HOW TO FIND THE CRACK.--Most mechanics know that by drilling a hole at the inner end of a crack in cast metal its extension can be prevented. But to find out the exact point where the crack ends, the _Revue Industrielle_ recommends moistening the cracked surface with petroleum, then, after wiping it, to immediately rub it with chalk. The oil that has penetrated into the crack will, by exudation, indicate the exact course and end of the crack. * * * * * FAST AND FUGITIVE DYES.[1] [Footnote 1: A paper recently read before the Society of Arts, London.] By Prof. J.J. HUMMEL. As it is with many other arts, the origin of dyeing is shrouded in the obscurity of the past; but no doubt it was with the desire to attract his fellow that man first began to imitate the variety of color he saw around him in nature, and colored his body or his dress. Probably the first method of ornamenting textile fabrics was to stain them with the juices of fruits, or the flowers, leaves, stems, and roots of plants bruised with water, and we may reasonably assume that the primitive colors thus obtained would lack durability. By and by, however, it was found possible to render some of the dyes more permanent, probably in the first instance by the application of certain kinds of earth or mud, as we know to be practiced by the Maori dyers of to-day, and in this way, as it appears to me, the early dyers learnt the efficacy of what we now call "mordants," which I may briefly describe as fixing agents for coloring matters. At a very remote period therefore, I imagine, the subject of fast and fugitive dyes engaged the attention of textile colorists. Our European knowledge of dyeing seems to have come to us from the East, and although at first indigenous dyestuffs were largely employed, with the discovery of new countries many of these fell slowly and gradually into disuse, giving way to the newly imported dyestuffs of other lands, which possessed some advantage, being either richer in coloring matter, yielding brighter or faster colors, or being capable of more easy application. Thus kermes gave way to cochineal, woad to indigo, and so on. Down to about the year 1856, natural dyestuffs alone, with but one or two exceptions, were employed by dyers; but in that year a present distinguished member of this Society, Dr. Perkin, astonished the scientific and industrial world by his epoch-making discovery of the coal tar color mauve. From that time down to the present, the textile colorist has had placed before him an ever increasing number of coloring matters derived from the same source. Specially worthy of notice are the discoveries of artificial alizarin, in 1868, by Graebe and Liebermann, and of indigotin, in 1878, by Adolf Baeyer, both coloring matters being identical with the respective dyes obtained from plants.