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272 Dr. S. A. Shorter on the Constitution 



light o£ the thermodynamical theory given above. Equation 

 (15) may be written in the form 



where A, it will be remembered, is the distance of the 

 " zero-plane" of d from that of C measured in the direction 

 of the <f>' phase. Now in the case of a system having a 

 density diagram of the kind shown in fig. 7, h is evidently 

 negative — as is also the case if only one of the liquids 

 exhibits surface compression. Hence the theory of surface 

 compression leads to the conclusion that the tension of the 

 interface separating two immiscible liquids is diminished by 

 increase of pressure. This result may, of course, be deduced 

 from quite elementary considerations. The change of pres- 

 sure produced by an extension of the interface at constant 

 volume must be such as to increase the tension. If the 

 liquids are compressed in the surface-layers this exten- 

 sion produces a diminution of pressure so that dr/dp is 

 negative. 



The effect of pressure on the interfacial tension has been 

 investigated by Lynde * in the case of a number of pairs 

 of immiscible or partially miscible liquids. In the case of 

 water and mercury it is found that the interfacial tension is 

 increased *74 per cent, by an increase of pressure of 5000 lb. 

 per sq. inch. This gives (assuming t = 370 dyne/cm.) for 

 dr/dp the value 8*2 x 10~ 9 cm. or '082 pp. 



This result is, of course, inconsistent with the view that 

 either the water or the mercury are compressed in the 

 surface-layer. It is interesting to calculate the order of 

 magnitude of drjdp required by Lewis's theory. Since the 

 mercury does not suffer any superficial increase of density 

 its " zero-plane " will be situated somewhere in the surface- 

 layer. Since the mean density of the water in the surface- 

 layer is double the bulk density, the ' w zero-plane " of the 

 water will lie in the interior of the mercury, a distance from 

 the nearer boundary of the surface-layer equal to the thick- 

 ness of the surface-layer. We therefore have 



dr 



=- > thickness of surface-layer. 



dp J 



If we assume the value 10 ~ 6 cm. for the thickness of the 

 * Phys. Eev. vol. xxii. p. 181 (1906). 



