THE NONMETALLIC MINERALS. 



339 



The following table* as compiled by Dr. Ries shows the variable char- 

 acter of the material from different sour<!es: 



aPogg. Ann., LXXVII, 1849, p. 591. gE. J. Riederer, analyst. 



6Klaproth.Beitr.,VoI.IV,1807,p.338. /(Standard Oil Company's property, E. J. Rie- 



c Dana, System of Min., 1893,p. 695. derer, analy.st. 



dGeikie, 1893, p. 133. i Howell property, E. J. Riederer, analyst, 



fi Penny Encyclopedia, XI, Dr. Thompson, analyst. j Morgan property, E. J. Riederer, analyst. 



/P. Fireman, analyst. 



Properties of clay. — To what the peculiar properties displayed by the 

 clays are due can not as yet be said to have been fully determined. 

 This is particularly the case with the property of plasticit}' and that 

 of becoming indurated when dried. ''Various explanations have been 

 oflfered, but none are yet advanced which make clear all points. It has 

 been ascribed to the impurities, to the alumina, to the combined water, 

 and to other causes, against each of which, examples can be cited that 

 seem to set it aside as inadequate. The impurities do not appear to 

 cause the plasticity, for the sand acts unfavorably to it. The alumina 

 is not responsible, or kaolins would be the most plastic of all, while 

 the flint clays of Ohio are many of them approximately pure kaolins, 

 and at the same time eminently non-plastic.^ The combined water 

 exerts some influence it is evident, as its expulsion entails permanent 

 loss of plasticity, but it can not be the sole cause of plasticity, as clays 

 equally hydrated are just as liable to differ in this respect as to agree. 

 No theory is so well received at present as that advanced by Cook. 

 He shows that the microscope reveals a crystalline structure which the 

 eye does not detect, and that this structure varies greatly in degree of 

 perfection in different samples. Some are composed of masses of 



' Seventeenth Annual Report of the U. S. Geological Survey, 1895-96, p. 880. 

 ^As is al.'^o kaolinite, the theoretically pure hydrous silicate of aluminum corre- 

 sponding to the formula A1.^03.2Si02.2H20. 



