PHYSICO-CHEMICAL PROCESSES 145 



precipitate; a sediment is a solid matter separated merely 

 by the action of gravity from a liquid in which it has been 

 suspended. A precipitate, on the other hand, is a solid 

 matter separated from a solution by chemical means. 



157. Clarification. Clarification is the process of sepa- 

 rating from liquids, without making use of strainers or filters, 

 solid substances which interfere with transparency. The 

 principal methods of clarification are: (1) by the applica- 

 tion of heat; and (2) through the use of gelatin and other 

 substances. Boiling facilitates the separation, since the 

 minute bubbles of steam adhere to the particles and rise 

 with them to form scum, which may be skimmed off. This 

 process takes place when milk is heated and the albumin 

 rises to the top. If albumin be added and heat applied to a 

 turbid (" milky") liquid, the albumin will, on coagulating, 

 envelop the particles and rise to the top with them. Acids 

 may be used to precipitate the casein (white curd) of milk, 

 and the precipitated casein will carry with it the insoluble 

 particles. If a cloudy liquid be agitated with paper pulp 

 and then allowed to stand, it will gradually become clear. 



158. Filtration. The commonest method of separating 

 solids suspended in a liquid is by filtration, i.e., by passing 

 the liquid through the pores of some substance called a filter. 

 The liquor that passes through is called a filtrate, and the 

 material that remains, the residue. Various kinds of mate- 

 rial, such as, paper, cloth, cotton, wool, asbestos, slag, sand, 

 and other porous substances, are used as filters. Cotton cloth 

 is often used by fastening it onto a wooden frame in such a 

 way that a shallow bag is formed into which the liquid to 

 be filtered is poured. The first portion of the filtrate that 

 comes through is cloudy, but the rest soon becomes clear, 



