32 



ELEMENTARY SUBSTANCES. 



Silex, or Siliceous Earth, exists nearly pure in large masses, form- 

 ing minerals, and even entire rocks, as rock crystal, quartz rock, and 

 flint : it communicates a great degree of hardness to all rocks or 

 stones into which it enters, in a large proportion. Such stones are 

 denominated Siliceous : they resist the point of a knife, or scratch 

 glass. In its combinations with other earths, Silex appears to act as 

 an acid. More than one half of the crust of the globe is composed 

 of siliceous earth eitber pure or combined. In some thermal waters, 

 siliceous earth occurs, either in a state of minute division or in solu- 

 tion ; and the waters of the boiling springs, or geysers, in Iceland, 

 deposit siliceous incrustations of considerable thickness. 



Alumiiie, pure argillaceous Earth, (Lat. argilla, Fr. argille,) is a 

 substance which in a mixed state is well known, but pure unmixed 

 clay is one of the rarest substances in the mineral kingdom. This 

 earth is soft, smooth, and unctuous to the touch; it strongly absorbs 

 water ; where it exists in the proportion of thirty per cent, it com- 

 municates in some degree these properties: such rocks are called 

 argillaceous ; they generally contain a notable portion of iron, which 

 appears to have a greater affinity for this earth than for any other.* 



Lime (Lat. calx, Fr. cha^x) is a well-known earth combined with 

 carbonic acid, in which state it forms limestone, marble, and chalk: 

 these differ from each other only by different degrees of hardness or 

 of crystallization. Mountains composed of lime are denominated cal- 

 careous. When lime is united with sulphuric acid, it forms the stone 

 called gypsum, which is softer than limestone, and does not, like it, 

 effervesce with acids. Calcareous earth, rnixed with common clay, 

 forms marl. 



Magnesia has, rarely, been found pure in a native state. It en- 

 ters into the composition of some of the primary rocks, to which it 

 generally communicates a soapy feel, a striated or striped texture, 

 and sometimes, a greenish colour. It occurs, also, in various hme- 

 slones in different proportions. 



Iron appears to be more abundant than magnesian earth : it forms 

 a constituent part of numerous rocks and stones; to it they most fre- 

 quently owe their colour : the earths, when pure, are white. Iron, 

 when in combination with the earths, is, like them, an oxide, or a 

 metal united with oxygen. To the presence of iron, the increase of 



* Though alumine or pure clay communicates a soft quality to most stones of 

 which it forms a principal constituent part, a very remarkable exception to this 

 is offered in adamantine spar and the sapphire, which nearly equal the diamond 

 in hardness. Klaproth, one of the most laborious and eminent chemists of the 

 present age, has analysed these stones : the former contains 90 parts in the 100 of 

 pure clay ; the latter 95 parts in the same quantity. " What a high degree of co- 

 hesive power (he observes) must nature command, to be able to transform such a 

 common substance as clay (aluminous earth) into a body so eminently distinguish- 

 ed and ennobled as the sapphire by its hardness, brilliancy, and its resistance to 

 the action of fire, of acids, or the effects of all-destroying time ! ''—Klaproth's Es- 

 says. 



