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the thickness of a shilling, all over the woody part with the bark ; 
the marks of the axe also remaining very conspicuous, with this 
petrified crust upon it. By what means it should thus happen can- 
not well be conceived, in regard there is no water near it, the part 
above the ground, and out of the weather ; the tree yet growing ; 
unless being cut at some season, when the sap was flowing, the 
oozing of the sap might become petrified by the air, and the tree 
grow rotten and hollow inward since that time ; which how long since 
is not known. 
‘‘ A piece of that part cut was presented, together with this 
account, to the Royal Society, for their repository*.” 
Aluminous fossil wood is mentioned by several authors : Mr. Walch 
describes it as being of a brown colour, of a light weight,^ and pos- 
sessing sufficient of its vegetable structure to demonstrate its origin ; 
he also relates, that when considerable quantities of it are exposed 
to the air, that it will take fire spontaneously. Besides being found 
in various parts of England, it is found in considerable quantities 
at Commadau and Altsattel, in Bohemia : near Hainfield, in Lower 
Austria ; at Weisner, in Hesse ; and near Duben, in Saxony. 
The spontaneous inflammation of this species of wood, and the 
use to which it is applied, the fabrication of alum, manifest that it is 
a pyritous wood blended with clay. The chief component parts of 
alum (the sulphate of alumine) are the acid of sulphur and the earth, 
alumine ; the former of which is derived from the sulphur contained 
in the pyrites ; and the latter from the clay, of which it constitutes 
by far the greatest part. 
What other earths enter into the composition of fossil woods, and 
in what proportion, must be determined by future analyses. 
Yours, &c. 
* Philosophical Transactions, No. 19, Nov. 1666. 
