JoLY — Some Sedimentation Experiments and Theories. 393 



The final disappearance of the upper boundary is thus not a suddenly- 

 attained state, but is reached by a gradual delay in the precipitation of the 

 finer particles. This is also revealed in the fact that before final disappearance 

 this boundary appears first at points which are ever lower in the tube and 

 at stages in the settlement correspondingly advanced ; also with increasing 

 indistinctness. 



If a sediment which has settled under the action of a strongly concentrated 

 salt be again shaken up, the phenomena first observed are repeated. There may 

 be a small decrease in the rate of descent of the bounding surface. On again 

 shaking up, the rate may show a further very small but distinct decrease. Finally 

 the rate becomes apparently constant and a little less than the initial rate. 

 If, however, the experiment of re-distributing a sediment be tried in the case 

 of concentrations approximating to those which fail to produce a bounding 

 surface, say to such as on first settlement leave the supernatant liquid so 

 turbid as to give it a tallow-like opacity, then the interesting fact is revealed 

 that on second or third shaking the boundary may utterly fail although distinctly 

 produced on first settlement. There has, in fact, occurred some loss of effective- 

 ness of the ions or change in the proi^erties of the suspension, which forbids the 

 repetition of the first effect. With each re-disturbance of the sediment the ionic 

 action is less marked. Finally, the tube may be almost indistinguishable in its 

 behaviour from one containing only silt and distilled water. 



Table I. contains experiments on the rate of falling of the sediment-surface in 

 presence of a monad metallic ion at various concentrations, and after successive 

 redistributions of the sediment. The sign (?) indicates that no sediment-surface 

 was distinguishable. It will be seen that at concentrations of 0'007 gram, 

 equivs. per litre the obliteration of a surface of settlement occurred upon second 

 and third settlements. 



[Table I. 



2M2 



