744 Mr. A. E. Garrett on Electrical 



The emission of the negative ions from the salts of calcium 

 and barium is interesting, since this has been made a special 

 study in the case o£ calcium oxide by Wehnelt * and also by 

 Dr. Horton f. The latter finds that the decreasing resistance 

 of calcium oxide and barium oxide to the passage of elec- 

 tricity with rising temperatures is partly due to the increasing 

 number of free corpuscles contained by those oxides. Under 

 the conditions of experiment, no negative ions were detected 

 in the cases of cadmium, bismuth, and lead iodides. 



In the cases of the iodides of calcium and barium, no 

 positive ions could be detected while the salts were dry ; but 

 if the salts were left in the apparatus and then tested, it was 

 found that the velocity of the negative ions had been reduced 

 to about one-third of their original value, and the presence 

 of positive ions was detected in the case of calcium iodide. 



It will be seen that the velocity of the negative ions is 

 many times greater than that of the positive. This being 

 the case, the nature of the ions must be different, since the 

 conditions under which their velocities were obtained were 

 similar. It was noticed during the experiments that in every 

 case in which the positive ions were detected a deposit 

 (apparently due to sublimation) could be seen on the upper 

 electrode B, and the relatively slow movements of the positive 

 ions made it probable that the ions in this case might be 

 charged centres surrounded by particles of the salt. Bloch { 

 came to the conclusion that dust particles were intimately 

 connected with the conductivity of recently-prepared gases, 

 and the conductivity produced by phosphorus. 



In order to ascertain whether the conductivity could be 

 due to such a cause, I heated some of the salts in a large 

 tube which was fitted with two corks, through which passed 

 two equal-sized, insulated, thick copper wires, which were 

 connected to the opposite poles of a battery of 200 storage- 

 cells. 



The material was heated slowly up to about 250° C. by 

 means of a Bunsen flame, which was kept moving so as to 

 ensure more regular heating. The electrodes were first care- 

 fully balanced on a pair of scales. 



With iodides of bismuth and cadmium it was found that, 

 after heating up, the negative electrode weighed one or two 

 milligrams more than the positive each time the experiment 

 was tried. 



With lead iodide the negative electrode was always heavier 



* Annalen der P/iysik, Band si v. (1904). 



t Phil. Mag. April 1906. 



X Ann. de Chemie et de Physique, Tome iv. 1905, pp. 127 et seq. 



