34 Transactions of the 



oxide of silicon. Clay is an opaque and non-crystallised body, of 

 dull fracture, soft enough in all states to take a mark from iron ; 

 when breathed on, it exhales an odour, which, from its peculiarity, 

 takes its name from the material, and is termed argillaceous. This 

 is owing to the oxide of iron with which it is mixed, as clay, when 

 absolutely pure, does not exhale any odour. 



Clay forms with water a plastic paste, having considerable 

 tenacity, which by the action of heat is brought to a very great 

 degree of hardness ; it is compact and smooth, almost unctuous to 

 the touch, and when dry may be easily polished by the finger. It 

 is not soluble in water, but mixes readily with it in all proportions, 

 parting with difficulty from the last portion of that which it has 

 absorbed ; it will adhere to the tongue. The description of clay 

 employed by potters is infusible in the heat of a porcelain furnace, 

 when some kinds, owing to their being combined with oxide of 

 iron, assume a red colour, while others have a pure white. The 

 highest temperature to which clay can be exposed tends only to 

 increase its density, hardening its substances, and diminishing its 

 volume. 



The second ingredient, silica, or pure flint, is commonly found in 

 most parts of the world in primitive mountaius, frequently em- 

 bedded in chalk, in great abundance. In Scotland and Ireland it 

 occurs in secondary limestone. Plint abounds in alluvial tracts in 

 the form of gravel ; an inexhaustible supply of excellent quality 

 might be collected on some parts of the sea-coast of England, and 

 particularly in the neighbourhood of Brighton, where there is 

 enough of this material known under the name of shingle to serve 

 the whole manufacturing wants of England for ages yet to come, 

 while its removal would be attended with advantage to the place 

 where it is now situated. Flint is silica in a state nearly approach- 

 ing to purity, its constituents being — 



Silica, 98'00 



Lime, "50 



Oxide of iron, . . - . . '25 



Alumina, •25 



Loss, 100 



100.00 



It is usually of a grey colour, with occasional striped delineations 

 occurring in its substance. It is obtained chiefly in rolled pieces, 

 but often occurs in irregular blocks. Internally it has a glimmer- 

 ing lustre, its fracture being conchoidal, and its fragments sharp- 

 edged. Silica may be found existing in solution in the Bath 

 waters, and in some mineral springs in Bohemia, while the cele- 

 brated fountain at Rykum, in Iceland, deposits so considerable 



