392 . Joty—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 ¢.ms., the liquid above being nearly 
limpid. At the expiration of twenty minutes from starting the surface will 
have fallen to about 2:6 c.ms.; 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 c.ms. If observations are carried on for a further ten minutes 
generally a rapid 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, 
