586 
CHEMISTR Y: HARKINS AND HUMPHER Y 
in 1906, Lohnstein^ applied the general theory to the special case of the 
hanging drop just before its fall and the residue left after its fall, and 
from the difference he obtained the magnitude of the falling drop itself. 
The equation which he used for the relation between the weight (W) 
of the falling drop and the surface-tension a is 
where r is the radius of the tip and a is the square root of the capillary- 
constant, and i{r/a) is a function of r/a. Since i{r/a) varies from 1.0 
to 0.6, it is evident that neglect of this correction may cause errors as 
great as 40% in the surface-tension. It seems somewhat remark- 
able that in work with this method the correction has been neglected 
almost as often as it has been applied. Lohnstein determined the values 
of the function of r/a for different values of r/a, and his calculation of 
these corrections from a theoretical standpoint would seem to make it 
possible to use the drop-weight method as an independent method 
for the determination of surface-tension. 
Unfortunately, however, when it becomes desirable to determine 
surface-tension, it is found that Lohnstein did not carry his calculations 
to a sufficient degree of accuracy to make this method available for 
determinations where an accuracy greater than 4% is desired. It 
is therefore important that this correction, which is a very funda- 
mental one in work on capillarity, should be determined with a greater 
degree of exactness. Forms of apparatus devised by the writers seemed 
to make it possible to determine this correction experimentally under 
conditions which are more ideal than is possible than when usual methods 
are used. Thus the substitution of experiments on the liquid-liquid 
interface for the ordinary method in which a liquid-air interface is used 
makes it possible to compare the drop-weight results with those obtained 
in a capillary tube of large bore (1.5 mm.) when the capillary rise is 
great enough (150 mm.) to give accurate measurements. Not only can 
the diameter of the large capillary be determined more accurately than 
that of the smaller tube used for measurements on a single liquid, but 
in addition it is much easier in the large tube to keep the surface of the 
meniscus in a pure condition. Then in the drop- weight determination 
itself the drop falls very much more slowly when it falls into a liquid 
than when it falls into a gas, so that there is less disturbance in the drop 
at the time of fall than when it breaks away at a relatively high speed. 
The corrections determined in this way are given in Table 1, and are 
shown in the form of a curve in figure 1. In the figure the ordinates 
(1) 
