210 J. A. POLLOCK. 



ketones, aldehydes, aromatic hydroxy compounds and 

 essential oils, the substances being used, in the practical 

 application, up to an amount which, in proportion to the 

 water, is generally but a very small fraction of one per cent. 



A number of solutions on the other hand notably lack 

 the frothing property, and it will now be shown that 

 only those dilute solutions can yield a lasting froth 

 which have a surface tension which diminishes with 

 increasing concentration. 



Molecules near the surfaces of matter, from the fact that 

 they are not equally surrounded by molecules of their kind* 

 are subject to an attraction towards the body of the 

 material not experienced by those not thus uniquely placed. 

 The extra restraint, under which molecules exist in this 

 situation, changes in degree from the surface inwards until 

 it vanishes at a depth equal to the radius of molecular 

 influence, the molecules at this depth being as similarly 

 acted on by forces in all directions as those well within 

 the mass. The surface stratum, equal in thickness to the 

 range of molecular action, is thus the seat of special mole- 

 cular conditions, and really wants a name indicative of 

 some one of its characteristics, preferably its depth ; it will 

 be referred to here merely as the surface layer or stratum. 

 So rapidly does molecular attraction diminish with increas- 

 ing distance that, adopting Quincke's estimate of the range 

 of molecular influence, the thickness of the stratum may 

 be considered of the order of five millionths of a centimetre. 



As the molecules within the surface layer are subject to 

 additional restraint, there exists in the stratum a greater 

 molecular potential energy, bulk for bulk, than in the mass 

 of the substance. The excess of the molecular potential 

 energy which exists within the surface stratum over that 

 which would be associated with the layer if the additional 

 restraint were removed is called the surface energy ; where 



