wrinkled than crystalline, and with variegated colours,light blue, violet, and golden yellow. "3. Tin 50, Zinc 50.-Texture, pallid white; surface of the ingot is very smooth, granular, and lamellar, without the appearance of shrinkage; the edges are somewhat round, and do not show plainly the iridescent colours; the fracture is bright and finely granular, upon a tin-white ground; and clogs the file a little; the alloy is well mixed, tough, and malleable, without being soft. "4. Tin 70, Zinc 30.-Texture white; and somewhat shining; exhibits no settling; feebly sonorous; surface granular, and dead white with slightly yellow spots; difficult to break; bears hammering well; easily worked with the chisel, which takes off long chips; it clogs the file; the fracture, like that of tin, is without brightness and crystallisation; when polished it is less bright than tin; the alloy is more complete and uniform in texture than the preceding ones. "5. Tin 75, Zinc 25.-Texture tin-white, but without brightness; exhibits no settling; surface granular and studded with bright particles; the upper surface has a tint changing from yellow to reddish-blue; clogs the file more than No. 4; very malleable, although resisting the hammer and chisel more than No. 4; bends without the crackling sound of tin. "6. Tin 10, Zinc 90.-The bar shows at the fracture the characteristics of a zinc bar; clogs the file more than zinc ; the fracture is not of so dull a gray; the bottom of the button is soft and easily receives the impression of a punch; like No. 2, tin appears to settle, and the metal at the bottom is softer than pure tin. "7. Tin 90, Zinc 10.-The bar presents the jagged fracture of tin, and the runner could only be separated by cutting it; the alloy clogs the file less than pure tin; the button settles sensibly in the middle, although the edges are sharp; the alloy is very malleable, although not very soft under the hammer. "8. Tin 1, Zinc 99.-The fracture is like that of zinc, but the facets are less; lustre is slightly brighter after filing; middle of the bar had settled; the button also settles in the middle, and the lower part was soft like No. 6, although not so thick, on account of the small proportion of tin in the alloy; the soft portions are bluish like lead, and are easily streaked by the nail. “9. Tin 99, Zinc 1.-The fracture is slightly granular, and not so dull and jagged as that of tin; when polished is not so bright as tin; there is more shrinkage on the ingot than on the button, and the surface of the latter presents dark iridescent colours. "General Observations.-The alloys where the proportion of zinc is the greatest, present in their fracture a crystallisation whose facets shine like graphite. Very small proportions of tin added to zinc cause this crystallisation. In similar circumstances the exterior of the castings is covered with a yellowish-white moiré. "In thick castings, where zinc predominates, there is a tendency for the metals to separate at the bottom of the mould; and, what is remarkable, this tendency grows greater as the proportion of tin becomes smaller, which is exemplified by the separation being more sensible in No. 8 than in No. 6. We may add as a singular anomaly that the tin which has passed through the zinc, and has become precipitated, loses its distinctive qualities and acquires the softness and bluish dull colour of lead. "The colour of the alloy of zinc and tin, whether simply cast, or after being filed, becomes brighter in a direct ratio with the proportion of tin contained in it. "The alloys already rich in tin become granular when the proportion of zinc is increased. The alloy No. 3—tin 50, zinc 50-has the fracture of iron, but its colour is duller. "The alloy No. 9-tin 99, zinc 1-has a fracture presenting no longer the jagged appearance of tin, and is dull-gray and finely granular in structure. "The specific gravity of the alloys of tin and zinc is in proportion to the mean specific gravity of the two metals, therefore the alloys where tin predominates are more dense. "The waste is greater where zinc is in excess; the tin having been put into the crucible after the fusion of the zinc, we infer that most of the waste comes from the zinc. 66 The addition of 1 per cent of tin to zinc is sufficient to impart to the latter metal a greater resistance without diminishing its hardness. 1 per cent of zinc added to tin impairs the flexibility of the latter, and, what is remarkable, prevents its peculiar crackling noise. These two alloys when the combination is intimate, present no other sensible changes. "The alloy of tin 50, zinc 50, is the best as regards stiffness and economy. More zinc would produce an alloy not so well mixed, more crystalline, and brittle; more tin would give a metal too soft and clogging the file. However, for thin and resisting castings an alloy of tin 70, zinc 30, is well adapted. The alloys kept between these figures and the proportion of half and half are very resisting and tenacious. Their malleability increases with the proportion of tin. "The alloy of zinc 1, tin 99, without impairing the malleability of the latter metal, increases its hardness and tenacity for castings. "The alloys where the maximum of zinc is employed are useful in foundries only for thick pieces; they are then very economical. Up to the portions of tin 30, zinc 70, they remain nearly as brittle as zinc itself. The proportion of tin 25, zinc 75, produces an alloy not so flexible as and less brittle than zinc, which could be adopted for foundry patterns. "The alloys Nos. 6 and 8 appear to us more brittle than zinc. In those experiments tin passing through the molten mass into the mould had become precipitated to the bottom. We may infer from this that a quantity of tin, sensibly less than 1 per cent, is sufficient to change the nature of zinc. "In the proportions of tin 40, zinc 60, the alloy possesses but little malleability." An alloy of 11 parts tin to 1 part zinc, beaten out into leaf, forms spurious silver-leaf. According to Rudberg ZnSn, (1 part zinc to 10 parts tin) solidifies completely at 204° C., but all the other alloys separate on cooling from a state of fusion into two portions, the one consisting of ZnSne, not solidifying till cooled to 204° C., while the remainder, consisting of an alloy containing a larger proportion of one or the other metal, solidifies at a higher temperature: thus :- Variable point Sn12Zn SngZn Sn4Zn SngZn SnZn SnZn ... 210 A metal, now largely employed for buttons, is prepared by rolling a thin sheet of tin on each side of a thick sheet of zinc. The metals become firmly welded together, and the pressed work has the advantage of a coating of tin. § 96. Tin and Lead. These metals are easily melted, and unite together in all proportions, forming a series of valuable alloys. Lead leaves a dark mark when drawn across paper, and when only a limited quantity of tin is alloyed with it, this property is still retained; but if a certain limit be exceeded, the alloy no longer has the property of producing the dark mark; hence it is possible to roughly estimate the quantity of the lead present by this test, taken in conjunction with the behaviour of the metal under the hammer, file, and chisel. Tin 90 parts, and lead 10 parts, does not streak paper. Between Tin 75, lead 25, gives a very faint mark. these limits no mark can be observed when the alloys are drawn across paper. All lead-tin alloys containing less than 75 per cent of tin have the power of marking paper. Alloys of lead and tin shrink, or settle less on cooling than either of the metals taken singly; they are not so fluid when melted, and the castings have not the same sharpness. The effect of lead on tin is to increase its malleability and ductility, but to diminish its tenacity and toughness. In the alloy, tin 90, lead 10, tin preserves the crackling noise, but in a less degree than in pure tin. On the contrary, 1 per cent of zinc in tin is sufficient to destroy the crackling noise when the metal is bent. The following table contains the results of Kuffer's experiments with respect to the specific gravities and melting points of lead-tin alloys. The author has added the percentage compositions. 66 * Sn=118. Pb 207. Alloys of lead and tin are distinguished by the facility with which they ignite and burn. The alloy of 4 or 5 parts lead and 1 part tin burns like charcoal at a red heat, the combustion continuing like that of an inferior peat, with the formation of cauliflower excrescences. The action appears to be due to the affinity which exists between the two oxides.” 1 § 97. Pewter is essentially an alloy of lead and tin, to which small quantities of other metals are sometimes added. 1 Watt's Dict. of Chem. p. 534. |