386 The Flocculation of Soils. Ill 



4. The ignition of a soil for a few minutes over a bunsen flame in- 

 creases the amounts of iron and aluminium dissolved by acid. Evidence 

 is brought to show that this is due to a destruction of the colloids which 

 bind the particles together, and a consequent exposure of a larger surface. 



5. The effect of a partial ignition on the base absorbing power of soils 

 and subsoils is described and the results are claimed to be in agreement 

 with the view that the particles in the aggregates are bound together by 

 gelatinous colloidal matter. 



Appendix. 



The preparation of clay suspensions. 



For experimental work on flocculation it is frequently necessary to 

 prepare clay suspensions without the use of excess of ammonia solution 

 which is employed in routine mechanical analysis. Two notes on the 

 making of such suspensions are appended : 



1 . In order to obtain, after sedimentation, the maximum amount of 

 clay in suspension it is necessary to adjust the relative amounts of soil, 

 etc. and water. All soil and clay particles are largely aggregated. If 

 therefore the proportion of clay to water is very large the deflocculated 

 particles are entrained by the aggregates and the whole settles down 

 leaving clear water above. On the other hand, if the proportion of clay 

 to water is small the amount of clay in suspension will be small. 



This can be strikingly demonstrated by arranging a series of tubes 

 containing a large amount of a stiff clay at one end and gradually 

 decreasing amounts through the series. If water is then added to the 

 same height in each tube, it is found after agitation that the supernatant 

 liquid is quite clear in a few minutes where the amount of solid is greatest. 

 It becomes gradually more turbid down to a certain point in the series 

 beyond which it becomes less turbid again. 



Only by such trial can the most suitable relative amounts of solid to 

 water be ascertained. 



2. An almost neutral suspension of clay is easily prepared by ex- 

 tracting the soil with dilute acid, washing, agitating with excess of lime 

 water, filtering at the pump and thoroughly washing with hot water. 

 The wet soil will then give good clay suspensions. 



Soils treated in this way, or after extraction with acid, should not be 

 dried after washing if a maximum clay suspension is required. 



(Received August llth, 1922.) 



