Method for determining Surface- Tensions. 633 



true " standard " value. Thus any hypotheses concerning 

 molecular complexity which depend upon a knowledge or! 

 the absolute value and temperature variation of k, are seriously 

 vitiated by the presence of an unknown and variable quantity 

 cos 6. Indeed, it is quite possible that part at least of the 

 variation in the "constant" value of k for unassociated 

 liquids may thus be accounted for. Further, il is quite 

 possible that variations in k for any liquid with change of 

 temperature which have been interpreted as showing a 

 variation of molecular complexity may be due, wholly or 

 partially, to temperature variations in the contact-angle. 

 For these and other reasons it is therefore advisable to 

 -employ only those methods for surface-tension measurements 

 which are quite independent of any assumption as to the 

 existence and magnitude of a contact-angle. 



During recent years the " drop-weight " method has been 

 seriously studied and brought to a very high state of accuracy 

 xis regards its practical details by Mr. J. L. R. Morgan*, who 

 has determined by its aid the surface-tensions of more than 

 one hundred pure organic liquids. Since the method, as 

 developed by him, is both rapid and very self-consistent in 

 practice, and is free from many errors inherent in the 

 capillary-rise method, it is of very great interest to know 

 whether the method gives results which are independent of 

 the contact-angle. A priori, there does not seem to be any 

 reason why the contact-angle should enter into the result; 

 but Mr. Morgan lays distinct emphasis on the fact that 

 surface-tensions as found by this method are in agreement 

 with those found by the capillary-rise method f. Further, 

 some writers of physico-chemical text-books in discussing the 

 theory and practice of the method are wont, to bespatter 

 their formulae with an assortment of terms in cos 6 without 

 giving any apparent theoretical or practical justification for 

 so doing. It is true that, by appealing to the " wetting of 

 glass," these inconvenient terms are usually, at the close 

 of the argument, dismissed to the limbo whence they came; 

 but as long as such a way of looking at the matter is common, 

 it would be well if the question of the dependence of the 

 method on the existence and magnitude of the contact-angle 

 could be finally settled; and, as Mr. Morgan emphasizes, 



* See various papers by Morgan and colleagues in the ' Journal of the 

 American Chemical Society ' for 1909, and later years. 



t " The weight of a falling; drop of liquid is thus found to be strictly 

 proportional to its surface-tension, determined by capillary rise or by any 

 other accurate method." Journ. Amer. Chem. Soc. xxxii. p. 349 (1911). 



