We have said that the grinding room is the _only_ dangerous room in such an establishment as this. There is one other process than grinding which was formerly considered as extremely unhealthy, and that is the process of polishing. The polishing of steel is performed by means of what are called _emery wheels_, which are wheels bound on their circumference by a band of leather, to which a coating of emery, very finely pulverized, is applied, by means of a sizing of glue. These wheels, a large number of which are placed side by side in the same room, are made to revolve by means of machinery, with an inconceivable velocity, while the workmen who have the polishing to do, taking their stations, each at his own wheel, on seats placed there for the purpose, and holding the piece of work on which the operation is to be performed, in their hands, apply it to the revolving circumference before them. The surface of the steel thus applied, receives immediately a very high polish--a stream of sparks being elicited by the friction, and flying off from the wheel opposite to the workman. Now although in these cases the workman was always accustomed to take his position at the wheel in such a manner as to be exposed as little as possible to the effects of it, yet the air of the apartment, it was found, soon became fully impregnated with the fine emery dust, and the influence of it upon the lungs proved very deleterious. There is, however, now in operation a contrivance by means of which the evil is almost entirely remedied. A large air-trunk is laid beneath the floor, from which the air is drawn out continually by means of a sort of fan machinery connected with the engine. Opposite to each wheel, and in the direction to which the sparks and the emery dust are thrown, are openings connected with this air-trunk. By means of this arrangement all that is noxious in the air of the room is drawn out through the openings into the air-trunk, and so conveyed away. The sparks produced in such operations as this, as in the case of the collision of flint and steel, consist of small globules of melted metal, cut off from the main mass by the force of the friction, and heated to the melting point at the same time. These metallic scintillations were not supposed to be the cause of the injury that was produced by the operation of polishing, as formerly practiced. It was the dust of the emery that produced the effect, just as in the case of the grinding it was the powder of the stone, and not the fine particles of iron.