402 JoLY — Some Sedimentation Experiments and Theories. 



It thus appears likely that in electrolytes above a certain concentration the 

 expulsive action is the principal cause of flocculation, and that but little change 

 in its rate of action is to be expected in the increased numbers of ions, owing to 

 the rise in viscous resistance, and the effects of this on the motion of the silt 

 particles: possibly, also, owing to increased aggregation of ions upon the silt 

 to a degree retardative of clumping. In these cases the silt is rendered neutral 

 towards tlie medium almost immediately. The "standard rate" of pi'ecipitation, 

 as we have seen, obtains over a wide range of concentration, and considerable 

 changes in the coefficients of ionisation. 



Summary. 



The rates of settlement of suspensions consisting of 5 grammes of finely powdered 

 solid in 12 c.c. of water containing ions in various degrees of concentration, 

 indicate that above a certain concentration the rate of fall of the surface of the 

 suspension is fairly independent of the degree of concentration. Below certain 

 concentrations (about five times greater for monad positive ions than for diad) a 

 distinct surface to the descending suspension fails, and the sediment is only seen 

 to collect from the bottom of the vessel upwards. A suspension precipitated at 

 a concentration so low as to be near the point of failure to show surface will, 

 if re-shaken, not again precipitate with a distinct surface. On removing the 

 electrolyte from such an "exhausted" suspension after it has stood sufficiently 

 long to settle, it is found that the liquid is as effective as at first in producing 

 surface if a fresh sample of the powder is used. On the other hand, the original 

 powder will not again show surface when treated with fresh electrolyte of the 

 same strength, but it will require a considerably more concentrated electrolyte to do 

 so. The failure is therefore to be traced to some alteration in the solid particles. 

 On testing the fresh powder it is found that this is negative towards distilled 

 water ; the used powder is apparently neutral or nearly so towards its salt solution. 



An exi)lanation of sedimentation is advanced, based on the low specific 

 inductive capacity of the solid particles compared with the specific inductive 

 capacity of the water, the charges on the ions being assumed to exert an expul- 

 sive action consequent on the increased energy required to establish the electric 

 field in the medium of low specific inductive capacity. In other words, the solid 

 particles have a de-ionising influence, and experience a reaction in consequence, 

 which will tend to retain in juxtaposition particles which, from any cause, are 

 once approximated. A principal cause of aggregation upon first precipitation is 

 to be ascribed to the negative sign of the particles leading to motions all in the 

 end favourable to aggregation, seeing that the state of aggregation is alone stable 

 in the medium. On second disturbance the particles are neutral or nearly so, 

 and aggregates are not formed with sufficient rapidity to lead to a general and 

 simultaneous descent of the suspension. 



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