EARTHENWARE AND PORCELAIN. 521 



prisms in the lava of Vesuvius, forming the mineral nephelin. 

 Other silicates of soda and alumina are : 



Mesotype or nafrolite, NaO, Si O 3 + 3(A1 2 O 3 , Si O 3 )+2HO. 



Sodalite, 3NaO, 2Si O 3 + 2(A1 2 O 3 , SiO 3 ). The latter con- 

 tains also chlorine. 



The two silicates of lithia and alumina are : 



Petalite, LO, 2Si O 3 + A1 2 O 3 , 3Si O 3 . 



Triphane or Spodumene, LO, Si O 3 + A1 2 O 3 2Si O 3 . 



Harmotome is a silicate of barytes and alumina, containing 

 water. 



The silicates of lime and of alumina combine in many differ- 

 ent proportions, forming a great variety of minerals. Most of 

 them contain water, in consequence of which they froth when 

 heated before the blow-pipe, and hence are called zeolites. 

 One of these named stilbite, from its shining lustre, corresponds 

 in composition with felspar, but contains in addition 6 atoms 

 of water; its formula is CaO, Si O 3 + A1 2 O 3 , 3Si O 3 + 6'HO. 

 A small portion of one or other of the alkalies is often found 

 in these minerals, besides small quantities of protoxide of iron 

 and other magnesian oxides, replacing, it may be presumed, the 

 lime in part. This extensive class of minerals has been very 

 fully studied by Dr. Thomson, who has added considerably to 

 their number.* 



Silicate of alumina and magnesia, forms the mineral called 

 soapstone, from its resemblance to mottled soap and being 

 unctuous to the touch. The formula assigned to this mineral by 

 Berzelius is 3MgO, 2Si O 3 + A1 2 O 3 , 2Si O 3 + 6HO. The for- 

 mula of dichroite, another combination of the same elements, is 

 3MgO, 2Si O 3 -f 3(A1 2 O 3 , Si O 3 ). 



EARTHENWARE AND PORCELAIN. 



The silicate of alumina is the basis of all the varieties of 

 pottery. When moistened with water, clay possesses a high 

 degree of plasticity, and can be extended into the thinnest 

 plates, fashioned into form by the hand, by pressure in moulds ; 

 or, when dried to a certain point, be modelled on the turning 

 lathe. It loses its water also in drying, without cracking, pro- 

 vided it is allowed to contract equally in all directions, and ac- 

 quires greater solidity. When heated more strongly in the 

 potter's kiln, in which it is not fused nor its particles agglu- 



* Outlines of Mineralogy and Geology, vol. 1. 



