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the Truth of this Syftem; which may alfo extend 
perhaps to many other Hills as well as this. 
As the Workmen are now obliged to dig this 
Loam at 26 Feet deep, inftead of about 14, at which 
Depth they long found it, and muft hereafter, as they 
are obliged to afcend the Hill, dig it at 38 or 40 
Feet, the Price, of it will, I am afraid, rob us of 
it, before the Vein is exhaufted. I think it would 
be a Matter worthy Confideration, whether, from 
examining the Parts it is compofed of, a Succeda- 
neum might not be found for it, by an artificial 
Mixture of fimilar Subfiances. In order to attempt 
this, I have, by means of Water, difunited its Parts, 
and procured them feparate; and, on comparing 
them with the various Earths and Sands from different 
Parts of England , which I have at times procur’d, I 
think that I can exa&ly match the Sand with one 
from Hampftead'Heath, and the Clay with one 
from a Pit near the lower End of Highgate p The 
Proportions may be eafily learned, by accurate Ob- 
fervation of the Quantities of each, where difunitedj 
and a Succedaneum on thefe Principles eafily made. 
I intend to attempt it, and recommend the Trial of 
it to Dr. Mortimer , who is well acquainted with 
the genuine, in Chemical and Metallurgic Opera- 
tions, in both of which he is very converfant 5 and 
I hope to find it equally ferviceable. 
It is evident to me, that the only Reafon why it 
endures the Fire fo much better than other Clays, is 
the extreme Hardnefs and great Quantity of the 
Sand it contains : And as I imagine it eafy to throw a 
Sand of equal Hardnefs, and in equal Quantity, into 
an 
