FOSSIL INVERTEBRATED ANIMALS. 475 



these worms were fully authenticated by zoologists ; for, with 

 the assistance of fancy^ or preconceived theory, foreign bodies 

 might very easily be taken for them, though, certainly, there 

 would be nothing very extraordinary in the fact of their exist- 

 ence. 



The entire system of M. Patrin is briefly this : — He thinks 

 that petrifaction is a genuine transmutation of the parts them- 

 selves — of the organized body — into siliceous matter : so that 

 a body was by so much the less susceptible of petrifaction, in 

 proportion as it was more decomposed at the period in which 

 it was buried. The petrifaction took place in an almost sudden 

 manner. It must be regarded as a chemical operation, and a 

 combination of gaseous fluids with the constituent principles 

 of organized bodies : an operation which very rapidly changes 

 the latter into stony substances, without touching, in any way, 

 the arrangement of their molecules ; so that neither the forms 

 nor the colours are at all altered by this modification. 



We may form a just idea of petrifaction by comparing it to 

 congelation — with this difference, that ordinary congelation 

 takes place by the simple abstraction of caloric, whereas petri- 

 faction is a coagulation occasioned by the introduction of a 

 foreign fluid. With respect to the weight which petrified 

 bodies acquire, there is nothing in that in opposition to this 

 theory ; for we know how much density the most subtile gaseous 

 fluids may acquire when they come to be solidified ; — such as 

 oxygen, for example, when combined with metallic substances. 

 A striking instance of this may be observed in vitreous tin, 

 which is an oxide of tin without mixture of any other matter; 

 and there is only wanting 3 to 4-100 to make its specific weight 

 equal to that of the pure metal, although the oxygen alone 

 makes more than 21-100 of the mass. It is, therefore, suscep- 

 tible of condensation to the degree of acquiring a much greater 

 weight than that of any stone; and it is infinitely probable that 

 it is the oxygen which plays the principal part in the pheno- 

 menon of petrifaction, by its combination with the phosphoric 



