FORMATION OF CLAYS. 91 



nesia, and protoxide of iron.* When we compare the other 

 kinds of clay with the porcelain clays, we find that the infusible 

 clays, or clays poor in potash, are of rare occurrence. The 

 clays diffused through the most kinds of rocks, those occurring 

 in arable land, and those in the beds of clay interspersed with 

 the layers of brown and mineral coal, contract when exposed to 

 heat, and become vitrified in a strong fire. Loam also melts in 

 a similar manner. When the oxides of iron are not present in the 

 clays, their fusibility is in direct proportion to the amount of their 

 alkaline ingredients. Clays arising from the disintegration of the 

 potash felspars, are free from lime ; those formed from Labrador 

 spar (the principal component of basalt and lava), contain lime 

 and soda. 



The limestones containing much clay are proportionally the 

 richest in alkaline ingredients. The marls and stones used for 

 cement belong to this class of minerals. They differ from other 

 limestones by possessing the property, after moderate burning, 

 of hardening when in contact with water. During the burning 

 of marl and of many other natural cements, the constituents of 

 the clay and lime act chemically upon each other, giving rise to 

 an anhydrous apophyllite, or an analogous compound of silicate 

 of potash and silicate of lime, which, being brought in contact 

 with water, forces the latter into chemical combination in a man- 

 ner similar to burnt gypsum, and crystallizes along with it.f 

 When a fragment of chalk is moistened with a solution of silicate 

 of potash, the latter forms a new compound on the surface, and 

 this becomes hard and stony. The lime of the chalk takes the 

 place of potash in the silicate of potash, and a certain quan- 



* COMPOSITION OF PORCELAIN CI.AYS. 



St. Yvreux. Meissen. 



f Formula of Apophyllite— Ko, 2 Si 3 -f 8 Ca 0, Si O 3 + 16 aq 



