Ionization Potential of Mercury Vapour. 753 



Owing to the harmonic motion of the piston its velocity near 

 the end of the stroke is low, and consequently towards the 

 end of the compression stroke the actual compression line 

 will approach the ideal ; we may assume that the two coin- 

 cide at b'. During the early part of the expansion stroke, 

 owing to the low velocity of the piston, the actual curve will 

 tend to coincide with the ideal, falling below it as the piston 

 velocity increases. The actual curve would then take the 

 form a'mb'nc 1 ' . 



Clerk's method of finding the specific heat should give 

 fairly accurate results, provided ac (fig. 4a) is taken suf- 

 ficiently far from the end of the stroke, since then the effects 

 on compression and expansion will largely cancel each 

 other. 



Professor Hopkinson*, in a report of some work of his on 

 this subject, has stated that as the end of the compression 

 stroke was approached the heat loss increased rapidly, and 

 that in the early part of the expansion the curve practically 

 followed the adiabatic. The explanation of his observation 

 is not that the heat loss behaved in this extraordinary way, 

 but that the lag in the gas behind its equilibrium state, 

 combined with the variable speed of the piston, gives the 

 curve the form nib' n (fig. 46), which has all the appearances 

 of heat loss observed by Hopkinson. 



It will be seen that many otherwise complex phenomena 

 become simple when the idea of lag in the gas behind its 

 equilibrium state is accepted, and this may betaken as strong- 

 evidence of the presence of such a lag. 



LXXIX. Note on the Ionization Potential of Mercury Vapour. 

 By F. H. Newman, B.Sc, A.P.C.S.', Imperial College, 

 S. Kensington-^. 



§ 1. Introduction. 



THE following experiment was made with the object of 

 finding the smallest potential difference through which 

 an electron must fall before it is able to ionize a molecule of 

 mercury by collision, While the work was in progress and 

 before the results could be published, Franck and Hertz % 

 published their results obtained in a similar experiment. 



* B. A. Report of Gaseous Explosions Committee, 1908. 

 t Communicated by the Hon. K. J. Strutt, F.R.S. 

 X Verh. d. B. Phys. Ges. xvi. p. 457 (1914). 



