Tension of Soap Solutions for Different Concentrations. 665 



measure the surface tension by blowing bubbles, and finding 

 the excess of pressure within the spherical bubbles so formed. 

 This method, besides being more simple and elegant in itself, 

 is free from all assumptions of an uncertain nature, and is 

 one which can verj conveniently be applied for an accurate 

 determination of surface tension of soap solutions at different 

 concentrations. The experiment therefore essentially con- 

 sists in finding the excess of pressure within a spherical 

 bubble, and as this is found to be very small, extreme care 

 is taken to measure it as accurately as possible. This has 

 been effected by a differential micro-manometer, the essential 

 part of which — the bed or the inclined plane, the inclination 

 of which with the horizontal can be varied by an accurately 

 cut micrometer screw — was made by C.S.I. Co., England, 

 and designed to measure angles correct to a micro-radian. 

 The manometer is an almost straight tube of 4 mm. diameter, 

 Avith its arms making an angle of nearly 180 degrees and 

 having a suitable length of xylene (density O850 grm. per 

 c.c. at 28° centigrade) for the manometric liquid, which has 

 been chosen for its extreme mobility and relative involatility. 

 The tube is mounted on a triangular bed of steel resting on 

 a hole-groove-and-point support and hinged about the first 

 two. The platform can be tilted by working a screw having 

 four threads to a mm. and its drum-head divided into 240 

 equal divisions. It is found by the authors that the whole 

 arrangement far surpasses all other methods of measuring- 

 pressure differences both in delicacy and quickness of action. 

 The manometer is connected to one end of a T-piece, the 

 other ends of which are joined to a compressed air-chamber 

 and a glass tube of 5 mm. diameter respectively. The tube 

 is enclosed in a glass case with sliding doors, and is fed with 

 the minimum quantity of the soap solution. By setting up 

 communication with the compressed air-chamber, a bubble 

 of suitable size is blown. On account of the difference in 

 pressure the manometric column is displaced laterally and 

 the position of the meniscus of the inner end is marked by 

 focussing a comparator, arranged in front of the manometer. 

 When the bubble is off the manometric column resumes its 

 original position and the screw-head is now turned until, as 

 judged by the comparator, the meniscus takes up the same 

 position as when the bubble is on. The readings of the 

 drum before and after the adjustment give the angle of 

 tilt, I, of the foot-board. If I is the length and d the density 

 of the manometric liquid, it is evident that the difference in 

 pressure is d.g.l&m I. 



