Positive Ionization from Hot Salts. 



689 



Barium Sulphate, BaS0 4 . — In all the experiments with this 

 substance exactly one gram of it was used. The specimen 

 of salt first used was supplied by Kahlbaum. Davisson's 

 experiments have shown that the positive ions emitted by 



Fisr. 4. 



10 15 



PftfSSURE fa *>s.) 



this substance are atoms of barium with a single electronic 

 charge. The current-E.M.F. curves were similar to what 

 would be obtained by taking the average of these given by 

 the salts already examined. 



The relation between positive emission and air-pressure 

 below 25 mm. is shown by the dotted curve in fig. 4. The 

 unit of current is 2'5 x 10 ~ 9 amp. and the temperature during 

 this experiment varied only between 760° 0. and 762° C. 

 The mean curve is similar to that given by beryllium sulphate 

 (or oxide) except for the absence of the minimum at about 

 1 mm. It will be observed that the ascending and descending- 

 branches cut one another. This was frequently observed 

 with barium sulphate, but not with any other of the salts 

 tried. This salt seemed to evolve less gas than most of those 

 previously examined as it was possible to pump down to 

 pressures as low as *001 mm. 



Barite, BaS0 4 . — It seemed desirable to test a sample of 

 barium sulphate from some other source ; so a specimen of 

 the mineral barite from Hartz, Prussia, was examined. One 

 oram of: it, finely powdered, was placed in the bottom of the 

 platinum tube. The mineral has not yet been analysed. 

 It looked pure, but was not transparent. 



