214 
Proceedings of the Royal Society of Edinburgh. [Sess. 
of the quantities used. We shall adopt electro-magnetic units. We have, 
therefore, 
e=l-6x 10~ 20 . 
1 = 1-8 x 10~E 
m 
m = 8-9 x 10~ 28 . 
Take 
c = 3 x 10 10 = velocity of light. 
\ = 6x 10~ 5 
where A is the wave-length of incident light. 
'jTTC JjTT . 
p = — — — in our units, 
X A 
since c is taken as the unit of velocity. 
.•. p = 10 5 approximately. 
The value of in the case of hydrogen is known by electrolytic deter- 
mination to he 
9-6 x 10 3 , 
and e being identified with the charge on an electron, 
M/j = M for hydrogen = T7 x 10~ 24 . 
(These values are taken from O. W. Richardson’s Electron Theory of 
Matter, chapter i.) 
The radius of the hydrogen atom lies between '92 x 10~ 8 and 1T9 x 10 -8 . 
(Jeans* Kinetic Theory of Gases, p. 347.) 
Hence the order of magnitude of 
4 E e 
t = fr-TTpe = — f = ^2*6 . 10+ 18 j 
3 a 6 
where a, the radius of the atom, is taken as 10 ~ 8 . 
We shall make the assumption, which is in accordance with a large 
body of experimental evidence, that the mass of an atom is proportional 
to the number of electrons in it, and hence when neutral to the positive 
electrification value. 
M __n 
■' M h' 
where h — number of electrons in the hydrogen atom. 
M = - . 1 "7 . 10 -24 . 
h 
