N. M. Comber 373 



ferric phosphate (imignited), basic slags (after the removal of free base), 

 finely powdered raw bones, and rock phosphates^ (after removal of free 

 hme). 



2. Aluminium phosphate suspensions were subjected to further ex- 

 amination. With three different samples, two purchased and one prepared 

 in the laboratory, most of the experiments were carried out that have 

 been described in the two previous papers in connection with clay 

 suspensions. The aluminium phosphate behaved in many respects like 

 clay. It was flocculated by calcium compounds better in alkaline than 

 in neutral suspensions; the use of ammonium hydroxide in conjunction 

 with a calcium salt produced a much larger volume of coagulum than 

 when the calcium salt was used alone ; at very low concentrations calcium 

 hydroxide was inferior to calcium chloride as a flocculant, and a con- 

 centration of ferric chloride could be found which gave an optimum 

 flocculation. 



B. The effect of colloidal silica on the suspen.sibilit!/ of small particles. 



Reference has been made in the earlier papers to the effect of very 

 small amounts of colloidal silica on the flocculation of suspensions of 

 ferric oxide. 



The ef[ect of siUca on the suspensibility of jDarticles has been frequently 

 noticed throughout these experiments. Specific observations were made 

 by weighing out 0-25 gm. portions of the powder concerned, suspending 

 one portion in f c.c. distilled water and the other in 10 c.c. of a silica 

 sol containing 0-0031 gm. SiO.j in the 10 c.c. 



The silica increased in a marked manner the suspensibility of two 

 different samples of ferric oxide, zinc carbonate, ignited soil particles, 

 kaohns and other lean clays. With other sub-stances, e.g. zinc phosphate, 

 powdered granite, powdered felspar, no eft'ect was visible. 



C. The effect of concentration on the flocculating poiver of calcium 



hydroxide. 



In the previous paper an experiment was described which shows that 

 in very low concentrations, calcium hydroxide has a flocculating power 

 less than that of calcium chloride, while at higher concentrations the 

 reverse is true. Similar experiments with other clays and with kaolin 

 have shown a similar result. Two of these experiments will be quoted. 



1. A London Clay subsoil was extracted with 5 per cent. HCl, and 

 thoroughly washed. One portion of the extracted subsoil was shaken 



' Nine mineral phosphates were kindly sujiplied by Dr G. Scott Robertson. 



25—2 



