THE NON-LIVING WORLD 143 



we call clay, with granite, quartz and various other 

 stones, are formed of " silicates." 



Arsenic is a metal of a steel-gray colour and consider- 

 able brilliancy, and forms with oxygen arsenious and 

 arsenic acid. 



Iron is the most abundant and widely diffused of all 

 metals, but is rarely, if ever, met with pure. It is found 

 in various combinations, or "iron ores" The natural 

 magnet, before spoken of,* is such an ore in the form of a 

 crystalline oxide. Carbonate of iron mixed with various 

 proportions of earthy matters forms alone one-third of 

 the iron ore of Great Britain. Iron and sulphur, or iron 

 pyrites, also exists in enormous quantity. There are 

 also chlorides of iron. Steel consists of iron united with 

 carbon. 



We have spoken of certain bodies being " crystalline," 

 which means that they are made up of crystals, large 

 and small. Now a crystal is a solid mineral substance 

 of a definite geometrical figure, being bounded by sur- 

 faces, or faces, which meet so as to form sharp edges 

 and angles. The angles formed by these faces are 

 characteristic of different crystalline substances, though 

 there is no constancy as to the size of the crystals, or 

 the proportionate size of their several faces. Snow is 

 a very familiar example of a crystalline body, and is one 

 which can only exist as such at a low temperature. If a 

 crystal be suspended in water which holds in solution as 

 much as it can contain of the same material as that 

 whereof the crystal is composed, then if the liquid be 

 evaporated, fresh solid material may be deposited, from 

 the liquid, upon the surface of the crystal, which will 

 thus increase in size. If a crystal, so suspended, be 



* See ante, p. 125. 



