6 OPERATIONS AND INSTRUMENTS 



and has given rise to the doctrine of definite 

 proportions. 



Here it must be remarked that chemical com- 

 bination and mechanical mixture are very different ; 

 since, although bodies only combine in definite 

 proportions, yet, they can be mixed together in all 

 proportions. 



In general, before substances can be made to 

 act chemically on each other, one of them, at 

 least, must be in a fluid state ; and, that solids 

 may be acted on more easily, they are generally 

 mechanically divided into small pieces, or reduced 

 to a powder. 



By trituration, pulverization, and levigation, is 

 meant the reduction of solids into powders of dif- 

 ferent degrees of fineness. Brittle substances are 

 reduced to powder by means of hammers, pestles 

 and mortars, stones and mullers. Mortars and 

 pestles are made either of metal, glass, porcelain, 

 marble, agate, &c, according to the hardness and 

 properties of the bodies to be pounded. WEDGE- 

 WOOD'S ware affords a most excellent kind of mor- 

 tar for most purposes, as it is very strong, and not 

 liable to be acted upon by acids. Many bodies 

 cannot be reduced to powder by the foregoing 

 methods : such are fibrous substances, as wood, 

 horns of animals, elastic gum, and mettles which 

 flatten under the hammer ; for these, Jiles, rasps 9 

 knives, and graters, are necessary. 



The separation of the finer parts of bodies from 

 the coarser which may want farther pulverization, 

 is performed by means of sifting or washing. 



A sieve for sifting generally consists of a cylin- 

 drical band of thin wood, or metal, having silk, 

 leather, hair, wire, &c. stretched across it. They 

 are of different degrees of fineness* 



