Count de Bournon on BardigUone. 381 



least affecting its figure or dimensions. It then proceeds graduallr, 

 till the decomposition and regeneration are complete. Specimens of 

 pyrites are found, in which only the surface to a very slight depth is 

 in the state of hepatic iron, as well as prisms of phosphate of lead, 

 which are precisely in the same circumstances. We find also crystals 

 of these two substances, in which, though the centre has participated 

 in the same decomposition and regeneration, particles of greater or 

 less bulk, that are nowise altered, remain interspersed here and there 

 in the regenerated substance. In phosphate of lead, which has passed 

 into the state of galena, we frequently observe one or more laminae, 

 of different degrees of thickness, parallel to the planes that form the 

 exterior surface of the hexahedral prism, which have still retained their 

 primitive form.* In the interior of these prisms the galena is in a 

 state of confused crystallization with small laminse, frequently lying 

 in different directions, so that the fracture, which is irregular and 

 granular, and has no resemblance to what we should expect in sul- 

 phate of lead or phosphate of lead, exhibits nothing but shining 

 lamina of galena without any determinate direction. Frequently 

 too we observe that in the two transitions of which I have been 

 speaking, when they are completed, there are several small cavities, 

 in which the decomposed substance has not been replaced. 



In the two examples quoted, though a perfectly exact explanation 

 of the means employed by nature is very difficult, yet we can con- 



* In such prisms of phosphate of lead as have passed entirely into the state of galena, 

 we also very frequently observe concentric hexagonal laminae, the sides of which are 

 parallel to the faces of the prism, and which sometimes even leave intervals betweea 

 them. This observation alone would lead me to doubt, whether this substance actually 

 has for its primitive crystal a pyramidal dodecahedron with triangular faces, as has been, 

 supposed. If to this we add the indications of natural joints parallel to the bases of the 

 hexahedral prism of phosphate of lead, which I have often observed, I am strongly ia*. 

 clined to believe, that this prism is itself the form of the primitive crystal. 



