See Chemistry, no 151, 204, 248, 280, 397, and Metallurgy.

Black Lead, a mineral dug in Britain, and, as Dr Woodward observes in the preface to his Method of Fossils, more plentiful and of a better kind than there is in any other part of the world. According to Dr Plot's account in the Philosophical Transactions, no 240, it is found only in Kefwych in Cumberland; and is there called *caud* or *kellow*, by which last name an earth like the black chalk is distinguished in other places.

The colour of black-lead, rather a deep, shining, bluish-grey, than a black, may be seen, diluted a little, in the black melting-pots when broken or the surface scraped off, and entire in the genuine sort of black pencils. It differs not a little in goodness, some sorts marking paper freely, and others very difficultly or scarce at all. It is smooth, and as it were unctuous to the touch; and hence is used sometimes instead of oil or soap for giving slipperiness to the rubbing parts of machines. Acids neither dissolve it nor alter its colour or unctuousness.

Black-lead has not been found to contain any of the metal from which it receives its name, and its composition appears to be of a very singular kind. From its known resistance to vehement degrees of fire, whether urged by itself in close vessels, or made with clay into melting pots, and placed among the burning fuel, it should seem that it could not partake largely of any volatile substance, and it has been generally supposed to consist chiefly of a talc-like earth. But Mr Quilt relates, in a curious paper of experiments on black-lead published in the Swedish transactions for 1754, that having exposed many different specimens of this mineral to a strong heat, on a scorifying dish under a muffle, they all yielded sulphureous fumes and flowers in abundance; and that there remained behind, from one sort, only a fifth part of its weight, and from another no more than a 20th part, of a yellow or brown calx, which, being treated with inflammable fluxes, yielded seven tenths of its weight of a metallic mass, which seemed to be a mixture of iron and tin. Agreeably to these experiments, in Cronstedt's Mineralogy black-lead is classed among the sulphureous minerals, and called *sulphur saturated with iron and tin*.

Dr Lewis kept 168 grains of the finest black lead used by our pencil-makers in a moderately strong red heat on a scorifying dish for three hours, with the common precaution of covering the vessel for a time, lest the matter should crackle, and some particles be thrown off from it in substance. He found it reduced to about 120 grains, and all the pieces changed on the outside to a rusty sparkling brown calx; of which a considerable part was attracted by a magnetic bar, the internal parts continuing of the same colour as at first. Being then broken into smaller pieces, and exposed to a like heat for two hours, it suffered the same change as before, and was reduced to about 60 grains. Being further broken, and calcined with a moderate red heat for 10 hours, it was diminished to 30 grains; and, by a repetition of the operation, to 12 grains, or $\frac{1}{4}$ of its original weight.

The remarkable dissipation in these experiments, of a substance which in close vessels resists intense fires, may be somewhat illustrated by the known property of charcoal, which when excluded from the action of the air, whether by being inclosed in a vessel, or mixed with clay into a mass, remains unconsumed and unaltered in the fire. Masses of black-lead seem to calcine and suffer a dissipation only on the surface; the internal part remaining long unchanged, unless the mass be broken, or the calx rubbed off, so that fresh surfaces may be exposed to the air. The common black-lead melting-pots made of clay, and the coarser kinds of black-lead powdered, like those made of clay and charcoal powder, lose their external blackness with part of their weight, and thus have their staining quality destroyed by strong fire.

**Black-Lead Pencils.** Black-lead, in fine powder, stirred into melted sulphur, unites with it so uniformly and in such quantity, that though the compound remains fluid enough to be poured into moulds, it looks nearly like the coarser sorts of black-lead themselves. Probably the way which Prince Rupert is said to have had, mentioned in the third volume of Dr Birch's history of the Royal Society, of making black-lead run like a metal in a mould, so as to serve for black-lead again, consisted in mixing it with sulphur or sulphureous bodies.

On this principle the German black-lead pencils are said to be made; and many of those which are hawked about by certain persons among us, are prepared in the same manner: their melting or softening, when held in a candle, or applied to a red-hot iron, and yielding a bluish flame with a strong smell of burning brimstone, betrays their composition; for black-lead itself yields no smell or fume, and suffers no apparent alteration in that heat. Pencils made with such additions are of a very bad kind: they are hard, brittle, and do not cast or make a mark freely either on paper or wood, rather cutting or scratching them, than leaving a coloured stroke.

The true English pencils, (which Vogel in his Mineral System, and some other foreign writers, imagine to be prepared also by melting the black-lead with some additional substances, and casting it into a mould) are formed of black-lead alone, sawn into slips, which are fitted into a groove made in a piece of wood, and another slip of wood glued over them: the softest wood, as cedar, is made choice of, that the pencil may be the easier cut; and a part at one end, too short to be conveniently used after the rest has been worn and cut away, is left unfilled with the black-lead, that there may be no waste of so valuable a commodity. These pencils are greatly preferable to the others, though seldom so perfect as could be wished, being accompanied with some degree of the same inconveniences, and being very unequal in their quality, on account of different sorts of the mineral being fraudulently joined together in one pencil, the forepart being commonly pretty good, and the rest of an inferior kind. Some, to avoid these imperfections, take the finer pieces of black-lead itself, which they saw into slips, and fix for use in port-crayons. This is doubtless the surest way of obtaining black-lead crayons whose goodness can be depended upon.

**Milled Lead.** See Chemistry, no 400.

**Poison of Lead.** See Poison.

**Leaf,** a part of a plant extended into length and breadth in such a manner as to have one side distinguishable from the other. This is Miller's definition. Linnaeus denominates leaves "the organs of motion, or muscles of the plant."—The leaves are not merely ornamental to plants; they serve very useful purposes, and make part of the organs of vegetation. The greater number of plants, particularly trees, are furnished with leaves; in mushrooms, and shrubby horsetail, they are totally wanting. Ludwig defines leaves to be fibrous and cellular processes of the plant, which are of various figures, but generally extended into a plain membranaceous or skinny substance. They are of a deeper green than the foot-stalks on which they stand, and are formed by the expansion of the vessels of the stalk, among which, in several leaves, the proper vessels are distinguished by the particular taste, colour, and smell, of the liquors contained within them.

By the expansion of the vessels of the stalk, are produced several ramifications or branches, which, crossing each other mutually, form a kind of net; the meshes or interstices of which are filled up with a tender cellular substance, called the pulp, pith, or parenchyma. This pulpy substance is frequently consumed by certain small insects, whilst the membranous net remaining untouched, exhibits the genuine skeleton of the leaf.

The net in question is covered externally with an epidermis or scarf-skin, which appears to be a continuation of the scarf-skin of the stalk, and perhaps of that of the stem. M. Defauvure, a judicious naturalist, has attempted to prove, that this scarf-skin, like that of the petals, is a true bark, composed itself of an epidermis and cortical net; these parts seem to be the organs of perspiration, which serve to dilute the superfluous juices.

The cortical net is furnished, principally on the surface of the leaf, with a great number of fleshy or absorbent vessels, destined to imbibe the humidity of the air. The upper surface, turned towards heaven, serves as a defense to the lower, which looks downward; and this disposition is so essential to the vegetable economy, that, if a branch is overturned in such a manner as to destroy the natural direction of the leaves, they will, of themselves, in a very short time, resume their former position; and that as often as the branch is thus overturned.

Leaves, then, are useful and necessary organs; trees perish when totally divested of them. In general, plants stript of any of their leaves, cannot shoot vigorously; witness those which have undergone the depredations of insects; witness, likewise, the very common practice of stripping off some of the leaves from plants, when we would suspend their growth, or diminish the number of their shoots. This method is sometimes observed with corn and the eulent grasses; and, in cold years, is practised on fruit-trees and vines, to render the fruit riper and better coloured: but in this case it is proper to wait till the fruits have acquired their full bulk, as the leaves contribute greatly to their growth, but hinder, when too numerous, that exquisite rectifying of the juices, which is so necessary to render them delicious and palatable.

When vegetation ceases, the organs of perspiration and inspiration become superfluous. Plants, therefore, are not always adorned with leaves: they produce new ones every year; and every year the greater part are totally divested of them, and remain naked during the winter. See Plant.