JoLY — Some Sedimentation Experiments and Theories. 401 



Many of the experimental facts are explicable on views now advocated. We 

 find that the finest silts are the most resistant to flocculation. The failure of 

 the ions to produce flocculation as the concentration is diminished is first shown 

 in the continued suspension of the finer particles. Thus the overlying liquid 

 grows gradually more turbid, with diminished concentrations. In the case of 

 NaCl 0-225 gram, equivs. leaves a nearly limpid liquid; at 0-112 gram, equivs. 

 the liquid is hazy ; at 0-028 it is translucent only ; at 0-014 it is nearly as 

 opaque as tallow. In the case of a diad salt the same effect is noticed. 

 MgCl2, traced from concenti-ations of 0-01 gram, equivs. downwards to 0-0008, 

 gradually fails to clear down the finer sediment, leaving more and more in 

 suspension, till finally surface is lost. Again, if the results of tlie present paper 

 be compared with those obtained in the case of the much finer and more attenuated 

 silts dealt with in my former experiments {loc. cit.), a similar law is found to 

 obtain. The optimum in the case of the monad salt, NaCl, was near the point 

 of saturation ; in the case of MgCU it was at about 0-032 gram, equivalents. 

 This may be due, in the case of fine as vjell as attenuated suspensions, in 

 part to comparative rarity of encounters under conditions suitable to the 

 neutralization of repulsive forces and to the effectiveness of the expulsive 

 forces in the medium. But generally the electrostatic properties of the silt 

 will tend to retain ions of both signs upon the finest particles ; j^i'obably there 

 is quite an atmosphere of such ions around the particles, tending to check all 

 close aggregation. It may, therefore, well be that the electrostatic properties 

 to which we have referred may, in cases of extreme subdivision, actually retard 

 the effectiveness of the expulsive forces arising from these properties. 



By re-shaking a precipitated sediment, we bring once more into suspension 

 particles which are no longer at a marked negative potential. Linder and Picton 

 have shown that ions, attaching themselves to minute colloidal particles, do so 

 with such tenacity that even washing the precipitate with distilled water fails to 

 remove them. We may, therefore, consider the actions which occur on second 

 jjrecipitation as almost uninfluenced by charges on the silt exerting attraction 

 upon either ion. Evidently the clumping effects of the ions must now be mainly 

 restricted to exjjulsive actions. When chance brings silt particles sufficiently into 

 mutual approximation to establish a preponderating electrostatic effect urging 

 them further together, tliis expulsive action of the ions will be available to forward 

 the process of clumping. If the electrolyte is one of low concentration these 

 actions may be feeble, without assistance from electric reactions between the silt 

 and ions. The discharged silt exerts now no attraction upon the ions of either 

 sign. It is probable, indeed, that the effects of increased ionic attraction across 

 the silt medium is now most effective in securing adherence of ions of both signs 

 upon the silt ; and that herein is to be found another factor in delaying the 

 clumping of a discharged silt. 



