718 Dr. S. A. Shorter on a 



currents and the ionization currents found to be approximately 

 inversely proportional to the rate of the air current, as they 

 ought to be. 



This point was being tested more accurately when the 

 experiments were brought to an end by the melting of the 

 platinum strip, and as it seemed unlikely that very much 

 more information would be obtained the work was dis- 

 continued. 



The chief uncertainty about the results is probably the 

 question as to the time taken in deemanation of the radium 

 him. This being so there seemed to be little object in testing 

 the activity of emanation whose average age was nominally 

 less than 10 seconds, as might have been done by modifying 

 the arrangement, since it is quite likely that several seconds 

 are occupied in setting free the emanation. 



The general result obtained is that the activity of radium 

 emanation is the same within a few seconds of its formation 

 as at some minutes later. A variation of 5 per cent, would 

 probably have been detected. Thus no variation from the 

 Exponential Law has been detected. 



In conclusion I wish to thank Dr. Joly for his kind interest 

 and valuable suggestions during the progress of this work. 



Physical Laboratory, 



Trinity College, Dublin. 



LXXXIL On a Classification of Foaming Solutions. By 

 S. A. Shorter, D.Sc, Assistant Lecturer in Physics in 

 the University of Leeds* '. 



IN order for solutions to be capable of forming durable 

 foam, the surface tension must be capable of local 

 variations. It has been recognized for a long time that this 

 power of local variation of the surface tension is due to the 

 existence of a surface pellicle differing in composition from 

 the interior portions of the liquidf. 



Investigation of the properties of the surface layers of 

 solutions has led me to the conclusion that though the above 

 general explanation is true, there are important differences in 

 the nature of the processes involved in the foaming of 

 different solutions. 



There are two distinct points to be considered with respect 

 to the phenomenon of foaming. One is the nature of the 



* Communicated by the Author. 



t See Rayleigh, Scientific Papers, vol. iii. p. 351. 



