TO THEIR STATE OF IONIZATION—MACGREGOR. 241 
differences between the values of (P,— P,) /n will be somewhat 
greater. As the (J —k)’s in this case, however, are more than 
twice as great as in the case of thermal expansion, the differ- 
ences in the values of (P,—P,)/n, if expressed as percentages 
of one of them, will be smaller than in the case of thermal 
expansion. Accordingly we find from Briickner’s observa- 
tions, 
Io ony — 0°5 1:0 15 2:0 25 
(P, =P.) /m = 0,116 0,122 0,126 = 0,128 °0,135 
For surface-tension 1 —k for NaCl is —0:096 and for KCl 
—0116. The approximation to constancy (judged by the per- 
centage criterion) will thus not be so close as in the last case. 
Rother’s observations give, by graphical interpolation, 
‘Hor sa = 10 15 2°0 
(e-pie = 016 0118 0105 
For refractive index 1 —k for NaCl is +0:0054 and for 
KCl +0:0091. Thus the values of (J. —hk)aa/An for the two 
salts are much more nearly equal than in the case of the other 
properties and consequently the differences in the values of 
(P,— P,) /n will be smaller than in the case of the other 
properties. Bender’s observations give for the D line, 
Ror ay — 1:0 20 3°0 
(P,-P,)/n 0,29 0,17 0,24 
If the value for n=2 be omitted from consideration, as being 
probably in error, (P,— P,)/n is seen to be more nearly con- 
stant so far as absolute magnitude is concerned than in the other 
cases considered. As the values of (P,— P,)/n however, are 
small, their differences when expressed as percentages are com- 
paratively large, and the approximation to constancy, viewed in 
this way is less than, e. g., in the case of density. 
The above account of this phenomenon may be further 
tested by the aid of Kohlrausch’s observations of electrical 
conductivity ; for in this case 1—k is the molecular con- 
ductivity at infinite dilution (usually written w.). The 
