396 JoLY — Some Sedimentation Experime7its and Theories. 



so that all in the one group received the same amount of agitation when 

 the sediments were being distributed and re-distributed throughout the liquid, 

 and were approximately at the same temperature. The test tubes were selected 

 to be as closely as possible of the same dimensions. 



To what is the remarkable change appearing on re-disturbance of the sediment 

 to be ascribed ? To an abstraction of ions from the solution by the silt or to some 

 physical change in the silt. The first explanation seems a priori most probable, j^ 



To investigate the matter I decanted the over-lying liquid from tubes 

 containing sediments which had experienced frequent disturbance till all 

 surface during settlement was lost, till sufficient was got to apply the 

 liquid to fresh silt, not before used. All quantities and conditions being 

 alike with those obtaining in the first experiments, it was now found that this 

 solution was just as vigorous as it was upon the occasion of the first usage ; 

 revealing a distinct boundary surface to the descending silt and the same 

 turbidity above. On re-disturbing those new tubes they gradually, on the second 

 or third precipitation, lost their bounding surfaces just as in the case of the first 

 use of the solution. This exjjeriment being several times repeated with the 

 same results, leaves no room to suppose that any appreciable change in the 

 solution is responsible for the effects noticed. 



The next step was to examine the sediment. The "ineffective" sediment, 

 as for brevity I will call that which has lost the property of revealing a bounding 

 surface during descent, left behind in the tubes which furnished the used solu- 

 tion for the last exjjeriments was supplied with fresh solution, the concentration 

 of which was the same as at first used. It was now found that the loss of surface 

 persisted. The fresh solution left the appearance quite unchanged. In fact it 

 it is evident that to some change in the properties of the sediment the effect 

 observed is to be ascribed. 



Experiments were now made to try if increased concentrations of the 

 ions would not restore the surface to the '* ineffective " sediment. It was 

 found that increased concentration had this effect. Thus silt which had lost 

 sm-face by re-disturbance in a O'OOo gramme-equivalent solution of NaCl 

 recovered it first in a solution of O'OIO gramme-equivalents and one which 

 in 0'00083 gramme-equivalents of MgCL had lost surface regained it first visibly 

 in a solution of O'OOlll gramme-equivalents of the same salt. 



The experiments referred to may be summarised as follows : — 



On the Electrohjte. 

 NaCl 0'005. 12 c.cs. which had become ineffective with 5 grams, silt were withdrawn, and 5 grams, fresh 

 silt added. Well-defined sm-face : depression iu 10 mius. 1'8 cms., in 30 mins. 4-8 cms. 

 Second settlement, some surface still ^oroduced. 

 Third settlement, surface almost indistinguishable. 



