436 Mr. R. A. Couper on the Chemical Composition of 



sited, are supposed to have been produced in a similar manner 

 to the china clay ; the rains having washed from the hills the 

 decomposed rock into a lake or estuary, where it has subsided 

 and gradually displaced the water, and become in the course 

 of time perfectly firm and solid, forming fields of clay. The 

 clay is found in layers or strata lying over each other, each 

 layer possessing some distinctive property from the othei*, 

 which renders each clay fitted for a peculiar purpose. 



2. Sandy clay (stiff or ball) is the upper layer of clay, and 

 is used by itself for making salt glazed ware ; it is well adapted 

 for this kind of ware, in consequence of the considerable quan- 

 tity of silica or sand which it contains. By analysis of this 

 clay, I found it to be composed of — 



Silica 66-68 



Alumina 26*08 



Protoxide of iron .... 1 "26 



Lime 'S* 



Magnesia trace 



Water 5'1* 



100-00 

 being previously dried at 212°, specific gravity = 2-558. 



3. Pipe clay is the second layer, which is used in making 

 tobacco pipes. This clay is not employed in manufacturing 

 earthenware, owing to its possessing the property of contract- 

 ing more than sandy clay. It was analysed by Mr. John 

 Brown, who obtained — 



Silica 53*66 



Alumina 32-00 



Protoxide of iron .... 1*35 



Lime '40 



Magnesia trace 



Water 12-08 



99-49 



4. Blue clay is of a grayish colour, and is considered the 

 best layer of clay in the whole series, owing to its burning 

 perfectly white, and approaching in character nearest to the 

 china clay. As analysed by Mr. John Higginbotham, it was 

 found to consist of — 



Silica 46-38 



Alumina 38*04 



Protoxide of iron .... 1*04 



Lime 1-20 



Magnesia trace 



Water 13-57 



100-23 



