508 Mr. W. C, M. Lewis : Experimental Examination 
Similarly for another liquid 
o-i = K r -. 
Hence 
°L = r h£. 
As a result of the exhaustive work of Guye and Perrot * y 
Kohirausch f, and Lohnstein J, a determination of surface- 
tension in absolute measure by means of the drop-pipette 
can be obtained as follows : — 
The expression giving the weight of a drop in terms of 
the tension is 
g = ra<&, 
where a = tension in millgrms./mm. 
g — weight of a drop in milligrams. 
r= radius of the orifice in mms. 
<3> is a function of / - |, where 
a 2 is given by - -, 
p being the density of the liquid in the pipette. 
This applies to cases of liquid-gas tension, but it may 
perhaps be extended to the present case by taking into con- 
sideration the density of the medium in which the drop of 
oil is formed, i. e. the water. 
Calling the density of the water p m we have 
<7 — 
Rv — Pi) 
We have now to evaluate <I>. The ordinary method of 
using the Lohnstein-Kohlrausch formula is to take some 
approximate value for a ; from this calculate a, and hence 
(— V Kohirausch § has given a table showing the values 
of 3> for different values of ( - ) and by inter- or extra- 
polation <E> can be evaluated. We then compare the value 
of g given by the expression ra4> with g actually obtained 
by experiment ; and finally, by a series of approximations, 
* Guye & Perrot, Archives d. Sc. Phys. 4th ser. vol. xi. p. 225 (1901) ; 
vol. xv. p. 132 (1903j. 
t Kohirausch, Ann. d.Physik, vol. xx. p. 798 ; vol. xxii. p. 191 (1906). 
X Lohnstein. Ann. d. Physik, vol. xx. pp. 237, 606; vol. xxi. p. 1030 
(190G). 
§ Kohirausch, loc. cit. p. 805. 
