440 Mr. II. A. Couper on the Chemical Composition of 



of the colouring agent with a quantity of clay slip ; whilst the 

 two last-mentioned dips are mixed with water to produce the 

 slip state, in which state they are employed. 



V. There are several kinds of bodies manufactured ; but 

 they may be all classed under two heads, viz. porcelain and 

 earthenware. 



1. Porcelain or china is a rich, very smooth and transpa- 

 rent ware, and is the finest quality that has yet been manu- 

 factured. It is a fused body, and owes its transparency to 

 this circumstance ; it also requires a very high temperature to 

 burn it, and is manufactured in this country from flint, Cor- 

 nish stone (granite), china clay, and bone-earth ; the lime 

 employed acting as a flux, partly fusing it. By analysis of 

 two pieces of china from different manufactories in Stafford- 

 shire, I found them to be differently composed. The last of 

 these pieces was also analysed by Mr. Crichton, the three 

 analyses being as follows: — 



No. 1, by R. A. C. No. 2, byR. A. C. No. 2, by W.C. 



Silica 39-88 



Alumina .... 21*48 



Lime 10'06 



Protoxide of iron ~\ nc.AA, 



Phosphate of lime J 



Magnesia 



Alkali or difference 2* 14 



100-00 100-00 100000 



2. Foreign manufacturers do not employ bone-earth ; but 

 instead of it they use felspar, the alkali of which supplies the 

 place of the phosphate of lime. The Germans make the best 

 porcelain for chemical purposes, as that body is more vitrified 

 and less liable to be acted upon by acids, as well as being 

 capable of standing a very strong heat; and hence it is exten- 

 sively used by chemists. By the analysis of some specimens 

 of foreign porcelain, I obtained the following results: — 



Berlin. 



Silica 72-96 



Alumina and protoxide of iron 24-78 



Lime 1-04 



Alkali 1-22 



100-00 



Specific gravity 2-419 



VI. Earthenware is a very porous and less compact body 

 than china or porcelain, owing to its containing little or no 

 alkali, which is the great difference between these bodies. I had 

 a piece of ware manufactured, resembling in appearance porce- 

 lain, as regards the absence of porosity and its compactness, 



