of Gibhss Tlieory of Surface- Concentration. 509 
a is given such a value as to make the calculated and ob- 
served values o£ g identical. 
Now a determination by Pockels * gives the value 48*3 
dynes as the tension between water il and a petroleum oil." 
Assume in the present case 
a = 48 dynes per cm. 
= 4*89 milligrams per mm. 
Then a 2 = 97'8; 
.-. a=9'8, 
r = l*5. 
Hence 
©=»■ 
152. 
Unfortunately this value of I — J is a little beyond the lower 
limit of Kohlrausclr's table, viz. : — 
r 
a 
*. 
; 300 
0-288 
0-200 
4-45 
4-48 
4-66 
For ( - J = *152, the value of <J> by inspection of these 
numbers (assuming the values do not change in direction 
down to (-) = '152) would be 
3> = 472; 
hence g = 34*62 milligrms. 
Now g found experimentally = 35'00 milligrms. 
The value for the interfacial tension was therefore taken 
to be 48 dynes per cm. To distinguish this oil from another 
hydrocarbon oil which was found to have a tension of 
33*6 dynes,, they will be throughout designated oils " A " 
and " B," viz. :— 
Interfacial f Water-oil A 33' 6 dynes per cm. 
tension \ Water-oil B 48 dynes per cm. f 
* A. Pockels, Wied. Ann. lxvii. p. 668 (1899). 
t Note. — It may he as well to state here that although oil A was as 
carefully tested as oil B, the writer would lay greater stress on the 
accuracy of the determinations made with oil B. 
Phil Mag. S. 6. Vol. 15. No. 88. April 1908. 2 M 
