440 SCIENTIFIC RECREATIONS. 



bined with silica, is the chief component of our clay. Silicic acid and this 

 base combine to form many minerals, and contains nearly all the precious 

 stones. Corundwns consist of pure alumina, and crystallize in the hexagonal 

 system. The following stones are varieties of this mineral : Sapphire, a 

 beautiful blue ; ruby, a red oriental ; topaz, yellow oriental ; amethyst, violet ; 

 all being sapphires more or less. The finest crystals are found in the East 

 Indies in the sands of rivers and diluvial soils. The common corundum is 

 very hard, and is used for polishing. Emery is well known, and is found 

 in mica-slate. It is of a bluish-grey colour, and is also a polisher. 



ALUM forms another family, of which we may first mention aluminite, 

 a "basic sulphate" of alumina and found in small quantities. Alum-stone is 

 found in Italy. Alum occurs in large crystallized masses. (See illustration, 

 fig. 464.) There are different minerals with a composition very similar to 

 alum, in which the potassa base of alum is supplied by others. Thus we 

 have the potassa alum, soda alum, manganese alum, ammonia alum all 

 being very nearly of the same constituents, and having similar crystals 

 in the regular system, and are thus termed isomorphous, or similarly-formed. 

 The potassa, or potash alum, is the commonest form, and is found abundantly 

 in England, on the Continent of Europe, and the United States. Soda 

 alum is called salfatarite, and magnesia alum pickeringite ; manganese 

 alum is apjoJmite ; phosphate of alumina is wavellite. 



There are compounds of alumina and magnesia called SPINELS. They 

 are hard minerals, and the same isomorphous changes take place with them as 

 are observable with the bases of alum. There are therefore varieties such 

 as the spinel ruby found in the East Indies, very red in colour ; the balas 

 ruby not so red, and the orange-red, termed rubicelle. Ceylon is remarkable 

 for some fine specimens of spinels. Chromite is like the spinel, but is known 

 as chrome iron. 



ZEOLITES are principally compositions of silica and alumina. They 

 contain water, and are white, vitreous, and transparent. There are several 

 varieties of them natrolite, stilbite, etc. We will now pass on to the Clays, 

 which are a very important family of the aluminum group. 



There are a number of hard minerals which, when disintegrated, form 

 certain earthy masses. These we term clay, or clays, which possess various 

 colours and receive certain names, according to the proportion of metallic 

 oxides they contain. All clays have an affinity for water, and retain it to a 

 very great extent. The earth has also a peculiar smell. Clay is used in 

 various ways ; pottery, for instance, we read in the Bible as having been an 

 employment from very ancient times. One attribute of clays, the retention 

 of water, is of the greatest use to the world in providing moisture for plants 

 and seeds. We may mention other characteristics of clay. It absorbs oil 

 very quickly, and therefore is useful for removing grease-spots. It cannot 

 be burned, so we have fire-bricks and fire-clay in our stoves and furnaces. 



