A stronger proof however in favour of my assertion, is what follows:--The Greeks and the Romans speak of no other than natural alum; but our alum is seldom produced spontaneously in the earth, and several of our most accurate mineralogists, such as Scopoli and Sage, deny the existence of native alum[491]. Crystals of real alum are formed very rarely on minerals which abound in a great degree with aluminous particles, when they have been exposed a sufficient time to the open air and the rain; and even then they are so small and so much scattered, that it requires an experienced and attentive observer to know and discover them. The smallest trace of alum-works is not to be found in the ancients, nor even of works for making vitriol (sulphate of iron), except what is mentioned by Pliny, who tells us that blue vitriol was made in Spain by the process of boiling; and this circumstance he considers as the only one of its kind, and so singular, that he is of opinion no other salt could be obtained in the same manner[492]. Besides, everything related by the ancients of their alum agrees perfectly with native vitriols: but to describe them all might be difficult; for they do not speak of pure salts, but of saline mixtures, which nature of itself exhibits in various ways, and under a variety of forms; and every small difference in the colour, the exterior or interior conformation, however accidental, provided it could be clearly distinguished, was to them sufficient to make a distinct species, and to induce them to give it a new name[493]. The celebrity which the ancient alum had, as a substance extremely useful in dyeing and medicine, was entirely forgotten when the alum of the moderns became known; but this celebrity was again revived when it was discovered that real alum could be often made from minerals containing sulphur compounds; or that where the latter are found there are generally minerals which abound with it. In many of these places alum-works have in the course of time been erected; and this circumstance has served in some measure to strengthen the opinion that the alum of the ancients and that of the moderns are the same salt; because where the former was found in ancient times, the latter has since been procured by a chemical process. Some historians of the fifteenth century even speak of the alum-works erected at that period, as if the art of making this salt had only been revived in Europe.