Formation, then, takes place by a _shooting together_, _i.e._ by a sudden solidification, not by a gradual transition from the fluid to the solid state, but all at once by a _saltus_; which transition is also called _crystallisation_. The commonest example of this kind of formation is the freezing of water, where first icicles are produced, which combine at angles of 60°, while others attach themselves to each vertex, until it all becomes ice; and so that, while this is going on, the water does not gradually become viscous, but is as perfectly fluid as if its temperature were far higher, although it is absolutely ice-cold. The matter that disengages itself, which is dissipated suddenly at the moment of solidification, is a considerable quantum of caloric, the disappearance of which, as it was only required for preserving fluidity, leaves the new ice not in the least colder than the water which shortly before was fluid. Many salts, and also rocks, of a crystalline figure, are produced thus from a species of earth dissolved in water, we do not exactly know how. Thus are formed the glandular configurations of many minerals, the cubical sulphide of lead, the ruby silver ore, etc., in all probability in water and by the shooting together of particles, as they become forced by some cause to dispense with this vehicle and to unite in definite external shapes. But also all kinds of matter, which have been kept in a fluid state by heat, and have become solid by cooling, show internally, when fractured, a definite texture. This makes us judge that if their own weight or the disturbance of the air had not prevented it, they would also have exhibited on the outer surface their specifically peculiar shapes. This has been observed in some metals on their inner surface, which have been hardened externally by fusion but are fluid in the interior, by the drawing off the internal fluid and the consequent undisturbed crystallisation of the remainder. Many of these mineral crystallisations, such as spars, hematite, arragonite, etc., often present beautiful shapes, the like of which art can only conceive; and the halo in the cavern of Antiparos[102] is merely produced by water trickling down strata of gypsum.