392 JoLY — Some Sedimentation Experiments and Theories. 



silt through the liquid it is allowed to stand. The phenomena now observed, 

 attending the settlement of the silt, will depend upon the concentration and 

 valency of the ions present. If above a certain concentration for the valency 

 of the positive ion present, there appears in less than a minute a bounding 

 surface to the silt beneath the meniscus of the liquid ; perhaps a millimetre 

 beneath ; which momentarily continues to sink, attaining at the end of ten 

 minutes a depression of about 1"2 cms., the liquid above being nearly 

 limpid. At the exjiiration of twenty minutes from starting the surface will 

 have fallen to about 26 cms.; after thirty minutes to 3*5 or thereabouts; 

 the rate of descent is often now further increased so that in forty minutes it 

 reaches about 5-5 cms. If observations are carried on for a further ten minutes 

 generally a i-apid convergence in the rate is observed due to the crowding of 

 the sediment towards the bottom of the tube. At the fiftieth minute 6'2 cms. 

 about may be reached. 



If this experiment be repeated with diminishing strengths of solutions, but 

 always using the same quantities of silt and liquid, the rate of fall of the 

 surface will be found well maintained, but it will be observed that with small 

 concentrations the limpid appearance of the liquid above the sediment, and which 

 is so remarkable a feature of the experiments at high concentrations, gradually 

 disappears. At successively decreasing concentrations this supernatant liquid 

 assumes a more and more milky and turbid appearance ; passing from a nearly 

 limpid appearance to the translucency which paraffin wax might show and finally 

 to that of tallow. The final stage of the phenomena being the complete dis- 

 appearance of all boundary between sediment and liquid. This stage is reached at 

 a degree of dilution which depends on the valency of the metallic ion, as will be 

 seen, and until it is reached the rate of descent of the visible upper boundary of 

 the sediment is fairly well maintained. But, as must be obvious, the sediment 

 sinking at this standard rate (as I may call it) is of a coarser character at the lower 

 concentrations of the electrolyte, the turbidity of the overlying liquid being due to 

 the continued suspension of the finer particles. At such low concentrations the 

 ions possess in fact a remarkable sorting effect, allowing the finer sediment to 

 remain so long in solution that when, in course of time, this too sinks to the 

 bottom, it rests as a sharply differentiated layer upon the sediment first deposited. 

 The stronger concentrations of salt solutions, leaving a clear liquid above, cause 

 all to fall together, and although there is some sorting of the largest particles, 

 those being most affected by gravity, there is no sharp line of demarcation 

 in the final sediment. Every intermediate stage between this almost perfect 

 intermingling of the particles of various sizes, and the very marked separation 

 obtained when a distinct upper surface to the falling sediment completely 

 fails, of course exists. 



