JoLY — Some Sedimentation Experiments and Theories. 397 



NaCl 0-005. As above. Depression in 10 mins. 2 cms., in 20 mins. 8 cms., in 40 mins. 5-4 cms. 

 NaCl 0"005. As above. First settlement, surface appears 2 mms. from meniscus. Second settle- 

 ment at 3-5 cms. below meniscus and indistinct. 



On the Sediment. 

 Silt from NaCl 0-005. 9 c.cs. fresh NaCl 0-005. No surface even after prolonged observation. 

 Silt from NaCl 0-005. 8 c.cs. NaCl 0-010 added. Eesult same. 



[As the slate occupies a true vol. of 2 c.cs. (sp. gr. 2-5), and fills 6 c.cs. of the tube, 



there are 4 c.cs. of interstitial solution of nearly the original concentration. The mean 



strength of electrolyte on second settlement is therefore 0-0083 gram, equivalents.] 

 Silt from NaCl 0-005. 8 c.cs. fresh NaCl 0-05 added. Final strength 0-035. This gives surface at once. 

 Silt from NaCl 0-005. 8 c.cs. fresh NaCl 0-025 added. Mean strength 0-0183. Good surface, upper 



liquid translucent as paraffin wax. Eetains surface on subsequent disturbances. 

 Silt from NaCl 0-005. 8 c.cs. fresh NaCl 0-0125 added. Mean strength 0-0100. Bad surface in 5 mins., 



not so good as with concentration 0-005. Second settlement, no surface till 20 mins., 



faint, some 3 cms. depressed. 

 Silt from MgCL 0-00083. 8 c.cs. fresh MgCL 0-00125 added. Total concentration 0-00111. Very bad 



surface, about 2-5 cms. depressed. 



As will be inferred from those experiments the loss of surface is reached at a 

 lower concentration of a diad positive ion than of a monad positive ion. By- 

 seeking in successive experiments for that concentration at which the salt first lost 

 it» power of producing a " surface," on first precipitation it was found that MgCl2 

 was 6 times as effective as NaCl, CaCl2 was 5 times as effective, MgS04 was 

 3 times as effective, and BaCls (acidified as usual) rather more than 5 times 

 as effective. 



TABLE IV. 

 Lowest Concentrations producing Surface of Separation in Falling Sediments. 



Giamiue equiyalents per litre. 



NaCl, 



MgCL, 



CaClj, 



MgSO,, 



Bad., 



0-005 



0-0009 



0-0011 



0-0016 



0-001 



On re- disturbance the sediment precipitated with those concentrations shows 

 no trace of surface. At slightly lower concentrations surface may appear 

 indistinctly at low levels on first usage. Thus MgCL 0'00083 on first use reveals 

 a surface 0"6 or 0"7 cms. below meniscus : on second settlement shows no surface 

 whatever. 



Experiments were made to determine the electric sign of the silt towards 

 water, and salt solutions. The silt (obtained in all comparative experiments from 

 Welsh roofing-slate), was placed in a U tube so as to close the bend. Into the 

 liquid above platinum terminals from a storage battery dipped. The e.m.f. was 

 22 volts. It was found that in fresh water the liquid rose round the negative 



