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LETTER XXIX. 



PETRIFACTION THEORIES RESPECTING SUBSTITUTION 



ADOPTED BY WALCH, KIRWAN, DAUBENTON, FOURCROY, &C. 

 ....THEORY OF DR. HUTTON AND MR. PLAYFAIR. 



ALTHOUGH the truth of your daughter's and our friend's observa- 

 tion 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 unnecessa- 

 rily. 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 ; 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 watry, 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 a nils .rue- 



