ashes and cinders, but still the crop was extraor¬ 
dinary, except where I took ofF all the clay that 
had become red in burning, where the burning lost 
most of its effects. Another circumstance 1 obser¬ 
ved, was, that where clay ground had been broken 
up in autumn, and exposed to the winter’s frost, it 
lost much of its adhesive quality ; after which if it 
was planted or sown the ensuing season, it general¬ 
ly produced the best crops. This adhesive quality 
of clay, seems to depend upon its union in some 
way with an acid. For on being burned, this qual¬ 
ity is diminished in proportion to the degree of heat 
employed. So far as to vitrifaction, beyond which 
I can say nothing relating to it. But on being dis¬ 
solved in vitriolic acid, (sulphuric acidj and possi¬ 
bly some other acids, it recovers its ductility again. 
And there is certainly an acid emitted from it in 
burning bricks, as is evident to the smell, after a 
certain degree of heat has been communicated to 
the kilo ; but of what kind this acid may be, I can¬ 
not say, tho’ I have seen curriers learn to their 
cost, that it would stain their leather black, if by 
any means it happened to get mixed in sufficient 
quantities with the astringent juice of the oak bark 
used in tanning. I suspect, however, that clay con¬ 
tains either the sulphurous or the sulphuric acid,, 
which only differ in their degrees of oxygenation ; 
and that the quantity of acid is too great for the 
purposes of vegetation. But as some acid seems 
