SCIENCE IN THE INDUSTRIAL WORLD 



heavier particles when mixed with lighter ones, tend 

 to settle first to the bottom when the substance is agi- 

 tated in water and then allowed to stand. In practice 

 there are various methods of applying this principle. 

 A very common one, in use where the raw material is 

 composed'of a mixture of several different-sized particles, 

 is to arrange currents of water and settling-tanks so as 

 to separate these particles into deposits of great uni- 

 formity. In an arrangement of a series of tanks through 

 which the liquid flows, the first tank will arrest and 

 collect the coarser particles; the second tank will arrest 

 the particles that are somewhat finer; the third will 

 arrest still finer particles, and so on until the last tank 

 receives only the very finest particles. This may be 

 taken as a typical method of levigation. And without 

 entering into details it may be said at once that this 

 process is perhaps the most important single one con- 

 nected with mineral-pigment manufacture. 



Reducing the pigment to a fine powder by grinding is 

 as old as recorded civilization itself. The kinds of mills 

 used for grinding do not differ, except in details, from 

 those used at various times for grinding other materials, 

 such as grain. Indeed, the mortar and pestle, quern 

 or hand-mill, millstones of various kinds, and, finally, 

 roller-mills, have been applied to color-grinding in 

 the order of their development, just as in the case of 

 grinding grain. 



Only second in importance to the pigments themselves 

 if, indeed, their place may be considered secondary 

 are the vehicles in which they are incorporated for 

 use in painting. Water is, of course, an important 



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