Before closing this subject there is one matter nearly connected with welding which has not received that careful attention that it demands, and which future interests will require - the selecting of iron suitable for welding properly together; not that there is any difficulty in welding Cleveland iron, for it is remarkable for the excellence of that quality, yet there are few districts which produce iron that is more laminated. Perhaps this may, in a measure, be the result of the prosperity of the past few years, which has prevailed in the iron trade, when quantity and not quality was the great desideratum. In welding hard and soft irons together, the difficulty is to get a heat suitable to both, as it is difficult to define the exact temperature for iron in the welding state, for it differs materially according to the different degrees of quality of iron. The amount of heat which a hard pure iron would absorb before arriving at a proper viscous or pasty condition would be sufficient to destroy a soft impure iron by burning. Iron may be welded at different degrees of heat, varying in colour from a greasy yellow up to a white heat, and if heated beyond this point it becomes burnt, through not being fusible when in an uncombined state.

Heat has great influence on iron in altering its condition. A high heat will change a fibrous to a crystalline iron, whilst a low welding heat will allow it to retain its fibrous character. Irons in a welding state possess great affinity or attraction for each other, and this is manifested in a greater or lesser degree according to uniformity of quality. If 2 pieces of iron are laid together in a welding condition, they readily stick to each other, and, if the surfaces are of moderate extent, it requires some force to pull them asunder. A striking - proof of this attraction may be seen in any forge, in the piling of very large masses of scrap containing thousands of pieces, which are heated to a welding state, and then brought out of the furnace, and held in suspension by the middle, between the points of a pair of tongs, and though it may weigh 2 or 3 cwt. there is no difficulty in transmitting it from the furnace to the anvil, during which time the particles composing the mass are held together by atomic attraction, but some mechanical force is necessary to bring the particles into closer metallic contact. The difficulty is not so great in welding hard and soft irons together as in keeping them together after they are welded.

In a sample containing 5 different kinds of iron, of varying degrees of quality, the welds, so far as can be judged from appearance, seem to be perfect, yet if this sample had. .much work out on it, if. it were upset under the steam - hammer, the harder knots would separate from the softer, their structural forms are so different. The fine crystalline form of the Low Moor iron cannot he thoroughly incorporated with the open molecular structure of the common Cleveland. Much of the defective welding found in Cleveland iron is due to this cause: we find on examination a layer of crystal and another of fibre in regular succession throughout the commoner kinds of iron. A low heat is adopted in its manufacture, purposely to retain its fibrous character; the result is the lamination spoken of. This evil is not confined to the manufacture of bars only; the rail trade has suffered from the same cause. Welding, properly performed, is neither a soldering nor gluing process. Neither of those words is applicable to the process.

It is possible to get a near approach to complete metallic contact by welding; but as the conditions are so varied by reason of the different chemical combinations in iron, it is impossible, in the present state of metallurgical science, to lay down any fixed rules; therefore, the skill and observation of the workman must supply this want, and be constantly directed to those affinities and combinations which are constantly taking place in all metallurgical operations, under fixed though perhaps undefined laws, which govern the results, and give good or bad work in proportion to the extent in which they are regarded or neglected. (Eng. Mech.)