SURFACE TENSION AND TEMPERATURE 75 



The constant change in the capillary rise due to tempera- 

 ture, is, of course, different for a different radius of the 

 capillary tube, but being, according to p. 74, inversely pro- 

 portional to the latter, the change 



Ahr 

 At 



is independent of the tube and characteristic for each 

 liquid. 



Relation to molecular weight. A striking relation between 

 surface energy and molecular weight was discovered by 

 Eotvos, Ramsay, and Shields, and shows itself in the 

 approximate constancy of the change in the molecular 

 surface energy with temperature. The molecular surface 

 energy is here defined as the work necessary to enlarge the 

 surface by an amount on which a mass proportional to 

 the molecular weight lies, the molecules being treated as 

 spheres. The quantity in question is therefore 



where M v is the molecular volume, i.e. the molecular weight 

 in grams divided by the density. 



We will first trace the constancy of the change in this 

 function with temperature, in the case of a definite sub- 

 stance, ether, the surface energy being in ergs instead of 

 gram-centimetres, by multiplication by 981. 



