On Ionization by Cumulative Action. 531 



matter is not in question, light simultaneities coincide with 

 these absolute simultaneities. 



The doctrine of relativity, as such, therefore breaks down, 

 and the mathematical transformations it introduces must, 

 it' they are valid, be susceptible of an interpretation in terms 

 of these absolute data ; while with regard to hyperbolic 

 Space-Time, in which the instantaneous co-directionality of 

 two moving lines is not an absolute property, but dependent 

 on particular frames of reference, it appears that intrinsic 

 contradictions are involved. 



JTote. — In the above discussion it might have conformed 

 more strictly to the criteria of relativity to have stated that 

 the condition satisfied by F' is that, in a finite region 

 and time considered, each point of the surface traced out 

 by L in F' is occupied by one point only of L and for only 

 one instant. 



The remark should perhaps also be made that the argu- 

 ment for the indifference of the coordinate frames can be 

 made definite by the introduction of the apparatus described 

 above, in which case we assume, as not open to question, 

 that for great velocities of R the same sense of motion of R 

 along L would be indicated whether 1? 2 were fixed in 

 F' or in F. 



LXI. Remarks on Ionization by Cumulative Action. By 

 K. T. Comptox, Professor of Physics, Princeton University*. 



A RECENT, very interesting paper by Professor F. 

 Horton and Miss A. C. Daviesf brings convincino- 

 proof of the importance of radiation in the production of 

 ionization in certain cases in which ionization by direct 

 impact is impossible or improbable. These cases include 

 the formation of arcs in metallic vapours and in helium at 

 voltages less than their minimum ionizing potentials. Ex- 

 periments now being conducted in this Laboratory by 

 Mr. Duffendack, to test the possibility of the production of 

 similar low volt;ige arcs in multiatomic gases, have indicated 

 that it is impossible to cause arcs to strike in such gases at 

 any voltages less than their minimum ionizing potentials, 

 even when stimulated by the most intense thermionic 

 currents which can be produced between heavy incandescent 

 tungsten strip electrodes heated by currents through water- 

 cooled leads. Experiments on low voltage arcs in atomic 



* Communicated by the Author, 

 t Phil. Mag-, xli. p. 7i6 (1921). 

 2 M2 



