The process of _case-hardening_, or changing the immediate surface of iron utensils into steel, is readily performed by covering their surfaces with such organic substances as contain carbon as a constituent, and then subjecting them to high heat for short spaces of time--thus the roller of a paper or sugar mill may be case-hardened by a coating of prussiate of potash, or of leather chips, and then subject the whole to high heat, excluded from atmospheric influences. By this process the gelatine and other constituents of the leather are reduced to carbon, and this enters the surface particles of the iron, converting them into steel. Many hypotheses are offered for this action, and among others, that "_the ultimate particles of matter are always in motion_," admitting the ingress of particles travelling in smaller orbits between them. The friends of this hypothesis offer as proof, that a fresh cast sash-weight when broken is a gray mass, while one taken from an old building, and broken, is beautifully crystalline, from the centre to the outside, like speculum metal. A freshly drawn piece of tin pipe when suddenly bent opposite the ear gives no crackling sound, and if broken has no crystalline structure, but if left at rest for one hour it has both. Barbers often tell us that razors get tired of shaving, but if laid by for thirty days they will then shave well. By microscopic examination it is found that the _tired razor_, from long stropping by the same hand and in the same directions, has the ultimate particles or fibres of its surface or edge all arranged in one direction, like the edge of a piece of cut velvet; but after a month's rest, these fibres re-arrange themselves heterogeneously, crossing each other and presenting a saw-like edge, each fibre supporting its fellow, and hence cutting the beard, instead of being forced down flat without cutting, as when laid by. These and many other instances are offered by the friends of the hypothesis named, to prove that the ultimate particles of matter are always in motion, and they say that in the process of welding, the absolute momentum of the hammer causes an entanglement of orbits of motion, and hence a re-arrangement, as in one piece; indeed, in the cold state, a leaf of gold laid on a polished surface of steel, and stricken smartly with a hammer, will have its particles forced into the steel so as to permanently gild it at the point of contact.