chemical " changes. Ionization by electron impact was the reaction first considered from this point of view (Townsend, 1901 ; Kirkby, 1902; Townsend, 1903, 15); it n-as then recognized that the experimental data, which relate to * Research Department, Messrs. Callender's Cable and Construction Co., Ltd.f Electrical Labora'tor).. Oxford. 3 Research Department, Me-ssrs. Metropolitan Vicktrs Electrical Co., Ltd. * For a case treated in detail. rze Lunt, Pearson and Topley, 1940. * The definite integral results from using the linear field X=Xc(l -z/d) and substituting, in V,./de (Fr,1)1'3 when d is eliminated (2), 3 =(Xc/Bp)(l -ai). Theupper limit becomes x=X,/Bp by means of (1). The value of the integral may be obtained from tables.