538 Mr. E. Cunningham on the 



high pressure show a departure from this rule, possibly o wing- 

 to unavoidable impurities. 



In the case of 00 2 it will be noticed that the effect obtained 

 is much more marked when some of the gas is actually 

 solidified during the cooling process. It might therefore be 

 urged that with C0 2 the nuclei are produced not while the gas 

 is approaching the solid state, but while changing from the 

 solid back again into the gaseous condition. This view, how- 

 ever, is improbable on the analogy of the results obtained 

 with the other gases. It is further disqualified by a fact 

 which we have not yet mentioned, namely, that the air derived 

 from boiling liquid air is perfectly nuclei-free, so that (as may 

 be seen from the figure) no glass or cotton-wool plug had to 

 be inserted between the reservoir containing the liquid air 

 and the experimental tube. This fact indeed confirms the 

 ordinary view of evaporation, that it is due to the escape of 

 separate molecules from the free surface of the liquid. 

 Evidently then in the case of air the formation of these nuclei 

 is not a " reversible " process. This point was not tested in 

 the case of liquid oxygen as there happened to be a glass- 

 wool plug already in position, but there is no reason for 

 supposing that oxygen would behave differently in this respect 

 from liquid air. The point raised in connexion with the 

 effects in C0 2 is at present left open ; we hope to investigate 

 it later on among other aspects of the problem. 



In conclusion we have pleasure in acknowledging our 

 indebtedness to Prof. L. R. Wilberforce for numerous 

 suggestions and for the facilities he so readily placed at our 

 disposal. Our best thanks are also tendered to Professor 

 F. G. Donnan for very helpful advice in regard to the 

 preparation of some of the gases. 



George Holt Physics Laboratory, 



The University of Liverpool, 



August 1, 1907. 



LI. On the Electromagnetic Mass of a Moving Electron. 

 By E. Cunningham, St. Johns College, Cambridge *. 



IN his discussion of the electromagnetic mass of a moving- 

 electron (Theorie der Elektrizitat, ii. p. 205), Abraham 

 raises an objection to the Lorentz conception of an electron 

 as having, at rest, a spherical shape, but in motion the shape 

 of an oblate spheroid the ratio of whose axes is ^/l — v 2 /c 2 , 

 v being the velocity relative to the sether, and c the velocity 



* Communicated by the Author. 



