1903.] Surface-layers of Solutions and "Suspensions? 157 



taking surface-tension considerations only into account, must be one 

 accompanied by minimal " surface energy." A dissolved substance, if it 

 increases the potential energy of a surface, will tend to leave that 

 surface, or if it diminishes it, to accumulate at that surface. This 

 principle has been recognised as holding good in crystalloid solutions* 

 but has not hitherto been shown to apply to colloid solutions and coarse 

 " suspensions." 



The same considerations may be applied in explanation of the 

 accumulations observed at the interfaces of liquids forming persistent 

 emulsions. 



The formation of surface pellicles, the separation of various solids by 

 mechanical treatment adapted to produce heaping up of surf ace- films, 

 the power of forming moderately persistent bubbles possessed by 

 various limpid solutions, and the power of forming persistent emulsions 

 possessed by various immiscible liquids, are all explained (in the 

 numerous cases where there is no evidence of chemical change) as due 

 to diminution of the surface-energy brought about by accumulation of 

 certain dissolved or suspended matters at the surfaces concerned, and 

 to the physical properties of the accumulated material. 



It is remarkable that the very common occurrence of these free- 

 surface accumulations has hitherto escaped general notice. Doubtless 

 this is due to the extreme delicacy and fragility of the solid coating, 

 and to its generally rapid re-solution when by contraction of the surface 

 it is heaped up in local excess, or when by the substitution of some 

 other surface, the reason for such accumulation has been removed. For 

 the production of visible masses of solid, or of deformed angular bubbles, 

 it is in fact necessary that the surface-solid shall either be rendered 

 insoluble by the mechanical treatment to which it is subjected, or shall 

 be heaped up more rapidly than it is re-dissolved. 



Full details of the methods employed, the control experiments made, 

 and the results obtained, together with references to the work of others 

 in the same field, I hope to publish shortly. Meanwhile a brief 

 summary of the main points will be found in the following observa- 

 tions : 



1. The presence of a free (i.e., gas) surface is essential for the 

 production of the de-solutions and coagulations described in my 

 previous paper. 



2. By simple and gentle mechanical means adapted to produce 

 heaping up of the surface-films, large masses of solid (" mechanical 

 surface aggregates ") can be separated out from all proteid solutions 

 and from a large number of colloid solutions and suspensions. (Vide 

 table at end of paper.) In the three cases where the dilution-limit has 

 been ascertained, solid " mechanical surface aggregates " have been 



* Vide J. J. Thomson, "Application of Dynamics to Physics anil Chemistry," 

 p. 251. 



