236 



Mr. A. T. Waterman on the Positive 



its power of emitting would regain it to some extent if 

 heated in air. Several attempts have been made here to 

 determine just how much this source of impurity entered 

 into the ordinary manipulation of the apparatus. 



A Pt strip was glowed until it showed no emission what- 

 ever below 1000°. The intensity of the emission was taken 

 at 1150°, and the strip was allowed to cool in the vacuum. 

 Dry air was admitted for one minute and immediately pumped 

 out again. A reading of the emission was taken at the 

 same temperature as before. The same thing was done after 

 exposing the strip to cold dry air for 25 hours. Then the 

 strip was removed and placed in a box for 42 days, exposed 

 to the ordinary air in the room, and a reading taken again. 

 The readings are given for the velocity of the electrometer 

 spot in decimetres per second. 



Then the same strip was tested in the same manner, being 

 touched between readings with the Pt wire used to place the 

 test salts on the strip. The wire always previously gave a 

 negative flame test. 



A further trial was made, placing a drop or two of dis- 

 tilled water on the strip between readings. 



The results follow : — 



Time. T°. 

 lirs. min. C. 

 Pt strip 1150° 

 alone. 



Intensity of 



emission. 



•290 



Remarks. 



Cold dry air admitted for one minute. 





1 1150° 

 4 



6 

 8 



•320 

 •303 

 •279 



•275 



Decaying 



fast." 

 Fairly 



steady. 



Air admitted for 25 hours. 







25 8 1150° 

 25 13 „ 

 25 18 „ 

 25 28 „ 



•376 



•351 



• -336 



•324 



Decaying. 



Fairly 

 steady. 



Exposed to air for 42 days. 



42 days 805° 



•161 





(See measurement (A) 



of ejm here.) 





hrs. min. C. 

 Pt strip 1050° 

 alone. 5 ,, 



Intensity. 

 •01 



•200 



Remarks. 

 Increasing. 

 Steady. 



Distilled water placed on strip with Pt wire. 



8 725° -01 



11 870° -45 



14 930° 2-50 



(See measuiement (B) of: e/m here.) 



