370
peculiarity of structure of the wood. This was an opinionfraught with so many obvious difficulties, that another modeof explanation was soon adopted.
“ The next theory which was proposed with the hope ofaccounting for these changes, was that of substitution ofstony in the place of organized matter, which was sup-posed by many of those who adopted this opinion to beentirely removed. Thus Berthollet says, ‘ petrifaction isnot a metamorphosis into stone, but a removal by petrifac-tion, or a precipitation of the vegetable or animal matter,in the same manner as happens in metals: copper being,for instance, sometimes thus substituted in the place ofiron.’ Mr. Watch also, in the supplementary part ofKnorrs splendid work on this subject, declares his adop-tion of this opinion to its fullest extent. ‘ Different speciesof petrified wood,’ he says, ‘ teach us, that all the particlesof any extraneous body may successively disappear, and bere-placed by foreign heterogeneous particles, with so muchregularity, that the body which is produced from them, maypossess perfectly the form of the original body, preserving-all its characters, and at the same time not retaining any(aucune de toutes) of its original particles.’ ” 36
Mr. Kirwan believed petrifaction to be performed thus,to which process he also applies the term substitution,describing it as—“ The introduction of stony, and some-times of metallic substances, into organic bodies, whetherof vegetable, or of the animal kingdoms, in proportion asthe particles of these organic substances are destroyed byputrefaction, so as to assume the place, and consequentlythe form and figure of these, as if cast in the same mould.The mineral substances thus moulded, are in the mostperfect sense of the word called petrifactions .” 37
Mons. Daubenton, is of opinion, that petrifaction onlytakes place in those bodies, whose composition is such, thata part of them already possess a stony hardness, for in-stance, such as the crustaceous coverings of certain animals,as shells, bones, scales, madrepores, &c. In substanceslike these, which he considers of an analagous nature, liethinks the process of petrifaction can take place, which hethinks is performed by the following operation:—" Beinglong left in an humid situation, the earthy part will separate