t As the varnish soon becomes cracked and detached from the foil, it is 



