EFFECT ,0F FINENESS OF GRINDING. 27 



to be an economical substitute for concentrated and sohible potash 

 salts remains to be determined by systematic investigation. It may 

 have been noticed in reading the account of the foregoing experi- 

 ments that in only a few cases was the fineness of the feldspar mate- 

 rial described. Each investigator considered ground feldspar fine 

 material, whether it passed an SO-mesh or a i200-uiesh sieve. A 

 standard sieve containing 200 meshes to the linear inch has openings 

 0.0020 inch square. A sieve of 80 meshes to the linear inch has open- 

 ings about 0.007 inch square. It is never the case, however, that a ma- 

 terial which has been ground so that it Avill just pass a certain sieve 

 will consist of particles of uniform size corresponding to the size of 

 mesh. All ground material contains a certain proportion of very 

 fine particles, down to those Avhich might be characterized as sub- 

 microscopic in size. The smaller the coarser particles of a given 

 powder which has been ground in hulk the higher will be the pro- 

 portion of very fine material. This is a very important point, because 

 it is doubtless true that the auiount of decomposition caused b}'^ the 

 action of Avater on ground feldspar is directly proportional to the 

 active surface area ^jresented by unit weights of the powder. To one 

 who has not studied the matter, the rapidity with which surface 

 area rises with fine grinding is very surprising. In a 200-mesh feld- 

 spar powder, such as is supplied by the grinder for the pottery trade, 

 the finest particles bear the same relation to the coarsest present as 

 these latter Avould to fragments about 2 inches in diameter. As the 

 surface area of a powder increases in inverse ratio to the diameter of 

 thetparticles, it can be seen how quickly the availability due to active 

 surface must increase with fineness in grain. No one would for an 

 instant consider the possibility of fertilizing land by scattering on it 

 feldspar fragments of an aA^erage diameter of 2 inches. Yet in 

 grinding these fragments to an 80-mesh powder, Ave have not in- 

 creased the active surface area as much as Ave should do by pushing 

 on the grinding of an 80-mesh poAvder to the ultimate fineness attain- 

 able in mechanical processes. Grinding is making surface, and it 

 can be shown that the availability of potash in ground feldspar 

 increases Avith the surface area. If feldspar is ground so that it will 

 pass an 80-mesh sieve, it Avill of course contain a certain proportion 

 of very fine particles, some of Avhich approach the limits of A'isibility 

 under a poAverful microscope. If we carry on the grinding of the 

 material from 80-mesh to 200-mesh, the proportion of the very small 

 particles is enormously increased. The smallest jDarticles which we 

 need to consider here are those Avhich can be measured by a microme- 

 ter device connected Avith the microscope. These smallest particles 

 have a diameter of about 0.0001 millimeter. Now, in order to make 

 a specific example, Ave Avill consider the surface areas presented by 1 



104 



