The fossil alkali crystallises from a dissolved state, in combining itself with a large portion of the water, in the manner of alum; and, in this case, the water is essential to the constitution of that transparent crystalline body; for, upon the evaporation of the water, the transparent salt loses its solidity, and becomes a white powder.  If, instead of being gently dried, the crystalline salt is suddenly exposed to a sufficient degree of heat, that is, somewhat more than boiling water, it enters into the state of aqueous fusion, and it boils, in emitting the water by means of which it had been crystallised in the cold, and rendered fluid in that heated state.  It is not possible to crystallise this alkaline salt from a dissolved state, without the combination of that quantity of water, nor to separate that water without destroying its crystalline state.

But in this mineral specimen, we have a solid crystalline salt, with a structure which, upon fracture, appears to be sparry and radiated, something resembling that of zeolite.  It contains no water in its crystallization, but melts in a sufficient heat, without any aqueous fusion.  Therefore, this salt must have been in a fluid state of fusion, immediately before its congelation and crystallization.

It would be endless to give examples of particular facts, so many are the different natural appearances that occur, attended with a variety of different circumstances.

There is one, however, which is peculiarly distinct, admits of sufficiently accurate description, and contains circumstances from which conclusions may be drawn with clearness.  This is the ironstone, which is commonly found among the argillaceous strata, attendant upon fossil coal, both in Scotland and in England.

This stone is generally found among the bituminous schistus, or black argillaceous strata, either in separate masses of various shapes and sizes, or forming of itself strata which are more or less continuous in their direction among the schistous or argillaceous beds.

This mineral contains, in general, from 40 to 50 per cent. of iron, and it loses near one third of its weight in calcination.  Before calcination it is of a grey colour, is not penetrable by water, and takes a polish.  In this state, therefore, it is perfectly solid; but being calcined, it becomes red, porous, and tender.

The fact to be proved with regard to these iron-stones is this, That they have acquired their solid state from fusion, and not in concreting from any aqueous solution.

To abridge this disquisition, no argument is to be taken from contingent circumstances, (which, however, are often found here as well as in the case of marbles); such only are to be employed as are general to the subject, and arise necessarily from the nature of the operation.

It will be proper to describe a species of these stones, which is remarkably regular in its form.  It is that found at Aberlady, in East Lothian.