84 
Transactions of the Royal Society of South A frica. 
experiments with ordinary light are concerned ; but for X rays Richardson * 
says: " So far it has only been demonstrated as a limiting condition — that is 
to say, when matter is subjected to the action of radiation of frequency v, 
no electron ever acquires a total energy, kinetic (\mu~) and potential (w), in 
excess of hv.'' Very little is known about (w), which, according to Bohr, is 
the energy radiated away from the atom during the previous binding of the 
electron into it. 
In the experiments hitherto made on the maximum speed of ejection 
of electrons by X rays, it follows that they must have come from the 
periphery of the atom. The important conclusion drawn, that v depended 
only upon v and in no way upon the nature of the substance emitting the 
electrons, does not satisfy the complete photo-electric equation. 
Whatever be the source of the ejected electrons in the case of X rays, it 
is indisputable from the experimental evidence at hand that for the highest 
speed electrons w must be very small compared with the other two terms. 
The very small variation in Beatty's absorption coefficients for the particles 
in air with variation of the nature of the metal of the screen from which 
they emerge, if the incident wave-length is constant, indicates the approxi- 
mate truth of the last statement. 
If the equation Imv- — hv (where w is neglected) apparently holds good 
for all screens, it should be reversible. Identifying the kinetic energy of a 
particle with the product of its charge and the potential through which it 
has fallen, the experiments of Rutherford, Richardson and Barnes f show 
that this abbreviated equation is by no means true, but that there is a 
limiting frequency obtainable from a given target. Likewise Duane and 
IIunt+ show that Ye = hv accurately, over a small range of wave-lengths, 
their value of h being smaller by about 2 per cent, than Millikan's accurate 
value 6' 55 x 10 "'-^^^ pointing to the possibility that a small constant negative 
term might be missing from the right-hand side of their equation. 
The potential energy term in the photo-electric equation seems to acquire 
reality in the case of the slower group of electrons described herein. Their 
velocity should be given by — 
■Imv'iovv = aJI'^k — AuC/i'^L + etc.). 
Putting m = 8-9 x IQ-sSgm., h = 6-55 x 10-27 erg. sec, a.^^k = 5-855 x lOis 
(mean frequency of a, 3, y lines after Bragg), au^^ = 2-538 x 10^^, ^u^^m = 
,,,^-^^^ = -417 X lOiB (^„.,,^= 2-748 x 10^^ and ,,v^ = 2-331 x 10^8 and ^u^^^ 
is taken as the mean of these) we obtain for the velocity of the slower 
corpuscle a value 65 x 10^ cm. /sec. If the missing terms of the equation 
were known, the velocity would be slightly less than this amount. 
Now Thomson has shown on theoretical grounds that for these particles 
* Loc. ext. 
t ' Phil. Mag.; [G] xxx, p. 352 (1915). 
X ' Phys. Eev./ [2] w\, p. 169 (1-915). 
