440 Notices respecting New Books. 



Therefore 



mtc \cr 2 / 



The value — is the same for the molecule as for the 

 m/c 



cluster. For hydrogen, then, the ratio of the diameter of 



the cluster to the diameter of the molecule is given by 



"340=5-7. 



10-4 



For oxygen D = *21 and, assuming its charge is double 

 that of hydrogen, we get a theoretical value of the velocity 

 of 85 cms. per second. This corresponds to a cluster of the 

 diameter of 5'5 molecules. 



In the case of chlorine, assuming the charge on the ion is 

 the same as that on hydrogen, we get a cluster of diameter 

 2" 7 molecules. We see, therefore, that to explain the ob- 

 served results the carrier need not be greater than 5 times 

 the radius of the molecule. 



Further experiments, which are not yet completed, have 

 been made to find the velocity of the ions of a gas conducting 

 under the influence of the radiation given out by uranium 

 and its salts. It can be shown that the velocity of the ions 

 in the conducting gas is the same as when the gas is acted 

 on by Rontgen radiation, so that the carrier in the two cases 

 is identical. Further results, however, must be reserved for 

 a future paper. 



In conclusion I desire to express my thanks to Professor J. 

 J. Thomson for many valuable suggestions during the course 

 of this investigation. 



Cavendish Laboratory, 

 July 19, 1897. 



LV. Notices respecting New Books. 



The Potentiometer and its Adjuncts. By W. Clark EisnER, 



A.M.I.C.E. London, " The Electrician " Publishing Co. 

 ^PHE usefulness of the potentiometer in the measurement of 

 - 1 - electrical quantities, and more especially in connexion with 

 continuous-current systems, is now so fully recognized that a 

 treatise on the instrument and its applications is in no wise super- 

 fluous. The author of the present volume has endeavoured to 

 increase the value of such a treatise by the addition of chapters on 

 standards of electromotive force and resistance, on galvanometers, 

 and on platinum thermometers ; in doing so he has collected much 

 information which, partly on account of its detailed character, 



