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LETTER XXIX. 
PETRIFACTION THEORIES RESPECTING SUBSTITUTION ADOPTED 
BY WALCH, KIRWAN, DAUBBNTON, FOURCROY, &C THEORY OF 
DR. HUTTON AND MR. PLAYFAIR. 
Although the truth of your daughter’s and our friend’s observation 
must be admitted, that you have been detained a long time from 
the consideration of the grand object of your inquiry, the nature 
and formation of 'petrifactions ; yet I will not, for a moment, admit 
the justice of the charge, that this has been done unnecessarily. 
On the contrary, I flatter myself that it will appear, that the 
process of bituminization, on which we have so long dwelt, has 
been the preparatory process to that of petrifaction, in most of the 
vegetable substances which have undergone this species of change. 
But, as the opinion I propose to offer, respecting the petrifaction of 
vegetable bodies, differs materially from the theories which have 
been framed by those, who are deservedly considered to be the first 
authorities in inquiries of this kind, I think it necessary, in the first 
place, to give you a sketch of those theories j and then to lay before 
you that hypothesis, which, according to my judgment, accords best 
with the several phenomena which these bodies yield. 
The earliest attempt to account for the petrifaction of wood, on 
chemical principles, proceeded on the idea, that the fixed, earthy, 
parts of the wood, deprived of their watery, oily, and volatile parts, 
on being penetrated by the lapidific fluid, would arrest the stony 
particles, and thereby so secure their arrangement, that the sub- 
stance thus produced should exactly represent the form and struc- 
