MOLECULAR WEIGHT AND SURFACE TENSION 73 



empirical modification, and consequently offer no certain 

 basis for molecular weight determinations. 



The phenomena, even at the critical point, are uncertain, 

 since they require the empirical rule of Young, given on 

 p. 23. Below the critical temperature the law of correspon- 

 dence gives a point of departure, since formation of double 

 molecules in the liquid, as explained on p. 26, causes a fall 

 in the concentration of simple molecules in the vapour ; 

 this may be concluded on kinetic grounds of a trustworthy 

 nature, so that if the vapour, as evidenced by its density, 

 consists only of simple molecules, a fall in its concentration 

 and deviation from the law of the rectilinear diameter must 

 occur in corresponding measure. Hence it is concluded that 

 molecular association in liquids is indicated by abnormally 

 large volume of the saturated vapour (p. 26), abnormally 

 small vapour pressures (p. 47), together with deviation from 

 the formula given on p. 47, abnormally high boiling point 

 of the liquid (p. 53), and abnormally high heat of evapora- 

 tion (p. 58) ; these four indications are necessarily asso- 

 ciated together, and must be distinguishable in the specific 

 heat as well. 



The methods for determining molecular weight which 

 do not belong to this group, such as that of Guye by the 

 critical coefficient, and that of Traube from the density, 

 are still less reliable a priori, although it must not be con- 

 cluded that they lead to false results. Granting this, we 

 may next discuss how far the conclusions of Eotvos, Ramsay, 

 and Shields on molecular weight of liquids from capillary 

 phenomena are justified. 



Surface tension. Starting from the capillary rise, it must 

 first be noticed that this depends on the bore of the tube, 

 so that the latter factor must be eliminated. The height 

 is independent of the nature of the capillary wall, whether 

 glass or not, if only the liquid wets it, so that the capillary 

 attraction proceeds from the film of liquid adhering. The 

 capillary being previously moistened, we have practically 

 a liquid skin, and when the meniscus rises h cm. the area 



