of Gibbs's Theory of Surface-Concentration. 483 



The following table (Table V.) contains the determinations 

 of the interfacial tension for caffeine solutions of different 

 concentrations, the tension being measured by the drop- 

 pipette. It will be noted that this oil is not quite identical 

 with that previously employed. 



Table V. 



Per cent. 

 Concentration 

 of Anhydrous 



Cafi'eine. 



Drop- 

 number. 



Kelative 

 Tension. 



Tension in 

 dynes/cm. 







146 



1C00 



48 



0-046 



153 



954 



45-8 



0092 



158 



0-924 



444 



0-114 



161 



0907 



43-5 



0229 



171 



0-853 



40-9 



0-457 



184 



0793 



381 



0915 



201 



0726 



34-8 



The interfacial tension is plotted against concentration, 



Eur. 6. 



48 



3 40 



80 



| P^gL I 



. 1 ' -i 5=te 



0-1 0-2 0-3 0-4 0-5 0-6 0*7 0*8 0*9 TO 



Grams of anhydrous Caffeine in 100 c.c. of solution. 



giving the curve on fig. 6. The tangent to the curve gives 

 the value of ^— at the desired concentration. 



dc 



2 L2 



