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the finest particles of clay usually keep the water somewhat 

 troubled for a day or two before they settle out completely. 

 But if the settlement of soil from the unmanured plots or 

 from the plots receiving ammonium-salts was complete in two 

 days, it would take three or four days, or sometimes an 

 indefinite time, to bring about the same clearness in the jars 

 containing soil from the nitrated plots. Now it had previously 

 been proved that there were fewer of the finest particles in 

 the nitrated soils, so that they should settle more quickly and 

 more completely were there not some other factor at 'work 

 hindering the precipitation. 



The settlement of clay from its turbid suspensions in water 

 has been frequently investigated, because it finds a good many 

 practical applications in such matters as the texture of the 

 soil, the fitness of clay for pottery and brick-making, etc., 

 and the important facts are that acids or certain salts hasten 

 the settlement greatly, whereas alkalis will retard or even 

 entirely prevent it. It is well known, for example, how 

 a little alum will bring about the clearing of turbid water, 

 and a trace of acid or of some salt of lime will produce the 

 same effect almost as rapidly. The process can be watched 

 under the microscope ; as soon as the acid or salt is introduced, 

 the very fine particles, which before were moving about in the 

 liquid without ever coalescing, suddenly rush together and 

 flocculate or coagulate into comparatively large and heavy 

 groups which will fall rapidly through the liquid. In clay soils 

 that are in good tilth the very fine particles are grouped 

 together in this flocculated condition, and the soil in conse- 

 quence behaves as if it was more coarsely grained, drying more 

 easily and into a friable condition ; whereas if the clay be 

 knocked about or tempered when it is wet, the groups are 

 broken up and the clay becomes deflocculated. The value of 

 lime in improving the texture of a soil, in rendering it dryer 

 and more workable, is due to the flocculating power of the 

 lime salts which begin to wash through the soil. 



Returning to the experiment, it was pretty clear that there 



