Electrification due to Heating Aluminium Phosphate. 573 



amorphous and crystalline phases described by Beilby *, 

 photoelectric fatigue being a gradual change from the amor- 

 phous to the crystalline form through an intermediate (labile) 

 phase 



The experiments described in this paper show that such a 

 view is untenable, at least in this simple form, and that we 

 must look to the gaseous films on the surface of the metal 

 for the explanation of the chief effects of photoelectric 

 fatigue. 



Wheatstone Laboratory, 



University of London, King's College. 



June, 1910. 



LXI. Positive Electrification due to Heating Aluminium 

 Phosphate. By A. E. Garrett, B.Sc.'t 



[Plate XL] 



I. Introduction and Experimental Arrangements. 



IN a paper published in the ' Philosophical Magazine ' for 

 October, 1904, by Dr. R. S. Willows and myself, it was 

 found that the halogen compounds of zinc when heated are 

 able to discharge both positively and negatively electrified 

 bodies. A more detailed examination of this phenomenon 

 was subsequently carried out by one of us {. 



In those experiments the temperature to which the salts 

 were raised was in no case higher than 3(30° 0., and no series 

 of observations at pressures lower than a few mm. were 

 undertaken. 



Sir J. J. Thomson § made some experiments to determine 

 whether the base or the acid is instrumental in producing the 

 ionization, and came to the conclusion that the nature of the 

 ionic charge is determined by the acid. Thus he found that 

 phosphates when heated produce a very large excess of 

 positive ions, halogen compounds produce an excess of 

 positive ions, and nitrates an excess of positive at first, but 

 when heated sufficiently to be converted into oxides they 

 produce an excess of negative. Incidentally he found that 

 aluminium phosphate gives off a very large excess of positive 

 ions. 



Now the halogen and other compounds used in the previous 



* G. T. Beilbv, Phil. Mag. viii. p. 258 (1904) ; Proc. Eoy. Sue. (A) 

 lxxii. p. 227 (1905); lxxix. p. 463 (1907). 



f Communicated bv the Physical Society : read June 10, 1910. 



X Garrett, Phil. Mag., June 1907. 



§ Cambridge Phil. Soc. Proc, p. 105, 1907. 



