due to Heating* Aluminium Phosphate. 585 



doublets cause them to break up, hence causing a large 

 positive current to pass between the electrodes. The initial 

 positive ions appear to increase in the number given off 

 per second as the temperature is raised until about 1050° C, 

 after which the output appears to remain constant. 



If, on the other hand, the salt is raised to a negative 

 potential, the initial positive ions are prevented from leaving 

 it, and so at reduced pressures, with electrodes a very small 

 distance apart, the doublets have smaller chances of breaking 

 up ; thus the current when the salt is negatively charged 

 is relatively small compared with that due to positive- 

 ions. 



That the potential to which the salt is raised has an 

 important bearing upon the relative number of positive and 

 negative ions present is clearly shown by the fact that the 

 current due to negative ions is not so small compared with 

 that due to positive ions when the salt is at zero potential 

 and the gas pressure is low. 



Also, at atmospheric pressure when the products due to 

 heating the salt were removed by an air blast, there was 

 always quite a large current produced by the negative ions. 



IX. Nature of the Ions at Atmospheric Pressure. 



For this purpose the apparatus used was two brass tubes 

 of 1*7 cm. internal diameter, insulated from each other, and 

 each having an insulated wire 0*33 cm. diameter and 28 cm. 

 in length along the axis. These wires could be in turn con- 

 nected with a Dolezalek electrometer of such sensitivity that 

 one volt caused a deflexion of 820 scale-divisions. 



The method adopted was to heat the phosphate on platinum 

 foil through which a current was passed, and to suck air 

 past the heated salt, and then through the two brass tubes 

 placed one behind the other. 



In order to avoid great fluctuations in the temperature, 

 the platinum and its leads were enclosed in a wide glass tube, 

 one end of which was connected air-tight to the testing- 

 apparatus, and the other closed loosely with cotton-wool. 



Readings were taken after the salt had been heated for a 

 sufficiently long time to bring it to the steady condition. 



The electrometer showed no initial leak even when one of 

 the electrodes was connected with its quadrants, and the tube 

 containing the electrode was raised to a positive potential of 

 650 volts. A saturation current was obtained when the 

 electrode nearer the heated salt was connected with the 



