8 OPERATIONS AND INSTRUMENTS 



tration, for separating solid particles which are 

 diffused through liquors. These are allowed to 

 settle to the bottom, and the clear fluid is gently 

 poured off. If the sediment be extremely light, 

 and apt to mix again with the fluid, by the slightest 

 motion, a syphon is used for drawing off the clear 

 fluid. 



Limviation is the separation by means of water, 

 or other fluid, of such substances as are soluble in 

 it, from other substances that are not soluble in it. 

 Thus, if a certain mineral consists of salt and sand, 

 or salt and clay, &c. the given body being broken 

 to powder, is placed in water, which will dissolve 

 the salt, and keep it suspended, whilst the earthy 

 matter falls to the bottom of the vessel, and, 

 by means of filtration, may be separated from the 

 fluid. 



Evaporation separates a fluid from a solid, or 

 a more volatile fluid from another which is less 

 volatile. 



Simple evaporation is used when the more vola- 

 tile or fluid substance is not to be preserved. 

 Various degrees of heat are employed for this pur- 

 pose, according to the nature of the substances, 

 it is performed in vessels of wood, glass, metal, 

 porcelain, &c. Basons made of Wedcewood*s 

 ware are very convenient, as they are not apt to 

 break by sudden changes of heat. Small flasks of 

 thin glass also : these are placed either over the 

 naked fire, or in a vessel filled with sand, which is 

 then called a sand-bath. This affords a more re- 

 gular degree of heat, and renders the vessels less 

 liable to be broken. 



When the fluid which is evaporated must be pre- 

 served, then the operation is called distillation. 



Distillation is evaporating in close vessels, when 



