On Contact Electricity of Metals. 145 



which is nearly the same as the temperature-variation of bare 

 aluminium in air. 



82. A plate of silvered glass used with the glass side facing a 

 standard plate gave almost the same potential as clean silver in air. 

 This plate, which was practically silver coated with glass, gave a 

 temperature-variation larger than that of silver in air. In this case 

 we have glass in contact with the silver surface instead of air, but 

 the change does not alter the potential. It may be mentioned that 

 the back of the silver film was painted black, and not coated with 

 glass ; but this is of small consequence, since it has been proved by 

 experiment that the condition of the back of a plate does not sensibly 

 affect the volta contact-potential ; or, more generally, that if parts of 

 a plate be in different conditions, the potential observed will be the 

 mean of the potentials of the different parts, the importance of each 

 part being proportional to its capacity. 



83. On account of the great attraction of sodium for oxygen, it 

 seemed of interest to measure its potential in circumstances which 

 excluded that gas from the surface of the metal. In order to effect 

 this two pieces of thin sheet-glass, each about 6 cm. square, were put 

 into a dish of melted paraffin-wax together with some clean sodium, 

 and a large drop of the sodium was put between the plates of glass 

 and squeezed out into a small plate of 2 or 3 square cm. area. The- 

 glass plates, with sodium between them, were taken out of the melted 

 wax and allowed to cool. Since the glass plates were of much 

 larger diameter than the sodium, the edges of the latter were pro- 

 tected by the wax which filled up the space between the plates not 

 occupied by sodium. The flat faces of ihe sodium were apparently 

 in contact with the glass. The sodium was connected to the elec- 

 trometer by a fine copper wire. The greater part of the sodium 

 surface was bright or only slightly tarnished, and it remained in 

 almost the same condition for many days, being protected by the 

 glass plates and by the wax which filled the space between them 

 unoccupied by sodium. The first measurements gave sodium 



2'86 volts 



positive to a tarnished zinc plate, i.e., about 



3'56 volts 



positive to the standard gold plate. This potential gradually de- 

 creased. 



84. Experiments were made to make sure that the result was 

 not due to temporary electrification of the glass. For instance I 

 breathed on the glass, causing a conducting layer of impure water to 

 form on its surface. Repeated measurements, made by the usual 

 method, showed that the potential at once fell to a small fraction of 



VOL. LXIII. M 



