43-S SCIENCE PROGRESS 



prevents its wider use seems to be the difficulty of measuring 

 exactly and varying arbitrarily the temperature of the drop. 



In the well-known method associated with the name of Jaeger, 

 T is determined from observations of the maximum pressure 

 required to release a bubble of air from the end of a capillary 

 plunged vertically into the liquid under examination. Neglect- 

 ing minor corrections, and supposing that the end of the capillary 

 tube (of radius r) is just touching the surface of the liquid, we 



have simply 



2T = rhpg .... (iii), 



where p is the density, and h the difference of level of the 

 surfaces of the liquid in the pressure-gauge. Taking into 

 account general convenience, rapidity, accuracy, and ease of 

 temperature-control, this appears to be one of the best of the 

 methods passed under review. It will be seen from the above 

 equation that if the liquid in the manometer be the same as that 

 examined, the difference of level observed in the manometer is 

 equal to the height to which the liquid would rise in a capillary 

 of radius r; and therefore, apart from the fact that it is indepen- 

 dent of the contact-angle, the Jaeger method possesses another 

 advantage over the capillary-rise method in that, by using a very 

 light liquid in the manometer, this height may be correspond- 

 ingly magnified, and therefore may be read off with a percentage 

 accuracy considerably higher than that of the measurement of 

 the corresponding height in the capillary-rise experiment. 



The only serious criticism that can be brought against the 

 method is that in the formation of equation (iii) statical prin- 

 ciples are brought to bear on what is really a dynamical 

 problem. But experience shows that the maximum pressure 

 observed is only a function of the rate of release of the bubbles 

 for comparatively high speeds. If the rate of liberation be slow 

 enough— one bubble every two or three seconds, or slower — 

 the maximum pressure observed is quite independent of the 

 rate of extrusion of the bubble, and may therefore safely be 

 taken to represent the true maximum. 



It will doubtless have been noticed that one method which 

 has lately come into prominence has not been included in the 

 table given — the so-called " drop-weight " method. If a drop of 

 liquid be allowed to form on and to fall slowly from a properly 

 constructed tip, it is known that, cceteris paribus, the weight of 

 the drop is proportional to the surface-tension of the liquid, and 



