Salts, bitumen, oil, and the grease of the sea, enter little or none into the composition of the shell; neither does the calcareous stone contain any of those matters; this stone is, therefore, only water transformed, joined to some little portion of vitrifiable earth, and to a great quantity of fixed air, which may be disengaged by calcination. This operation produces the same effect on the shells taken in the sea as upon those drawn out of quarries; they both form lime, with only a little difference in their quality. Lime, made with oyster or other shells, is weaker than that made with marble or hard stone; but the process of Nature is the same, as are the results of its operation. Both shells and stones, lose nearly half their weight by the action of fire in calcination; the water issues first, after which the fixed air is disengaged, and then the fixed water, of which these stony substances are composed, resumes its primitive nature, is elevated into vapours, drove off and rarefied by the fire, so that there remains only the most fixed parts of this air and water, which, perhaps, are so strongly united in themselves, and to the small quantity of the fixed earth of the stone, that the fire cannot separate them; the mass, therefore, is reduced nearly a half, and would probably be still more if submitted to a stronger fire. And what appears to me to prove that this matter, driven out of the stone by the fire, is nothing else than air and water, is the avidity with which calcined stone sucks up the water given to it, and the force with which it draws water from the atmosphere. Lime, by exposure either in air or water, in a great measure regains the mass it had lost by calcination; the water, with the air it contains, replaces that which the stone contained before. Stone then retakes its first nature, for in mixing lime with the remains of other stones, a mortar is made which hardens, and becomes a solid substance, like those from which it is composed. Thus, then, we see on the one hand all the calcareous matters, the origin of which we must refer to animals; and on the other, all the combustible matters proceeding from animal or vegetable substances; they occupy together a great space on the earth; yet, however great their number may be, they only form a small part of the terrestrial globe, the principal foundation of which, and the greatest quantity consists in one matter of the nature of glass; a matter we must look upon as terrestrial element, to the exclusion of all other substances, to which it serves as a base, like earth, when it forms vegetables by the means, or remains of animals, and by the transformation of the other elements; and it is also the ulterior term to which we can return or reduce them all.