PBEFACE vii 



have been fairly well established by experimental methods 

 have hitherto only appeared in different original papers, 

 and those who are interested in the subject have found 

 it difficult to get a general outline of the principles which 

 are involved. The text-books that have been written on 

 the conductivity of gases do not contain a good description 

 of the methods by which the fundamental principles are 

 established, and the attempts that have been made to 

 explain well-known phenomena by the aid of the theory 

 of collisions involve so many arbitrary assumptions, and 

 are so inaccurate, that the results obtained are in many 

 cases of no value. 



It is in the hope of removing some of these difficulties 

 that I have undertaken to write this account of the theory 

 of ionization by collision and to discuss some of the 

 phenomena in which the effects produced by collisions 

 appear to play a predominant part. 



In arranging this work for publication I have derived 

 great advantage from suggestions that have been made 

 to me by Mr. C. E. Haselfoot ; my best thanks are due 

 to him for the assistance he has thus rendered, and also 

 for having undertaken to correct the proofs. 



