On a i*. II-
COMPOSITION AND USE OF MINERALS.
669
securely fixed on a short cross-bearer ; in this is a notch in the upright side nearest theroller, which allows the treblct and pipe to passthrough, and also forms a hold for the steelplate through which the pipe is drawn: these steel plates or whirtles vary according tothe sizes of the pipes, and are movable at pleasure; they arc made rounding on one side,to allow a more ready exit and entry of the pipe, and diminish gradually, from the sizeof the rough cast pipe to that required. When the lead pipe is fitted to the treblet, it islaid upon the rollers on the bench, and the end of the treblet being put through the largestset of whirtles, it is hooked on to the carriage, and the whirtle lodged against the cheeksof the frame. The drum is then put into gear by means of the handles and levers, and,winding up the double chains, the pipe is drawn through the whirtle, and diminished insize as it is lengthened. After the pipe is drawn entirely through, the roller is cast off bvshifting the levers; the treblet is then unhooked from the carriage, and pushed back intoits former position, and a smaller whirtle being put on, the pipe is drawn through asbefore; the operation is repeated through smaller whirtles, until the pipe has acquiredits proper thickness: this is sometimes performed through a dozen, when it is madeperfectly even and smooth ; the elevation and two sections of the whirtles are shown abovethe machine. By this means lead pipes are drawn out in lengths of from 10 to 12 feet;after winch, by a process termed burning, they are united together; this is performed hvpassing through one pipe an iron core, which enters a few inches into the other, and asmall iron mould put together in two halves over the ends of the two pipes, which arebrought close together. Melted lead is then poured into the mould, which runs out bya bole in the bottom ; when the stream of lead has run a sufficient time to fuse the ends ofboth pipes, a slider is made to pass over the hole, and the mould being left full is sufferedto cool, when the pipe is removed.
Zinc or Spelter has a crystalline texture, is brittle at ordinary temperatures, and of abluish white colour: at 300°, it is both malleable and ductile, and at a white heat isconverted into vapour. When pure zinc is exposed to air and moisture, it acquires a dullcolour from partial oxidisement; and great electric action takes place when it is in contactwith copper, and the zinc decays in consequence. Its specific gravity is 7*, and it has agreat attraction for oxygen; the weight of a cubic foot is 439J pounds.
Oxide of Zinc is obtained by intensely heating the metal exposed to air; it takes fireat a red heat, if the air is freely admitted, burning with a very bright flame.
Zinc
.
.
- 1
-
32
80
Oxygen
-
-
- 1
-
8
20
1
40
100
Sulphurct of Zinc (Blende ) is found native, and is a brittle soft metal of a brown andblack colour; its primitive form is a rhomboidal dodecahedron, and it is a most abundantmineral. The pure metal is obtained from it by roasting the ore, and afterwards distillingit when mixed with charcoal.
Zinc, - - - - 1 - 32 - 66-5
Sulphur - - - 1 - 16 - 33*5
I 48 100*0
Carbonate of Zinc ( Calamine) .* when found crystallised, its primitive form is an obtuserhomboid.
Oxide of zinc - - 1 - 40 64*5
Carbonic acid - - - 1 - 22 - 3 5 5
1 62 100*0
Zinc is obtained from the sulphurct and carbonate; the ore w*hen broken is submitted to•a dull red heat in a reverberatory furnace, when the carbonic acid is driven off from thecalamine, and the sulphur from the blende : it is then mixed w’ith £ of its weight of powderedcharcoal, being first ground and thoroughly washed, and distilled by the application of ared heat; the metal being put into earthen pots with iron tubes cemented into the lowerparts, dipping into water, where it is collected, and afterwards cast into cakes. A barof zinc 12 indies long, and 1 inch square, weighing 3*05 pounds, expands in length at onedegree of heat ^ 55 , and melts at 648° ; it will bear, without permanent alteration, apressure on a square inch of 5700 pounds.
Zinc is used for the preservation of iron, by electro deposition. The iron is first renderedperfectly clean and free from oxide, by placing it in a bath of heated sulphuric acid andwater; then in a cold solution of sulphate of zinc. The positive pole of a galvanicbattery is attached to a zinc plate, and the negative to the iron to lie covered ; the puremetal is deposited, and the zinc and iron arc amalgamated. Wooden troughs are em-