736 Mr. A. E. Garrett on Electrical 



In some later experiments with pure zinc iodide, it was 

 found that at temperature 248° C, at a pressure of 2*5 mm. 

 of mercury a saturation-current could be obtained with a 

 difference of potential of about 260 volts in the case of the 

 positive ions, and with about 180 volts for the negative ions. 

 The brass tube used in place of iron tube F (fig. 3) had an 

 internal diameter of 1*4 cm., and the central electrode a 

 diameter of "25 cm. 



IV. The Effect of Moisture. 



On one or two occasions, when a series of readings were 

 required and the time at disposal was limited, some fresh 

 material was placed in the apparatus while the latter was at 

 a temperature between 250° C. and 260° C. On each occasion 

 when using zinc iodide, it was noted that the rate of leak 

 obtained was extremely small compared with previous results. 

 From this it appeared that either the method of heating or 

 the presence of water- vapour had an important effect on the 

 rate of leak. Some zinc iodide was therefore prepared 

 synthetically by heating pure zinc and iodine in distilled 

 water until the colour of the solution disappeared, then 

 evaporating to dryness, and leaving the residue in a desiccator 

 for a week. 



An addition was now made to the apparatus shown in fig. 3, 

 in the form of a trap of concentrated H 2 S0 4 between the 

 tube A and the water-pump. 



When the zinc iodide was placed in the apparatus and 

 heated up slowly at a pressure of 30 mm. of mercury, the 

 leak obtained was similar to that when commercial iodide of 

 zinc was used under similar conditions. 



When, however, some of this zinc iodide was placed in the 

 apparatus while the latter was at 256° O, no extra leak 

 could be detected with pressures as low as 30 mm. of 

 mercury. 



The material was left in the apparatus for a week, care 

 being taken to exclude moisture as far as possible. At the 

 end of that time, with the temperature raised to 263° C. and 

 the pressure reduced to 30 mm. of mercury, no extra leak 

 could be detected. Hence moisture is necessary to make 

 these salt vapours conductors of electricity. 



Other effects of moisture were noted in the later ex- 

 periments made to determine the velocities of the ions, and 

 also in the experiments on the rate of decay of zinc iodide. 

 These will be referred to in connexion with the parts 

 named. 



