576 Mr. A. E. Garrett on Positive Electrification 



steady readings were taken ; the pressure and temperature 

 were kept constant throughout. 



Experiments of this kind were carried out with air, 

 hydrogen, and C0 2 , as the gas in the tube. It was found in 

 every case when air or hydrogen was used that the manner 

 in which the current varied with the time for the first 

 20 minutes was peculiar. 



A typical curve to illustrate this is given (PL XL fig. 2). In 

 this curve the currents are plotted as ordinates and the times 

 as abscissae. 



This shows a rapid fall from A to B, then a rise to a 

 maximum at C, and lastly a decay C to D, in which the 

 current decreases roughly exponentially with the time. The 

 part of the curve near B sometimes showed still further 

 irregularities. 



With C0 2 as the gas in the tube a typical decay curve is 

 represented by ECD. The initial changes observed in air, 

 and hydrogen, are apparently due to water, since a pre- 

 liminary heating of the phosphate at a lower temperature 

 sufficient to expel the water, gets rid of them altogether. 



Further, in the case of C0 2 , that gas may possibly assist 

 in the removal of hygroscopic moisture, and so prevent its 

 action on the salt. 



After some hours' heating a more or less steady state was 

 reached. This state persisted for some months, nor did it 

 regain any activity if dry or moist air was admitted, even if 

 left for 2 or 3 days. Only on one occasion was a slight 

 temporary regain noted, and in this case the interval was 

 16 days. 



If after the steady state was reached the salt was moistened 

 with distilled water there was a large increase in the current 

 which quickly died away, and in about 10 minutes the steady 

 state was again reached. The decay of this, AF (PL XI. 

 fig. 2), being taken in conjunction with a typical decay curve 

 ECD, a curve of the form ABCD is obtained. 



This is further evidence that the initial changes are due to 

 water. These changes will not be further considered. 



A typical decay-curve over a longer period omitting these 

 changes is shown in PL XI. fig. 3. 



The part of the curve near C is exponential ; the portion 

 EDO is very similar to the curve given by Rutherford in the 

 2nd edition of ' Radioactivity/ p. 342, for the variation in 

 activity of the active deposit of Th due to a very short ex- 

 posure to the emanation. In the case of Rutherford's curve 

 this is known to be due to the decay of two substances Th A 

 and Th B, the former of which does not produce any rays, 



