Mr. J. Taylor's Lecturei on Metallurgt/. 373 



that they may be rendered as pure as these means will allow 

 before they go to the furnace. 



The following is a brief account of the processes of dress- 

 ing: 



The ores, when first raised from the mines, are in various 

 states; some in large masses free from other matter, other 

 large masses containing ores, spar, &C. ; small pieces both of 

 pure ore, and that which is mixed, and much that is crumbled 

 to small minute fragments. 



The first operations relate to separating them into jiroper 

 sizes and qualities by spulling or knocking, sifting or gridling, 

 and picking. 



The purer parts are then separated, and are often removed 

 at once for smelting after having been broken down to a cer- 

 tain degree of fineness or size. 



The other or mixed parts are submitted to various processes 

 of washing, but first must be put into a proper state of di- 

 vision by being bruised and sifted to uniform sizes : this is 

 done either by cobbing and bucking, manual operations ; or 

 by crushing and stamping by machines. 



The processes of washing are very various, but depend on 

 one general principle, — that the metallic may be separated 

 from the eartlnj parts by their difierent specific gravities. Thus, 

 if both are agitated together in water, the heaviest will sink or 

 be deposited first. This principle is applied by throwing the 

 ores into streams of water running over planes gently inclined. 

 The ores settle at the upper, and the spar and earthy parts at 

 the lower. 



Another way much in use is by agitating the whole in sieves, 

 so fine as to permit but little of the sohd matter to pass, but 

 to admit die water freely. By this agitation in water the ore 

 settles to the bottom of the sieve, and the spar or matrix at the 

 top, where it is skimmed off and thrown away, and this is 

 called Jigging. When the ores are cleaned by dressing, they 

 are fitted tor the operations of reduction or smelting. 



The art of managing metallurgic processes with considerable 

 skill and effect is of the highest antiquity, which is rather to 

 be wondered at, seeing Unit so few of the metals offer them- 

 selves to our notice in a state fit for any useful purpose ; that 

 these occur in small quantities and in few localities ; and that 

 many of die ores have no appearances by which their metallic 

 contents could be judged of. 



They differ indeed in this respect : thus 



Galena has nmch the apjioarance of the metal it contains. 



Yellow copper has a metallic appearance, but not indicating 



(he metal. 



Tin, 



