88 



CHEMICAL MANUEES. 



protection sieve, which retains small fragments of material and only 

 allows particles of a certain fineness to pass through. The fine 

 grains then fall on a second sieve, which is the outside sieve, the 

 wire gauze of which is selected according to the degree of fineness 

 desired. The powder of the desired degree of fineness passes through 

 this gauze and falls into the discharging funnel at the bottom of the 

 machine, where it is bagged up. The cores from the two sieves are 

 collected inside the mill by two openings arranged for the purpose, 

 and again submitted to the action of the balls. The mill is fed 



Fig. 4. — Ball Mill on Masonry Foundation — the Sieves exposed. 



through a hopper fitted over one of the bosses, which has helicoid 

 blades. During rotation of the mill these bring the lumps into the 

 interior after the style of an endless screw. The wrought-iron 

 cover is connected by a cloth jacket with a chimney which maintains 

 the pressure of the air in the cover in equilibrium and evacuates the 

 dust which is given off during grinding. The weight of the balls 

 varies according to the nature and quality of the material to be 

 ground. The mills in general use have a diameter of 6^ feet and a 

 section 39-3 inches. The charge of balls is 1 metric ton. These 



