4 wiscoisrsix academy scien-ces, arts, and letters. 



" porcelain-clay.'" An exact statement of the geological and chem- 

 ical application of the term is not so easily given. Geologists have 

 commonly designated as kaolins only those clays resulting directly 

 from the disintegration of felspar-hearing rocks in place, as distin- 

 guished from the bedded or sedimentary clays. It is true that most 

 all of the kaolins used for porcelain making are obtained from de- 

 posits of the former nature, but it is equally true that beds, or 

 patches in beds, of sedimentary clays have the same composition 

 and properties as the ordinary kaolins. It has long been noticed 

 that clays possessing these properties tend to approach a type com- 

 position, and that they are frequently separable by a process of lev- 

 igation into a fine white scaly clay, and a sand composed of jDarti- 

 cles of quartz and undecomposed felspar. The white clay thus sep- 

 arable has always a definite composition, and, as shown by Messrs. 

 Johnson and Blake,''" is seen under the microscope to consist of 

 translucent or transparent, rhomboidal or hexagonal plates, which 

 are flexible and inelastic, isolated or aggregated in prismatic, curved, 

 or fan-shaped bundles, and referable to the orthorhombic system. 

 The bases of these scales are marked with lines arising from the 

 edges of super-imposed laminae. The hardness varies from that 

 of talc to about midway between that of selenite and calcite. 

 The mineral whose existence is thus rendered certain, has been 

 designated as kaolinite by these gentlemen, from the kaolin 

 in which it is most commonly found. These crystalline scales 

 are however found to occur, not only in the real kaolins, which 

 they chiefly make up, but also in small quantities in many ordi- 

 nary sedimentary fire-clays, or even in common brick-clays. In 

 these, however, they appear to be associated with other silicates 

 of alumina, or at least with an excess of silica over the amount 

 necessary to form kaolinite, which cannot be proved to exist 

 in the free state. The ordinary clays cannot therefore as yet be re- 

 garded as having a base of kaolinite. The composition of kaolin- 

 ite, Messrs. Johnson and Blake showed to be as follows: 



P. cent. 

 Silica 46 . 3 



Alumina 39.8 



Water 13.9 



100.0 



-"_" On Kaolinite and Pholerite." Am. -Jour. Sci. II. xliii. p. 3d1 et seq, as quot- 

 ed in Percy, p. 92 volume on Fuels etc. 



