Air by Uranium and its Compounds. 103 



drawn through the electric filter, and the deviation of the 

 electrometer-reading from the metallic zero of the instrument 

 when the two pairs of quadrants were insulated, was noted. 

 To give some idea of the efficiency of the filter the results 

 obtained when air was drawn from the neighbourhood of the 

 electrodes of a Ruhmkorff coil — a 10 inch Apps — will be given. 

 When the electrodes were near enough to admit of sparks 

 passing it was found that the air in the neighbourhood of the 

 positive electrode gave a positive electrification of + 1'5 volts 

 after one minute of pumping at the rate of 12 strokes per 

 minute. When the air was drawn from the negative electrode's 

 vicinity a negative electrification of —2*5 volts in half a minute 

 was observed. When the electrodes were drawn so far apart 

 that sparking did not take place, air drawn from the positive 

 electrode raised the filter to a potential of + '7 of a volt after 

 one minute of pumping ; air drawn from the neighbourhood of 

 the negative electrode gave a reading of — 1*5 volts in the 

 same time. 



When the negative electrode was connected metallically to 

 the cathode of a Crookes tube, air drawn from the neighbour- 

 hood of the cathode through the filter caused an electrometer 

 deviation equivalent to a potential of — *8 of a volt per minute 

 of pumping. Air drawn from the neighbourhood of the wire 

 connecting the negative electrode to the cathode caused a 

 deviation equivalent to — '7 of a volt. Air drawn from the 

 neighbourhood of the wire joining the anode of the Crookes 

 tube to the positive terminal of the coil gave a positive reading 

 on the electrometer less in amount than the corresponding 

 negative reading. 



§ 3. Electrification of Air by Metallic Uranium. — To 

 examine the electric state of the air in the neighbourhood of 

 metallic uranium charged to a given potential, the following 

 experimental arrangement was made : — A metallic cylinder 

 24 cm. in length, 8 cm. in diameter, was constructed. One 

 end of this was closed with a piece of the same metal ; in this 

 end was a hole into which a plug of tunnelled paraffin was 

 placed. From the paraffin a piece of indiarubber tubing about 

 1^ foot long led to the electric filter. The other end of the 

 metallic cylinder was closed with a disk of paraffin, in the 

 centre of which a hole was made to admit air from the 

 laboratory. This hole was filled with cotton wool. The 

 uranium, a circular disk 5'5 cm. diameter, *5 cm. thickness, 

 was insulated on paraffin inside the cylinder. The cylinder 

 itself was laid on a block of paraffin. The arrangement will 

 easily be understood from the following diagram. 



In the first instance a piece of lead of the same size as the 

 uranium was insulated on paraffin in the metallic cylinder. 



