due to Heating Aluminium Phosphate, 575 



The sensitivity o£ the galvanometer was such that a de- 

 flexion of one scale division represented a current of 

 2 x 10 -9 ampere. 



When taking readings a saturation voltage, obtained 

 from a battery of small accumulators the negative terminal 

 of which was earthed, was put on e (fig. 1) as soon as the 

 heating of the salt was commenced, and the deflexion of the 

 galvanometer noted from time to time. When the voltage 

 was taken off e in order to read the ther mo-current, time was 

 allowed, when the voltage was again put on, for the current 

 to become steady before readings were taken. 



When the heating-current had been continued for some 

 time, such a large amount of heat ha)(i been conducted along 

 the copper leads as to cause the melting of the wax which 

 was used to render the tube air-tight. This was remedied 

 by soldering a small metal tube over each lead as shown in 

 fig. 1, and making air-tight wax joints at h and k. 



During the course of the experiments in which the galvano- 

 meter was used many attempts under varying conditions were 

 made to detect the presence of negative ions, but with no 

 success. That such ions are present was afterwards proved, 

 but from the results obtained they must be less than i per 

 cent, of the positive. 



Now if the positive ions are produced by chemical change 

 brought about by the heat, then one would expect that a 

 decay in the current would take place with the time. It was 

 found that the current does decrease when heating is con- 

 tinued, so it was decided to obtain the curve of decay for this 

 substance. 



II. Decay of Positive Ionization. 



The aluminium phosphate was made into a paste with 

 distilled water and then placed on the foil e ; the foil was 

 heated slightly by the current before putting it in the tube, 

 this causes the phosphate to adhere to the platinum, and also 

 gets rid of the excess of water. To lessen the effect of the 

 contained gas the tube was evacuated, and the temperature of 

 the foil quickly raised to the degree desired. Varying con- 

 ditions of temperature and pressure were tested, and it was 

 found that the most expedient method was to reduce the 

 pressure to about O05 cm., and to use a temperature of about 

 1200° C. Under such circumstances it was possible to obtain 

 a record of the decay in 5 or 6 hours. 



The method of procedure was as follows : h60 volts were 



put on e (fig. 1), and as soon as the temperature became 



