1(5 
MR. W. R. BOUSFIELD : IONIC SIZE IN RELATION TO 
sizes, and are at liberty to simplify the equations by choosing a suitable unit radius. 
This we do by making C = 1 in equation (4), so that we have always 
or, at infinite dilution, 
r = l/vrj or 1 Jr — crj . 
E = I//. 
( 5 )> 
In other words, our unit radius is the radius of an imaginary monovalent water- 
coated ion which has unit mobility at infinite dilution. All ionic radii will be given 
in ionic units, as we may shortly term this unit. Moreover, for reasons which will 
more fully appear hereafter, we shall speak of the ionic radius as the “ radion.” This 
will be specially convenient, first, because the radion will be shown to be an important 
physical quantity, quite apart from our theory that it represents the radius of the 
hydrated ion; secondly, because, as was shown in the former paper, the value of the 
ionic radius which we obtain is the average value for the whole of the ions, and not 
the value for any individual ion ; and, thirdly, because the conception of the radion is 
useful, not only in reference to ions, but also to other hydrated molecules, and even 
to water itself. 
Slightly modifying the notation given in the former paper, we have 
u = 
and writing 
V = 
r. 
'A 
V 
r , 
n 
E, _ | , , 
- — I +</>!, 
n 
E 
"2 
= i +4> 
2 ) 
where (fix and c/> 2 are written as abbreviations for c/fi (A) and </), (A), so that 
— — — — 4(1 +^ 2 ) 
( 6 )> 
Ave net for the true value of a 
a = 
V 
A 1+0) 
where d> = 
b^i + 402 
A 
(7). 
In the case of KC1 it Avas sIioavu that ( h Avas of the form BA It will be 
hereafter shown that a function of the same form can be applied with a close approxi¬ 
mation to the truth to the case of NaCl. Assuming that and </)_> are of the same 
form as <fi, Ave get 
BA = Bib + B 2 T.(8), 
Avhen l\ and B 2 are the- constants for the separate ions, and we get for the radions 
the expressions 
1 ='? 1 (i + B 1 /r^ 8 ), I = 4(i + B 2 /r 23 ).(9). 
It aa ill be convenient to designate B, B l5 B 2 , &c., as the hydration numbers. 
