232 SCIENCE PROGRESS 



and TiQ'^ = 25-26, the values calculated from the formula 

 given on p. 229 being 29-20 and 25-27. Once the drop weight 

 of benzene has been obtained at a given temperature with a tip, 

 the calculation of the surface tension of a second liquid whose 

 drop weight has been determined with the same tip at the same 

 temperature is simple, e.g. with a tip 4-51 mm. in diameter, the 

 weight of a drop of benzene was found to be 0-024269 gram, 

 and that of a drop of quinoline 0-038487, both determined at 

 27-8° C. ; since the surface tension of benzene at this temperature 

 is 26-75, the surface tension of quinoline is 



0-038487 X 26-75 



■« 0> 27*8= 7 = 42-4. 



^' 0-024269 ^ ^ 



Morgan's drop weight apparatus is made by Eimer and Amend, 

 of New York. 



A method depending on the formation of bubbles of air in the 

 liquid is that described by Jager {Wein. Akad. Berichte, 100, 

 245). The pressure inside a spherical air-bubble in a liquid 

 exceeds that outside by an amount p given by the equation 

 p = 2 Tjr ; at constant temperature the product pr is constant. 

 Now, if a glass tube is placed vertically in the liquid and is 

 attached to an air vessel fitted with a manometer, on gradually 

 increasing the pressure, the air will pass down the tube and a 

 bubble will form at the end. If the pressure is then slightly 

 increased, since pr is to remain constant, the radius of curvature 

 of the drop must diminish as the volume of the drop increases ; 

 the drop thus continues to grow until it becomes hemispherical. 

 The excess pressure p above that of the atmosphere shown by 

 the manometer is then equal to the hydrostatic pressure plus 

 the pressure due to the curvature of the drop : 



i.e., p = dgh + ^Tjr 



where d is the density of the liquid. 



h is the depth of the end of the tube below the surface. 

 T is the surface tension. 

 and r is the radius of the tube. 



If the pressure is again slightly raised, since the radius 

 of curvature of the drop cannot be less than the radius of the 

 tube, the radius of the drop must increase as the volume in- 

 creases and the capillary counterpressure is reduced {pr being 

 fixed) ; thus a very slight increase in the pressure above the 

 value given in the equation causes the drop to continue to 

 increase in size until it detaches itself. The experiment there- 

 fore consists in finding the minimum pressure at which bubbles 

 will continue to form and detach themselves from the tube. 



