dm to Evaporation of Sodium in Air and other Gases. 185 



With these arrangements it was found that very soon after heat 

 was applied to the sodium, a negative electrification of 2 to 3 volts 

 was indicated by the electrometer. This electrification persisted for 

 & considerable time, but eventually the insulation broke down, owing 

 to the sodium vapour condensing in the glass tube G at the mouth of 

 the cylinder, sufficiently to cause a solid connection between the tube 

 and wire. 



AMien the same experiment was repeated tvitJiout the sodium, no 

 electrification was indicated by the electrometer. 



These results were confirmed by repeated experiments, and the 

 question then arose whether this negative electrification is due to the 

 evaporation of the sodium, or to oxidation of the sodium going on in 

 the iron cylinder. 



The latter seemed probable, as the electrometer showed electrifica- 

 tion almost from the moment when heat was applied to the sodium, 

 whereas sodium does not fuse till at temperature of 96° C. and boils 

 At about 400' C. 



To determine this point, the same experiment was repeated with 

 this difference, that the air in the cylinder was replaced by an atmo- 

 sphere in which the sodium could not oxidise. Carbonic acid gas 

 was first tried, being kept flowing into the cylinder after passing 

 through a drymg apparatus ; but in this gas the sodium became 

 ■coated with a white encrustation, and showed no signs of evaporation 

 or of boiling even at a red heat. 



Coal gas was next tried, and no difficulty was found in boiling the 

 sodium in this gas. To prevent accident by explosion, the apparatus 

 was set up in the fire-place, so that the escaping coal gas might pass 

 up the chimney, and not mix with the air of the room. Great care 

 was taken to ensure the complete removal of air from the iron 

 cylinder before heat was applied. With this atmosphere of coal gas no 

 electrification was obtained while heat was applied to the tube for over 

 an hour. The insulation was tested from time to time during this 

 period by giving a charge to the wire from an electrified vulcanite rod, 

 watching the rate of leak indicated on the electrometer scale, and 

 then discharging. It was found to be excellent ; but eventually, as 

 before, it broke down when some of the sodium vapour condensed in 

 the mouth of the cylinder between the wire and the glass tube. 

 Repetition of this experiment confirmed this result. 



This problem of the possible generation of electricity by evapora- 

 tion of a liquid, has been recently investigated for the case of water by 

 Pellat,* who found no trace of electrification. A similar result was 

 iound in the earlier experiments of Blake, f who used water and solutions 



* Pellat, * Seances de la Societe Francaise de Physique, 1899,* ler Fascicule, 

 p. 21. 



t Blake, * Wiedemann's Annalen,' vol. 19, 1883, p. 518. 



