AS 
Argil and Silex are infusible in all proportions. Kirwan, 
v. 1. p. 66, says, three parts of Argil and three parts of 
Silex with one of Magnesia form a Porcelain. Other pro- 
portions of the same substances remained as a powder, 
hardened only or formed a Glass, porous Enamel, Porcelain, 
&c. This shows the necessity of having a knowledge of 
the nice distinctions sometimes necessary in comprehend- 
ing nature. It was with a Clay of this kind that Wedge- 
wood formed his pyrometers; but it is said that after the 
mixture was exhausted he could not venture to make any 
more, as he could not find Clay that he could depend upon 
as having the same proportion of parts, and consequently 
could not produce any precisely of the same standard. 
Earths of this kind are found wherever Granite rocks 
occur in a decomposing state in greater or less abundance, 
differing in the size or quantity of the different ingredients, 
the Quartz in large or small grains, with more or less Talc, 
which holds Magnesia, and perhaps helps the decomposi- 
tion when in contact with the air, which last seems neceés- 
sary, as Granite is never considered as Kaolin till thus de- 
composed, although perhaps containing the same sub- 
stances. It is curious to remark that these substances, 
afforded by nature to snit our particular purposes, are ge- 
nerally known by some external characters which distin- 
guish them in a general way, and those used to it do not 
care to venture on any other. In this instance the substances 
may vary much, as is seen by the analysis ; and perhaps 
the less decaying rock might, by affording a more suitable 
proportion of parts, sometimes answer the purpose better, 
The decaying rock 1s, however, more easily gathered. This 
may be convenient to be known to those whose estates are 
contiguous to the other’necessaries for percelain manu- 
factories. 
The specimen figured shows irregular spots or grains of 
Quartz, with some shining greenish particles of Tale ; the 
remainder is chiefly Earthy Feldspar or Petuntse. . 
