44 Preparation of Substances 



and become so small that their rapid motion 

 ofsets the force of gravity. 



The rapid motion of any small particle 

 in suspension may be easily observd under 

 a high power microscope. The motion is 

 produced by the molecules of water which 

 strike the larger particles with sufficient force 

 and frequency to keep them in oscillation. 

 This motion is known as the " Brownian 

 movement." 



The action which the arsenate of lead 

 undergoes in becoming colloidal is said to 

 be as follows: When the soluble salts (elec- 

 trolytes) are sufficiently decreased by the 

 washing process some groups of mole- 

 cules of lead arsenate react with either the 

 hydrogen or the hydroxyl-ions of the water 

 combining with them. If the particles com- 

 bine with the hydrogen they become posi- 

 tively charged colloids and the hquid re- 

 taining the negativ hydroxyl group becomes 

 negativ. It is entirely possible that it is 

 the layer of atoms on the surfaces of the 

 particles of arsenate of lead which is active 

 in combining with either the H"^ or the ~0H 

 of the water. The action of salt in coagu- 

 lation, or precipitation, consists in neutraliz- 

 ing the electrical charges. As this takes 

 place the lead arsenate particles, which were 

 previously all of the same electrical condi- 



