670 Messrs. Narayan and Subrahinanyam on Surface 



In order to examine how the surface tension o£ soap 

 solutions varies with dilution at great dilutions, we have 

 thought it desirable to institute another series of experiments 

 with soap solutions. As no bubbles could be blown at these 

 great dilutions, i. e, below 0*22 gm. per 100 c.c. of solution, 

 in all these experiments measurements of surface tension 

 are made by the capillary tube method with the tubes used 

 in the former experiments. 



Table VI. 



Castyl Soap Solution : Surface Tension for different 

 concentrations (at great dilations). 



Concentration — Grins, 

 per 100 c.c. of solution. 



Height (corrected). 



! 



Surface Tension. 

 (Dynes per cm.) 



0-20 



5 - 44 cm. 



27-10 



0-175 



5-557 „ 



27-70 



0-150 



5-768 „ 



28-70 



0-125 



6-015 „ 



29-90 



0100 



6-653 „ 



33-10 



0-050 



8-025 „ 



40-09 



0-025 



9-212 „ 



4591 



0-0125 



10-525 „ 



52-36 



0-0062 



11-47 „ 



57-03 



0031 



12-475 „ 



61940 



0-00155 



1302 ., 



64 71 



It will be seen from comparison of the observations 

 recorded here with those already recorded, that the surface 

 tension of soap solution begins to increase w y ith dilution only 

 at great dilutions, up to 0'23 grm. per 100 c.c. of solution, 

 the surface tension being practically constant, and then 

 rapidly rising with dilution. 



The results of the experiments are graphically represented 

 on the diagram, where concentrations in grams per 100 c.c. of 

 solution are plotted along the X-axis, and the surface tension 

 values along the Y-axis. The curve, which may well be 

 styled the isothermal of Castyl soap solution at 28° C, is an 

 hyperbola. Further experiments are in progress — 



(i.) to construct the isothermals of different soap solutions ; 

 (ii.) to construct the isothermals of one and the same soap 

 (Castyl soap) for different temperatures from 30° to 

 100° C. 



