238 \Vettiii(j Poirtr and Its Detcnin'mifion 



accurate valuos. The intcrfacial tension existing between an oil and 

 an aqueous liquid is itself a somewliat indefinite quantity, and varies 

 with the age of the oil globule. At the same time, we feel that the 

 method will furnish a means, sufficiently accurate for the purpose, of 

 determining and expressing in numerical values the wetting power of 

 a solution, and therefore will supply a long felt need. The apparatus 

 required for the test is simple, and at the same time there is no call for 

 great manipulative skill in carrying out the determination. If attention 

 is given to the few simple precautions mentioned, comparable and 

 reliable results are easily obtained. It may be desirable to mention 

 once again that the method is only applicable to preparations con- 

 taining soap as a basis; with other bases, e.g. gelatine, saponin, etc., 

 it entirely fails. With these substances, surface concentration at the 

 interface of the oil and aqueous solution becomes so marked that 

 quasi-solid surfaces are produced, which completely vitiate the drop 

 number and therefore the value for the interfacial tension. We are at 

 present endeavouring to find a simple method by which the wetting 

 power of solutions of such substances may be compared, but up to 

 the present have been unable to evolve anything satisfactory. 



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