400 JoLY — Some Sedimentation Experiments and Theories. 



most general condition will be that particles separated by molecular distances will 

 experience preponderating clumping forces tending to retain tliem in juxtaposi- 

 tion, and to further retain in the group any particles urged into contact with it. 

 The particles are, in fact, everywhere expelled from the electrostatic field on the 

 energy principles referred to ; the phenomena being the inverse of those which 

 occur when a plate of sulphur is drawn further inwards (replacing air) between 

 the electrified plates of a condenser. Although we must look to the mechanical 

 forces arising from lines of force which are not symmetrically distributed across 

 silt jjarticles tending to straighten and expel laterally the particles of low specific 

 inductive capacity, we may state the matter more generally in the view that 

 the silt exerts an influence opposed to the ionising forces existing in the liquid 

 medium, and these forces consequently reacting on the silt, tend to reduce the 

 de-ionising influence of the silt by favouring its exjDulsion from the medium. 



In a medium exerting this expulsive force upon tlie suspension any mutual 

 attractions arising in the electric sign of the particles will go towards explaining 

 how silt particles possessing a charge might flocculate or clump more rapidly than 

 those deprived, or almost deprived of this charge. Such attractions might arise 

 in reversal of sign of some small jjarticles by attraction to the negatively charged 

 particle of positive ions ; or local reversal of sign in tlie case of large particles ; so 

 that particles become mutually attractive. Again, tlie preponderating attractive- 

 ness for -I- ions of the larger particles in a region where silt particles were adjust- 

 ing their positions under the electric forces may lead to ajjproximation of the 

 smaller silt particles to the larger. 



But obviously in a medium possessing these expulsive properties it is only 

 necessary to confer active motions ujjon the repelled particles to ensure their 

 rapid aggregation. For in such a medium final stability is only attained by 

 aggregation. The mutual approximation of the particles is a position of stability, 

 their separation is not. Hence each encounter reduces the number of separate 

 free particles. Now it is certain that in the existence of an electric charge upon 

 the silt there arises a cause of motions among the particles which a neutral silt 

 would not possess. Whether rejDulsions between negative silt and negative ions, 

 or attractions between silt to positive ions, the results are activity. Nor need the 

 mutual repulsion of the particles oppose final clumping, for it will everywhere 

 hajjpen that particles urged into collision are neutralized wholly or partially 

 before this repulsion can effect separation. 



We are in complete ignorance of the actual mechanism of the ionising forces 

 in a liquid such as water ; but that its high specific inductive capacity is probably 

 concerned in its remarkable ionising power by weakening the electric forces 

 between the ions has been pointed out by Professor J. J. Thomson. And it is 

 very certain that the efi'ect of the silt is to tend to undo this work of ionisation. 

 On these premises the foregoing remarks are based. 



