SORBY STRUCTURE OF CRYSTALS. 467 



so common in many slags of iron-furnaces, as shown by fig. 48 ; and 

 in some crystals of pyroxene in the slag of the blast-furnaces at 

 Masborough, good examples of stone-cavities are found, one of which 

 is represented in fig. 47. 



The connexion between the form of the crystals and that of the 

 cavities is precisely the same in the case of glass- and stone-cavities 

 as of fluid-cavities. Thus, in fig. 45, four, and in figs. 41 and 46 

 two straight sides are directly connected with planes of the crystals, 

 whilst in figs. 42, 44, 47, and 48 they are curved and scarcely in 

 any way related. There is thus a most perfect analogy between 

 glass- and stone-cavities and fluid-cavities, in every respect except 

 the nature of the included substances. These, however, differ from 

 one another as much as the fusing-points of the liquids from which 

 the crystals were deposited, and as the two processes of igneous 

 fusion and aqueous solution ; and are so essentially connected with 

 those processes as to point out most clearly how the crystals were 

 formed. 



When a perfectly pure homogeneous substance crystallizes on 

 cooling from a state of fusion, of course no cavities like those just 

 described can be formed ; but some substances in passing from the 

 liquid to the solid state give off gas that was soluble in them when 

 liquid, but cannot be dissolved by them when solid. This fact is 

 well illustrated by the freezing of water, the bubbles enclosed in ice 

 being gas-cavities produced in this manner by the cooling and 

 solidification of a substance fusing at a low temperature. When 

 heated to the melting-point, the ice thaws round about these cavities, 

 as described by Dr. Tyndall (Proceedings of the Royal Society, 

 vol. ix. p. 76) ; and they then contain both water and a bubble, like 

 the fluid-cavities in crystals deposited from solution in water at an 

 elevated temperature. It is, however, merely a deceptive analogy, 

 without any true affinity. The real relationship is to what would 

 in all probability occur if crystals formed by the cooling of a sub- 

 stance that becomes solid at a high temperature were heated to the 

 point of fusion, when probably it would fuse first round about the 

 few gas-cavities which such crystals often contain. 



The most important result that could be produced by the opera- 

 tion of great pressure, in the formation of crystals by igneous fusion, 

 is what might occur if it were so great as entirely to counteract the 

 elastic force of the vapour of water, and permit it to be present in a 

 liquid state along with the melted stony matter. It would be ex- 

 tremely difficult to prove by actual experiment what is the structure 

 of crystals formed under such conditions ; but I think the general 

 principles derived from the experiments already described enable us 

 to form a very satisfactory conclusion on the subject. Some crystals 

 might be deposited from solution in the highly heated water, and 

 catch up small portions of the fused stone, whilst others might be 

 formed by the crystallization of the melted stone, and catch up small 

 portions of the liquid water. In both cases the characteristic struc- 

 ture would be the presence in the same crystal of the peculiarities of 

 crystals formed from a state of igneous fusion, combined with the 

 peculiarities of those deposited from solution in water ; and it might 



