Positive Ionization jrom Hot Salts. 679 



experiment which gave rise to the preceding numbers, the 

 effect of water vapour on the emission w r as tried. Successive 

 increments in the pressure were obtained by connecting to a 

 small side chamber, which was exhausted except for the fact 

 that there was a small quantity of water in the bottom of it. 

 The approximate values of the pressures after the addition of 

 water vapour w r ere obtained from the depression of a baro- 

 meter column. The values of the thermionic current w r ith 

 increasing pressure were as follows: — 



Pressure (mm.) ... -01 65 11 14 16 18 21 22 



Current 86 203 144 118 102 93 84 76 



(1 = 1-25 Xl0~ 8 amp.) 



The first reading at O'Ol mm. corresponds to the air initially 

 present in the apparatus. On pumping out the water vapour 

 the following values of the emission were obtained at the 



pressures stated : — 



65 5 4 3 2 1-5-5 -33 

 174 230 260 306 340 456 550 520 



Pressure (mnis.) ... 22 



10 



Current 75 



130 



(1 = 1-25 Xl0 -8 amp.) 





Pressure (mms.) ... '15 -030 *005 



Current 490 410 220 



(1 =1-25 XKT 8 amp.) 



The maximum emission in water vapour is thus about five 

 times as great as in air at this temperature. The pressure of 

 maximum emission also appears to be somewhat higher 

 (about *5 mm. as against *2 mm.). 



This treatment with water vapour apparently produced a 

 permanent change in the salt, as the maximum emission in 

 air when measured at the same temperature was found to be 

 only 1*2 X 10 -7 amp. as against 1*2 X 10 ~ H amp. previously. 



The mode of variation with the pressure at very low 

 pressures has been examined very carefully, as it seemed 

 that this would be likely to throw some light on the nature 

 of these rather involved phenomena. The only conclusion 

 that I have been able to come to is that at very low pressures 

 the emission is a linear function of the pressure. This is a 

 state of things that would occur on almost any view of what 

 is going on. At very low pressures the phenomena seem 

 very variable ; sometimes a change of pressure from about 

 •00005 to *01 mm. would increase the emission by a factor 

 of eight, whilst at others it would scarcely affect it, I have 



