least affecting its figure or dimensions. It then proceeds gradually, 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 laminæ, 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.[1] In the interior of these prisms the galena is in a state of confused crystallization with small laminæ, frequently lying in different directions, so that the fracture, which is irregular and granular, and has no resemblance to what we should expect in sulphate of lead or phosphate of lead, exhibits nothing but shining laminæ 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 conceive
- ↑ In such prisms of phosphate of lead as have passed entirely into the state of galena, we also very frequently observe concentric hexagonal lamina, the sides of which are parallel to the faces of the prism, and which sometimes even leave intervals between them. This observation alone would lead me to doubt, whether this substance actually. has for its primitive crystal a pyramidal dodecahedral 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 inclined to believe, that this prism is itself the form of the primitive crystal.
