Phcnomeiia in the Solar System* 449 



The vitreous state of a body is nothing more than a fixing of 

 its molecules in the positions which belong to them in the liquid 

 •state, and probably represents the liquid in its greatest degree 

 of density. The crystallization of barley sugar, of wrought iron- 

 and of Reaumur's porcelain, are striking examples of the ten- 

 dency of molecules to group themselves in crystals, even in the 

 midst of solid masses, and we can thus readily understand the 

 absence of vitreous substances among the older crystalline rocks. 

 The great difficulty is to determine with exactness the propor- 

 tion of the vacant spaces resulting from this change, since these 

 will vary for each body, and probably also with the volume of 

 the mass. Sul^^hur fused in an open vessel crystallizes slowly, 

 the level of the liquid sinks a little, and after complete solidi- 

 fication, the surface is covered with hollows resulting from the 

 shrinking, whereas if cooled in a spherical shape these cavities 

 would naturally be formed at the centre. Water and bismuth, 

 as is well known, behave in a very different and remarkable 

 manner, the first dilating eight or ten hundredths at the mo- 

 ment of congelation, and the second one fifty-third. The only con- 

 clusion to be drawn from these facts is, that each body in solidifi- 

 cation behaves in a different manner, and that for the solution of 

 the question before us, we can only take into account the well 

 known porosity of rocks. The problem, however, appears to me 

 one of great importance in connection with theoretical geology ; 

 if we admit with Deville, that at the moment of crystallization, the 

 density of rocks is in all cases augmented, we are forced to con- 

 clude that all the crystalline masses formed at the surface of the 

 liquid globe must have sunk and accumulated at the centre. The 

 eS'ect of a similar action has been shown by physicists, who have 

 demonstrated that the cold of winter would freeze our lakes and 

 rivers from the bottom if the ice sunk at the moment of its for- 

 mation, as would the solidified parts of a lake of molten sulphur. 

 We should then have in place of a liquid globe surrounded by a 

 solid shell, a mass solidified to the centre, a conclusion which is per- 

 haps more in harmony with the feeble and local action which the in- 

 terior is known to exert on the surface. Since then the data are 

 wanting to fix the amount of shrinking in the crystallization of 

 rocks, we may find in an analogous phenomenon some terms of 

 comparison. The difference between the density of cast metals, 

 and the same after hammering, can only arise from a contraction 

 similar to that which takes place in igneous rocks. The surface 



Can. Nat. 3 Vol. VI. No. 6. 



