CHAPTER XXX 

 FLOCCULATION (Reports, XIV, 37; XVII) 



WHEN clay is puddled with water, and the turbid liquid is 

 allowed to settle, the grosser particles fall to the bottom more or 

 less rapidly, but the finer ones remain in suspension for a long 

 time, not settling for many days. If, however, a trace of any 

 acid or salt be added to the liquid, the fine particles settle much 

 more rapidly, and, when examined under the microscope, it is 

 found that the acid or salt has caused them to collect together 

 into groups, so as to form small floes, and is, therefore, said to 

 have caused the flocculation of the clay. 



Another remarkable change accompanies flocculation. The 

 minute particles while in suspension are seen under the microscope 

 to be in a state of perpetual motion, moving suddenly and errati- 

 cally, first in one direction and then in another, exhibiting what 

 is known as the Brownian motion : but on the addition of the 

 acid or salt, this motion at once ceases. Now, the Brownian 

 motion is known to be merely a consequence of the smallness of 

 the size of the particles, any substance, whatever its nature, 

 exhibiting this motion if the size of the particles is below a certain 

 limit ; and the fact that the motion ceases on the addition of 

 acid means, therefore, that the size of the particles must have 

 been increased by the acid. But no such increase in the solid, 

 visible particles can be noticed, so that the increase must have 

 occurred by these visible particles having become united to 

 something invisible, a composite particle, consisting of a solid 

 nucleus surrounded by liquid, having been formed, these composite 

 particles exceeding those limits of size for which Brownian motion 

 is possible. The increase in the space occupied by the clay after 

 flocculation was also established by measuring the volume occu- 

 pied by it when it eventually subsided ; and it was found that, 

 as the proportion of acid or salt was increased, so did this volume 

 increase, till, when flocculation became complete, as evidenced 

 by the absence of any suspended matter in the liquid, the particles 

 of clay occupied double or treble the space which they did before 



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