Chap, 13.] [ 71 ] 



CHAP. XIII. 



ARGILLACEOUS EARTHS, 



General Account ef Clays or Argils. Alum ; its Comfo/itioti. Na- 

 tural Hiflory of Alum. Attion of this Subftance on other Bodies. 



Uj'es of Alum in the Arts. Lac Lunts. Porcelain Clay Manner 



of making different Kinds of Pottery. Stone Ware. T~elltnv f or 

 Queen's Ware.-~Cbtna.Litbomarga. Terra Lemnia. j&o/f. 

 Zeolite. Lapis Lazuli.* Tripoli. Brick Clay. Slate. 



TH E argil, or earth of clay, is one of thofe fub- 

 ftances which abound moft in nature. There 

 are immenfe ftrata of clays, and they make a part of 

 every rich foil. The idea commonly entertained of 

 clay, is that of a natural fubftance, the pureft kinds of 

 which are firm, and have a fort of fatnefs or unftuofity, 

 and which by being rubbed by the finger receive a 

 polifh. When dry, they imbibe moifture, fo that 

 when applied to the tongue it adheres to them. From 

 their attraction for water, they arc always found moid 

 in a natural ftate. If more water is added, they form 

 a ductile pafte, which, when thoroughly burnt and 

 dried, becomes hard, ftony, and impenetrable to water. 

 On the regular contraction which clay undergoes from 

 the application of different degrees of heat, depends 

 the construction of a thermometer for meafuring in- 

 tenfe degrees of heat, and invented by Mr. Wedg- 

 wood *. 



As clays when wrought together become impene- 

 trable to water, they are made ufe of to retain water 

 in ponds, &c. This is the reafon that when cattle are 

 allowed to tread a clayey foil in wet weather, the 



* Sec vol. i. p. 115, 



F 4 plants 



