CLAYS, THEIR ORIGIN, COMPOSITION AND VARIETIES. 49 



tinguished from decomposed felspar, though a superficial examination 

 would easily confound the two products. As a necessary result of the 

 mode of origin already described, more or less of this felspathic mud is 

 ordinarily associated with deposits of clay. 



Chemical Properties of Kaolin. — The physical properties of kaolin 

 have been described in the preceding paragraph. It remains to describe 

 with equal brevity the chemical behavior of the mineral. Its most marked 

 chemical characteristics are its insolubility and its infusibility. Kaolin is 

 not affected by any of the ordinary chemical agents, nor by the high tem- 

 peratures that we are able to produce. Out ot the latter fact grows one 

 of its most important uses, viz: the supply of refractory materials for 

 the various needs of metallurgy and other lines of manufactures. Kaolin 

 shares this character of/ infusibility to some extent with one or two other 

 substances, as, for example, with quartz, which is pure silica, and also 

 with talc, which is silicate of magnesia. But it easily stands at the head 

 of the list of refractory substances and is indispensable in many of the 

 arts of practical life. 



But while kaolin, taken by itself, is infusible even when exposed to 

 the highest temperatures, in the presence of compounds of lime, mag- 

 nesia, iron, potash, soda and certain other elements, it readily combines 

 with these substances and forms double silicates, similar to those which 

 constitute the felspars of the granite rocks, the history of which we have 

 already briefly followed. The substances last named are accordingly 

 called, in these relations, fluxes. Named in the order of their effective- 

 ness in this work, the principal fluxes are potash, soda, iron, lime and 

 magnesia. Even small percentages of one or more of these substances in 

 admixture with the clay will destroy the value of the latter as a refractory 

 substance. So, also, with the felspathic mud, already described. Con- 

 taining, as it does, the potash, soda and lime of the original minerals, it is 

 ready to take part in the mischievous reactions which render the clay 

 fusible. On the other hand, the finely divided silica of the original min- 

 eral, which is sure to be left in greater or less amount from the decom. 

 posed granite in intimate admixture with the kaolin particles, detracts 

 nothing from the latter in its heat-resisting properties. In almost all 

 high-grade clays a notable percentage of free silica is found. This is the 

 only impurity that can be allowed in clay without detracting from its 

 value as a refractory body. For many uses the silicious clays are not 

 inferior to the purest kaolin. 



The fluxing elements already named, and particularly potash, lime 

 and iron, are so widely distributed in nature, and especially in the older 

 felspathic rocks, that it cannot be expected that they would be wholly 

 wanting in the products of decay of the latter. Only in the rarest 

 instances, as already stated, in another connection, will the kaolin parti- 

 cles be found without admixture. Whenever they occur, the deposit, if 

 easily accessible, acquires considerable commercial value. The great 



4. G. O. 



