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LEADVILLE

2041

LEAP

the whole surrounded by spent tan-bark or horse-refuse. The heat from these evaporates the acid, which, with the air, changes the surface of the lead to an acetate, this in time being converted into a carbonate by the action of the carbonic gas from the hotbed. The chief ore from which lead is obtained is galena. This is found through almost all of Europe and in many states of the Union, the largest deposits worked being in Idaho, Utah, Montana and Colorado. For ordinary lead-smelting the ore is crushed or almost pulverized by machinery, and placed upon the bed of a rever-beratory furnace. A part of the ore becomes an oxide, and some of it a sulphate, while sulphurous gases pass off. When the furnace-doors are closed and the mass is melted, the oxidized parts and the remaining sulphide react upon each other, forming sulphurous acid and metallic lead. Toward the end, some lime is thrown into the furnace to prevent the slag becoming too fluid, and the melted lead flows from the tap-door. Lead-ores are often smelted in blast-furnaces, particularly for the sake of obtaining the silver in the ores or in other ores mixed with them. The silver then goes into the metallic lead. Lead is used in many alloys — with antimony to make type-metal or with tin for solder and for the manufacture of pewter, Britannia metal and the like.

Lead'ville, a city of western central Colorado, the silver-mining center of the state and the county-seat of Lake County, is an important mining-town, situated in a valley 10,200 feet above the level of the sea. The city is located on California Gulch near Arkansas River, on the Denver and Rio Grande; Colorado Midland; and Colorado and Southern railways. The mines produce gold, silver, iron, copper, zinc, bismuth and lead, the latter to the extent of 70,000 tons or more per year. The town was incorporated in 1878, and contains many smelters, one of which is the largest in the world, stamp mills for crushing ore, iron foundries, a government nshhatchery, besides manufacturing machinery, jewelry and novelties. It has several fine churches, an excellent public-school system, which includes an high school, two banks, and is a modern and well-improved city. Pop. 7,508.

Leaf. Leaves are expanded organs of the higher plants for the purpose of displaying green tissue to the air and sunlight. Tnese ' organs are found in fern-plants and seed-plants, and very simple leaves also appear among the mosses and liverworts. The leaves of the two higher groups are exceedingly variable in form, but are of very uniform general structure. The ordinary leaf consists of three main parts: (i) the blade, that is, the essential expanded part; (2) the petiole, the stalk upon which the blade is produced, and which may or may not

be present; and (3) the stipules, pair of more or less conspicuous appendages at the point where the petiole joins the axis, which may or may not be present. A cross section through an ordinary leaf reveals three distinct regions: (i) the "epidermis" of the upper and lower surf aces, Jwhich usually consists of a single layer of colorless and close-set cells; (2) between the two epidermal layers the "mes-ophyll," the tissue whose cells contain the green chloroplast; (3) the cut ends of the "veins," which penetrate among the mesophyll cells. The epidermis is a protective layer, and through it the mesophyll cells carry on exchanges with the outside world. In the epidermis the peculiar breathing-pores or stomata are developed. (See EPIDERMIS.) The mesophyll is the working tissue of the leaf, and in ordinary horizontal leaves its cells are arranged differently on the upper and under side. Those next to the upper erjidermis are elongated and stand endwise, being close together and forming the so-called "palisade tissue." The mesophyll cells on the under region of the leaf are loosely arranged, leaving large intercellular spaces, through which there is a free circulation of air. The stomata open chiefly on the under surface of the leaf and into this system of intercellular passage-ways. The veins are conducting tissues, carrying materials from the stem into the mesophyll and receiving elaborated foods in turn from the mesophyll to be distributed to other parts of the plant. The leaves of seed-plants are often divided into two groups on the basis of the arrangement of the veins. In the monocotyledons, represented by grasses and lilies, the venation is said to be parallel, that is, the veins run approximately parallel from the base to the apex of the leaf. In the dicotyledons, on the other hand, the veins branch in various directions and the leaves are said to be net-veined or reticulate. The parallel-veined leaves are apt to be narrow and elongated, while the net-veined leaves incline to broader shapes. Among the net-veined leaves there are two types of venation : (i) the pinnate type, in which a

GEOTROPIC CURVATURE OF LEAVES