72 PHYSICAL PROPERTIES AND COMPOSITION 



tends to increase the area of the liquid surface meets a resis- 

 tance, and the shape of the liquid changes, with increase in 

 surface, until equilibrium is reached. 



In the numerous methods for measuring the surface 

 tension the force employed to increase the surface is mostly 

 the weight, and most simply by the use of a capillary tube. 

 We shall restrict ourselves to this method because, in the 

 form worked out by Ramsay and Shields, it is so far the 

 most important for the relations between surface tension 

 and other properties. 



We may next roughly describe the simpler apparatus 

 used for the purpose. 



It consisted l of a capillary tube which could be raised 

 or lowered at will, so that the meniscus of liquid sucked 

 up into it, when observed, was always at the same level, 

 thus making the results more comparable. The capillary 

 and the liquid were contained in a tube that could be 

 evacuated, and sealed up, and surrounded by a vapour jacket, 

 so as to maintain a suitable constant temperature. The 

 arrangement for raising and lowering was an electromagnet 

 outside the whole apparatus, acting on a piece of iron 

 attached to the capillary. 



B. Molecular Weight and Surface Tension. 



It is of importance to test the reliability of the assump- 

 tions on which depend the various proposals for determining 

 molecular weights of liquids. It has already been pointed 

 out that the methods discussed in this Third Part are 

 inferior to those dealt with in Part II, depending on 

 Avogadro's law, and the corresponding laws of dilute solu- 

 tions, since it is only the latter that rest on a satisfactory 

 theoretical basis. A similar research, not, however, yet 

 accomplished, lies in the characteristic equations of liquids, 

 which all need, in order to harmonize with the facts, a purely 



1 Zeitschr. f. Phys. Chem. 12. 458. 



