"This Patent describes an invention for welding iron tubs for steam boilers and other purposes, and it consists of using a long fixed bar or beak iron, supported at one end, on to which a prepared iron tube at the welding heat is placed, the edges of the metal ovcelapping in order to produce a lap joint, and then the weld is produced by external mechanical pressure, which is shown to be produced by a grooved roller, situated above the end of the beak iron by drawing it off the beak iron and beneath the roller; the beak iron must not be less than half as long as the tube, and the latter is welded at two processes. This invention has come into extensive use in making tubes of large diameter of thin plate iron with lap joints."
In concluding this notice of the manufacture of wrought iron tubes, the author has to observe that the great feature of modern times in the manufacture of tubes, is the being able to dispense with all internal support, and to complete the tube by external pressure alone, such pressure acting on all points of the circumference.
The mandril was quite indispensable, when gun-barrels were forged by means of the lateral blows of band-hammers upon anvils or swages, and the idea of the necessity for the mandril has been long retained under various modifications greatly to the prejudice of the entire manufacture of wrought iron tubes, as when the mandril fits tightly it hinders the progress of the tube over it and spoils the work. The mandril when now used is only employed as a supporting instrument, one that does not fit the tube but only serves as a holder or bracket to carry the tube in its heated and flexible state, and not in any respect as a means of forming or perfecting the bore of the tube.
On this point the strongest yet clearest judgment was pronounced by Baron Parke, in the trial on Whitehouse's patent, namely, " that the great novelty is the complete circumferential pressure, with motion, leaving out the mandril or any internal support.
Another point of great nicety in the manufacture is the reverberatory furnace, which, notwithstanding its length, requires to be heated most intensely yet uniformly throughout; sometimes a blast is used, but the description of Mr. Proeser's furnace will serve as a general explanation.
The furnace requires of course to be of the full length of the longest tube, and it has a door at each end for the entry and removal of the skelp; on the one side are several stoke holes for the introduction of the fuel, which is mostly coal, sometimes coke, and in the opposite wall, beyond the bridge of the furnace, are corresponding apertures leading into a longitudinal chamber parallel with the fire, and thence into the lofty flue; the dimensions of the apertures must be determined in some measure experimentally, until the furnace burns with equal intensity throughout its length.
The time the iron is exposed to the intense heat of the furnace likewise requires careful attention, as if accidentally exceeded, the iron is entirely spoiled.
The manufacture of thin tubes has recently obtained a great impulse, from the very general adoption of the tubular system in marine boilers. These tubes are usually about one-tenth of an inch thick, and as large as three inches diameter, to adapt them to the combustion of coal, the fuel of marine engines; whereas the tubes of locomotives in which coke is always burned are of only about half the bore of those for marine boilers; the tubes for locomotives, although more generally of brass, are also made of wrought iron.
The tubular constructions of boilers present a very great fire surface, and effect a proportionate saving in the dimensions of the boiler, and consequently in the weight both of the boiler and the water contained therein. Thick tubes, from their weight, would be altogether inapplicable either to marine or locomotive boilers.
Note U, page 256. - To follow the Foot Note. (Additional remarks on the late Sir John Robison's Workshop Blowpipe.)
"Articles heated in the flame of the workshop or gas furnace blowpipe (said its inventor), preserve their polish in the same way as by Mr. T. Oldham's process, if they have been kept in the flame. As the whole of the oxygen is taken up by the hydrogen of the gas, none is left free to act on the surface of the steel, and as there appears to be a tendency to a deposition of carbon on glass rods when submitted to this flame, it may be, that this may not only have an influence in saving the steel from oxidation, but may produce some chemical effect on its composition, as workmen suppose that gravers or turning tools hardened from this heat, are more enduring than those heated in the muffle or on a bar of hot iron."
As already noticed on page 440 the workshop blowpipe is figured and described in the Mech. Mag. for 1842, page 258.
Note V, page 283. - In continuation of the article on Silver. (Amalgams used by Dentists for stopping teeth.)
Dentists employ an amalgam containing silver for stopping carious teeth, it is prepared by rubbing together in a mortar, or even in the hollow of the hand, finely divided silver and mercury, and then squeezing out all the uncombined mercury, leaving a plastic mass, which feels to grate and crepitate under the fingers. When all the unsound parts of the tooth have been carefully cut away, the amalgam is thrust into the dry cavity, that the tooth may be hermetically sealed from the air, and in the course of a few Lours the amalgam appears to crystallize, and become considerably harder than lead.
The usual mode of preparation is to dissolve the silver in muriatic acid, and precipitate it as a fine metallic powder, by stirring the solution with a rod of zinc or iron. Some dentists file part of a shilling into dust, under the impression that the copper then also employed makes the amalgam harder, others rub in with the silver a little gold leaf or platinum leaf with the same intention. Precipitated palladium forms with mercury a similar amalgam to that with silver, but with the evolution of heat at the time of combination. These alloys, which have received various high sounding names, are seldom remelted, but then resume for some hours their plastic condition.