398 JoLY — Some Sedimentation Experiments and Theories. 



pole, the difference of level in them being from 1 to 3 cms. In the case of silt 

 in sea water no change of level was observed. According to these results the silt 

 is negative to fresh water and neutral to sea water. When the e.m.f. was raised 

 to 60 volts the fresh-water tube again shows the water rising round the minus 

 pole. In the case of the sea water violent evolution of gas, and heating of 

 the solution and the silt, render the experiment useless. The experiment 

 of seeking if there is a greater rate of settlement in one limb of the U tube 

 than the other, only a very attenuated sediment in fresh water being used, show 

 that cleai'ing seems hastened round + pole : and in this limb of the tube 

 precipitation is densest upon the bottom. This too confirms the negative sign 

 of the powder in fresh water. That some negative charge may linger in 

 silt which has become "ineffective" or lost surface, was indicated by an 

 experiment on the contents of a tube of MgCl2 (0'00125 gramme-equivalents) 

 which had, on repeated shaking, lost all surface. This transferred to a U tube and 

 tested by an e.m.f. of 22 volts showed a rise of liquid round the — pole of 1 m.m. 

 in an hour. The difference in electric potential between slate-silt to distilled water 

 is thus very considerable compared with what exists between the silt and a solution 

 of sufficient strength to deprive it of the power of settling with a stable surface. 



In seeking for an explanation of those results two facts must be kept 

 prominently in view : the existence of a potential difference between the silt and 

 the liquid medium around it, which only obtains upon first precipitation or is 

 then most marked, and the difference of the electrostatic properties of the 

 two media ; the silicates (composing the silt) and the water. The specific 

 inductive capacity of the former may be assumed to be between 4 and 7 ; of the 

 latter about 80. Furthermore, the view that the ions represent centres of electric 

 force, positive and negative — in fact, may be regarded as free charges — in 

 continual motion, disappearance and reappearance according as re-combinations 

 of ions or disassociations occur, will be assumed in seeking an explanation 

 of sedimentation. The sizes of the silt particles diminish downwards to ultra- 

 microscopic and even to molecular dimensions, probably a large proportion being 

 of dimensions comparable with the mean distance separating the ions.* 



The interaction of these forces is most complex and difficult to analyse. 

 The silt particles being initially negative in sign there will be attractive forces 

 between silt and positive ions, repulsive forces between silt and negative ions, but 

 on these forces must be superimposed those forces on the ions, arising from the 

 presence of matter of low specific inductive capacity in a medium of high specific 

 inductive capacity. If -f- and - ions find themselves separated by a silt particle, 

 the electrostatic field between the ions can only be established through the 



* See " Subsidence of Fine Solid Particles in Liquids," Carl Barus, Bulletin of the United States 

 Geological Survey, No. 36, 1886. 



