50 PRINCIPLES OF GENERAL PHYSIOLOGY 



capillary tube, according to whether it wets the glass or not. This change of 

 level is due to the curved shape of the meniscus or surface separating liquid 

 from air, so that the surface tension has a vertical component which pulls up 

 the liquid against gravity, or presses it down, according to whether the meniscus 

 is concave or convex. The best form of apparatus for this method is that of 

 Rontgen and Schneider (1886, p. 203), especially in the modification described 

 by Schryver (1910, p. 109). 



A means of rendering this measurement more accurate was pointed out to me by W A. 

 Osborne. Two capillaries of different but known diameters are taken, and the difference of 

 heights to which the two liquids to be compared rise in the two capillaries is measured. By 

 this device the measurement of the height of the meniscus from the body of the liquid is 

 unnecessary, a somewhat difficult and uncertain one. Since the total height in each case is 

 inversely proportional to the diameter of the capillary, and directly proportional to the 

 surface tension, the difference of the heights of the two liquids is also so proportional. We 

 know the diameters of the two capillaries and the surface tension of one liquid, so that it is 

 easy to calculate that of the other. This method is also recommended by Michaelis and Rona 

 (1909, p. 496). 



The formula for rise in capillary tube is : 



Height x radius of tube x density x 981 



The precise cause of the 

 existence of surface tension 

 is too complex for discussion 

 here. Briefly, one may say 

 that it is due to the forces 

 of attraction between the 

 molecules of a liquid, pro- 

 ducing what is known as the 

 " internal pressure " of Lap- 

 lace (1845, iv. p. 389). This 

 pressure can be calculated, 

 and amounts to several 

 thousand atmospheres (Stefan, 

 Wied. Ann., 29, p. 055). The 

 molecules in the body of the 

 liquid are exposed to these 

 forces equally on all sides. 

 Those at the surface are ex- 

 posed to unbalanced forces 

 tending to draw them in (see 

 Fig. 32). The result of this 

 is that the surface of a liquid 

 is always the least possible, or, in other words, is pulling itself together. One may 

 see the necessity of a minimum surface also from the point of view of energetics. 

 Since there are forces drawing the molecules inwards, work is required to bring them 

 to the surface, therefore the greater the surface, the greater the energy contained 

 in it ; but, as we have seen, free energy always tends to a minimum. For 

 further details see Freundlich (1909, pp. 6-14). The explanation of the properties 

 of the free surface, by regarding it as the seat of tension, is due to Thomas Young 

 (1805, p. 82), who speaks of unbalanced molecular cohesive forces at the surface 

 as the cause of the tension. 



The values of the surface tension of pure liquids vary greatly. The following 

 numbers in dynes per centimetre will serve to illustrate this : 



FIG. 32. DIAGRAM TO ILLUSTRATE THE ORIGIN OF SUB- 

 FACE TENSION FROM INTERNAL PRESSURE OF LIQUIDS. 



The molecule 4 is exposed to equal attractive forces on all sides. 

 The molecule B, at the surface of the liquid, on the other hand, 

 is exposed to unbalanced forces, of which the resultant is a 

 pressure in the direction of N. Equilihrium will result when the 

 number of molecules at the surface is the least possible ; that 

 is, the surface area tends to a minimum. 



(Errera, 1907, p. 16.) 



Water - 

 Alcohol - 

 Ether - 



73 

 22 

 16 



Since the surface tension is not altered by enlarging the surface, as in 

 blowing a soap-bubble, it follows that the pressure inside a small bubble is 



