G66

THEORY AND PRACTICE OF ENGINEERING.

Rook 11.

ore. A large bellows throws its blast into the hearth, whilst two men working togetherstir the melted lead, and gradually add more ore. There is a small channel in the hearthin which the melted lead flows into a pot at the side of the brickwork in whicli thehearth is fixed, from which the men lift up large ladles of the metal, and pour it intomoulds. At most smelting-mills the smelters are divided into three sets of two men each,who come in turns, ten hours each set at a time, so that each works ten hours and reststwenty. Dime is sprinkled on the edge of the hearth when melted slag is running off, whichhas the effect of uniting with the slag, and converting it into a solid.

Smelting-furnace is of the same description as the roasting-furnace, already described ;the roasting and smelting are both done in one heat, which occupies about five hours,coal being mixed with the ore to smelt it. A bing of ore is roasted and smelted at oneshift; the smelting furnace, requiring more fuel than the smelting hearth, is not used wherecoals cannot be readily procured. The process of roasting being effected, the doors of thefurnace are shut, and the heat is then increased sufficiently to melt the ore.

Of the horizontal Chimneys _It is important that the chimneys of smelting mills should

carry off the effluvia, so highly injurious to the workmen. About twenty years ago hori-zontal chimneys were first used, some of which were more than 100 yards in length.The chimney at the Derwent Company’s mines is a mile from the smelting mills, andproceeds under ground the whole of the way up the side of the hill to the foot of alofty turret, carrying off the destructive smoke, but which, falling upon the ground, rendersthe grass poisonous to the horses and cows partaking of it.

To prevent the land from being injured by the smelting-hearths an arched tunnel, a milelong, is usually conducted to a chimney shaft, which at the end of the year is cleaned, andthe matter smelted, by which means a sufficient quantity of lead is obtained to remuneratethe expense of making the tunnel.

The chimney in Allendale is 3 miles in length, from which many thousand poundsworth of lead are obtained, the farthest smoke being the richest in lead. This tunnel orchimney is 3 feet wide and 6 feet high, so that it may be effectually cleansed.

Tire smelting mills arc generally placed in a low situation, on account of the water ne-cessary to turn the wheels which give motion to the blasts. It is obvious that a tunnelcarried from such a situation up the side of a hill to a great height will have a strongdraught of air, and consequently draw off the smoke and effluvia from the metal, andthe noxious matter so ruinous to the health of the workmen.

Refining the Lead and Silver _The process of refining the lead and silver depends on the

principle, that lead exposed to heat readily imbibes oxygen from the atmosphere, andbecomes oxide of lead, whilst the silver remains unaltered. It is carried on in a rever-beratory furnace, which allows the flames from the fuel to strike against the lead and silver ;the lead is converted into litharge, and the silver, formed into a plate, remains below. Beforetile metal is put into- the furnace a test is made from the ashes of bones or those of fernsor brakes (Pteris aquilina). This plant, when burnt, yields a vegetable alkali, or potash,which constitutes its value as a test. A mixture is made of the bone and fern ashes, beatup with water, and afterwards moulded into an oval form, and placed within an iron framein the furnace, with the pig of lead upon it: some of the litharge is absorbed by thismixture, and its quality tested. The flames change the lead into a semi-vitrified oxide orlitharge, the melted lead abstracting oxygen from the air. On one side is an opening, andat the other the blast of a large bellows is introduced, which blows the litharge from thesurface, and occasions it to fall through an opening into an iron vessel placed to receive it,which, when nearly full, is removed, and another put in its place. The test absorbs intwo or three days such litharge as is below the silver, with a part of the silver.

Of the Separation of Lead and Silver . — The present mode of separating these two metalswas discovered by Hugh Lee Pattinson, of Alston, an agent employed by the trustees ofGreenwich Hospital to test the lead paid to them as their royalty, to ascertain the quantityof silver it contained, and to determine its value. In the course of conducting his opera-tions he observed that part of the lead crystallised before the rest, which induced him toattempt to discover the cause; and on analysis he found that the portion which continuedlongest liquid contained a larger proportion of silver.

The operation, as now practised, may be thus described. Three cast-iron pots arc setin brick along the middle of the chamber; when the lead is melted in pot No. 1., it isstirred with an iron rod, and every now and then the lead which adheres to the rod is removedwith a great hammer. On the other side a man with another rod, at the end of which is aladle full of holes, dips it into the pot of melted lead No. 1., and, pressing it on the edgeof the pot as a fulcrum, raises up the ladle nearly full of lead, curled, crisping, and frosted,which runs out in a liquid metal from the holes in his ladle. This is held above thesurface, and shaken till no more metal will run out, and then the lead is emptied into potNo. 2. This operation is continued until there be very little liquid metal left in pot No. 1.,if there be a breeze of wind through the room, the lead cools faster, and the work goes onmore rapidly. The metal in pot No. 1. is then brought into moulds and cast into pigs,