VI PREFACE. 



method is, therefore, repeated in Chapter V with greater detail. Con- 

 denser and fog chamber are now identical, and the data obtained are of 

 reasonable value. 



In Chapter VI, finally, the effect of incidental voltaic contacts occur- 

 ring in Chapter V is critically studied, with a view to using a condenser 

 whose parts are of different metals separated by an ionized medium for 

 the measurement of voltaic potentials. 



My thanks are due to Miss Laura C. Brant for efficient assistance 

 throughout the course of this work. 



CARL BARUS. 

 BROWN UNIVERSITY, 



Providence, R. I., August i, 1909. 



