ELECTRIFICATION OF AIR, VAPOUR OF WATER, AND OTHER GASES. 225 



sented in fig. 13, and electrifying by one needle point joined to the insulated terminal 

 of the Voss electric machine. The air was contained in a glass bell-jar, A, which 

 was coated inside with strips of tin-foil kept in metallic connection with one terminal, 

 G, of a high-resistance mirror galvanometer, the other terminal of the galvanometer 

 being joined to the sheath of the voltmeter, V, and to the disinsulated terminal, M', of the 

 electric machine. The stand of the bell-jar rested on a piece of paraffin. The pressure 

 of the air in the bell-jar, was measured by noting the height to which mercury rose in 

 the tube, B. The vessel containing the mercury was insulated by a paraffin block. 



Fig. 13. 



The pressure of the air after it had passed the electric filter (block-tin pipe with fine 

 brass filings) was also measured by means of the rise of mercury in tube, B'. A baro- 

 meter t\ibe not shown in the diagram gave us the atmospheric pressure. The differ- 

 ences of the heights of the mercury in tubes B, B', -and the height of the mercury in 

 the barometer, gave the pressures of the air in the bell-jar and on the exit side of the 

 filter respectively. 



All the tubing through which the air passed was block tin. 



Throughout each experiment the pressure of the air in the bell-jar was kept 



VOL. cxci. A. 2 G 



