228 INDUSTRIAL MINERALS DIAGRAM 20. 



and we therefore see the rain remain on the surface of the ground 

 for a long time in clayey districts. 



Bricks, and the different kinds of crockery and earthenware, 

 are made of the paste formed by the mixture of clay and wator. 

 Porcelain is simply made of a white kind of clay called kaolin. 

 All bricks and earthenware must be baked before they are suffi- 

 ciently solid to be fit for use. 



Fuller's Earth is a greenish clay, greasy to the touch, which 

 dissolves in water, and makes it soapy. It is used to remove 

 the oil from cloth, for it is necessary to apply oil to wool before 

 it can be spun and woven. 



Gypsum, or Plaster of Paris. Gypsum is a stone which has 

 much external resemblance to limestone, but it does not effervesce 

 with acids, and does not yield quicklime when burnt, but plaster. 

 Sometimes gypsum is found in the form of large crystals, shaped 

 like spear-heads, which also yield plaster when heated. This does 

 not require so intense a heat as the manufacture of lime. When 

 gypsum is taken from the kiln, it is easily reduced to powder, 

 which again becomes solid when mixed with water. If plaster 

 has been exposed to the air before using it, it is no longer 

 serviceable, because it has absorbed moisture from the atmo- 

 sphere. Plaster is used either to join brickwork or masonry, or 

 to make ornaments and stucco on walls. It is also run into 

 moulds, and is taken out with the desired form. This process is 

 called moulding, and is used to reproduce statues and busts. 



Rock Salt. A large part of the salt which we use is collected 

 by evaporating sea-water in shallow pools. This is called sea-salt, 

 or bay-salt. But salt is also found in the earth in layers. It is 

 sometimes deposited in thick beds -like rock, from which it is hewn 

 like stone. It is then very white and transparent, and called rock- 

 salt. At other times the salt is mixed with clay or sand in the 

 earth, and in this case water is allowed to flow into the mines, 

 which dissolves the salt, and afterwards deposits it on evaporation. 



Salt is not only very useful for food, for salting provisions, 

 making bread, etc., but is also used to make soda, a substance 



