﻿Thermions from the Salts of the Alkaline Earths. 131 



Barium Phosphate. 

 This salt of barium proved more refractory than any one 

 of the others tested. The curves were never smooth and 

 were always subject to rapid time variations. The samples 

 employed were prepared from the same supply of baiium 

 chloride used in preparing the other salts, and Kahlbaum's 

 ammonium phosphate. Satisfactory curves were obtained 

 but twice, and the values of ejm. from these were quite 

 different from the values obtained from the other barium 

 salts. The data follow : — 



Barium Phosphate. 



v. 



H. 



d. 2.r. 



1 



ejm. 



( e M) Ba + + 



283 



282 

 I 



2915 

 2015 



1 

 1275 -438 



1-275 ! -413 



1 



2C8 

 238 



142 



Mean = 253 



These very anomalous values might be disconcerting were 

 it not for the fact that anomalous values of about this 

 magnitude are to be found among the results for the salts 

 of all the alkaline earths. Taken together they point to some 

 common impurity. Similar values were found by Richardson 

 in his investigation of the salts of the alkali metals, and were 

 attributed by him to the presence of potassium as an impurity. 

 Potassium salts give rise to very large thermionic currents 

 and alkali metals are among the commonest of impurities. 

 The values obtained for ejm in this case agree with that for 

 potassium, which is 249. It is assumed, therefore, until 

 some better explanation presents itself that these and similar 

 results are due to potassium or its salts. 



Another difference between these experiments and those 

 on the other barium salts is that measurable thermionic 

 currents were obtained at lower temperatures. Currents 

 were measurable at 500° or 600° C, while in all other cases 

 a temperature of more than 1000° 0. was required. This 

 emission at comparatively low temperatures was found to be 

 characteristic of observations leading to these anomalous 

 results. In some cases it has been possible by continued 

 heating to rid the salt of these impurities and to finally 

 obtain a value of ejm characteristic of the salt. In the case 

 of barium phosphate, however, the difficulty due to time 

 variations seemed worse at higher temperatures, and further 

 experiment was abandoned. 



K 2 



