of the cylinder in the section belongs only to that part of the cylinder: the other part is perfectly flat, its purpose being to open a communication with the valve-cylinder.

Fig. 129.

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On the principal axis are three eccentric wheels, with iron straps, connected with a lever under the wind-chest, and these wheels are so arranged in respect to the corresponding crank that when the piston of any cylinder is either above or below, the lever is horizontal, and the valve exactly closes the hole h. When the piston in this figure begins to ascend, the end E of the lever continues to ascend also, and the other end F descends, and being connected E with the valve-rod at G, this also descends, and opens a communication | between the interior of the cylinder and the external air, which rushes in with a fresh supply. This valve continues to descend until the piston is half-way up; it then begins to ascend until the piston is at the highest point, when the valve has the position shown in the figure. The piston now descends, but the valverod continues to ascend, and opens a communication between the cylinder and the wind-chest, into which the air is forced by the piston. When the piston is half-way down, the valve-rod has reached the highest point, and then continues to descend with the piston until the latter is down, when the hole h is again covered with the valve. In this way the three cylinders are successively opened to the atmosphere and to the wind-chest, and a constant influx of air is produced. To preserve a steady action in the valve-rods, they are made to pass through guards level with the floor F. The fourth cylinder has no bottom, but is open to the wind-chest, and its piston, which weighs 700 lbs., serves only to regulate the pressure, which amounts to about lb. on the square inch. When the pressure exceeds this, the piston rises, and opens a safety-valve

BENZOIC ACID. When bitter almonds are subjected to great pressure, a fixed oil is obtained. By distilling with water the almond-paste which remains, a volatile oil is obtained, which is prepared in large quantities, chiefly for the use of the perfumer. This oil does not pre-exist in the almonds, but appears to be formed by the action of water upon a peculiar crystallizable substance called amygdaline, aided by the presence of the pulpy albuminous matter of the seed. The crude oil has a yellow colour, and contains a quantity of hydrochloric acid, which is formed at the same time as the oil. By agitating it with a solution of protochloride of iron, containing an excess of hydrate of lime, and then distilling the whole, the essential oil is obtained in a purified form. It is freed from the water which passes over with it by means of fused chloride of calcium.

This oil is supposed to be the hydruret of a basic substance named benzoyle, from its relation to benzoic acid, containing C, H, O2. Pure hydruret of benzoyle, or bitter-almond oil (C14 H5 O2+H), is a thin colourless fluid, of great refractive power, and of a peculiar and agreeable odour; its density is 1.043, and its boiling-point 356°. Its vapour is inflammable, and burns with a bright smoky flame. It is soluble in about 30 parts of water, but alcohol and ether dissolve it indefinitely. It absorbs oxygen rapidly from the air, and forms a mass of crystallized benzoic acid. It is doubtful whether the pure oil is poisonous, but the crude product is highly so, and yet this is sold for flavouring puddings, custards, &c.

Benzoic acid, or oxide of benzoyle, BzO (C11 H5 O2 + 0), is not usually obtained by the oxidation of bitter-almond oil. Several of the balsams, especially gum-benzoin, yield benzoic acid in abundance. By exposing this substance to a gentle heat in a subliming vessel, the benzoic acid is volatilized, and condensed in the upper part of the apparatus. The best contrivance for this purpose is a shallow iron pan, containing the substance to be sublimed, in a thin layer: a sheet of bibulous paper, pierced with a number of pin-holes, is stretched over the pan, and a cap of stout cartridge-paper, secured by a string or hoop, is drawn over the whole, as in Fig. 130. The pan is slowly heated on a sand-bath; the vapour of the acid condenses in the cap, and the crystals are prevented from falling back into the pan by the porous paper. The acid thus obtained is in the form of snow-white, light, feathery crystals, with a fragrant odour, which is due to the presence of a small quantity of volatile oil. A more abundant product may be obtained by mixing the powdered gum-benzoin with an equal weight of hydrate of lime, boiling the mixture with water, filtering, and evaporating, and then decomposing it with an excess

Fig. 130.

Benzoic acid is inodorous when cold, but has a faint smell when gently heated; it fuses just below 212°, and sublimes a little above that point: it boils at 462°, and emits a dense vapour, which is very irritating to the throat and eyes. It dissolves in about 200 parts of cold, and 25 of boiling water. It dissolves in about twice its weight of alcohol; it also dissolves in ether, and in fat and volatile oils. It combines with bases, and forms salts, which are called benzoates.

Benzoic acid is an ingredient in some kinds of perfumery, and in fumigating powders and pastiles. It was formerly used as an expectorant in asthma and dry cough. When taken internally, it is rejected from the system in the state of hippuric acid, which occurs in large quantity, in combination with potash or soda, in the urine of horses, cows, and other graminivorous animals. If the urine be slightly putrid, it yields benzoic instead of hippuric acid. Much of the cheap benzoic acid of commerce is prepared in this way, the perfume being superadded by subliming with a little gum-benzoin. Benzoic acid is a principal ingredient in friars'-balsam, useful for stimulating languid wounds, but mischievous when applied to recent wounds. A cosmetic, under the name of virgin's-milk, is prepared by mixing two drachms of the alcoholic solution of benzoin with a pint of

rose-water.

The other benzoyle compounds, such as the chloride, bromide, iodide, and sulphuret of benzoyle, possess great chemical interest, but are not of importance in a manufacturing point of view.

BILE. A secretion separated from venous blood by the largest internal organ of the body, the liver. It was regarded by the old chemists as a saponaceous compound, in which an organic acid was combined with soda. Modern chemists have re-adopted this view, regarding bile as a soda-soap, and, as such, the bile of the ox, or ox-gall, as it is called, is used in the arts, by painters in water-colours, scourers of clothes, and many others; but, from its green colour, it requires for many purposes to be clarified. This is done by allowing the fresh gall to settle for a day in a basin; the liquor is then poured off from the sediment into an evaporating-dish, and exposed to a boiling heat in a water-bath until it is somewhat thick. It is then spread upon a dish, and dried before the fire, in which state it may be kept for years in jelly-pots covered with paper. When required for use, a piece of the size of a pea may be dissolved in a table-spoonful of water. The gall may be made perfectly colourless by the following process: A pint of gall is to be boiled and skimmed, and an ounce of alum, in fine powder, or an ounce of common salt, added; the mixture to be kept on the fire until

if

portion of the gall prepared with salt, when the colouring matter will be coagulated and precipitated, leaving the ox-gall perfectly colourless. It may, necessary, be further purified by passing it through filtering-paper. This gall becomes purer by keeping, and never loses its good qualities, nor contracts a disagreeable smell.

Prepared gall combines with colouring matters and pigments, and gives them solidity either by being mixed with or passed over them upon paper. It adds to the brilliancy of ultramarine, carmine, green, and most delicate colours, and makes them spread more evenly upon paper, ivory, &c. Mixed with gum arabic, it thickens the colours without giving a glistening appearance; it prevents the gum from craking, and fixes the colours so well, that others may be applied over them without running into them. Mixed with lamp-black and gum, it forms a good imitation of China ink. A coat of ox-gall upon black-lead or crayon drawings prevents the lines from being effaced, and such drawings may be safely coloured by first mixing the colours with ox-gall. Miniature painters use it for removing the unctuous matter from the surface of ivory, and when ground with the colours it enables them to be spread easily and renders them fast. It is also useful for transparencies, by passing it first over the varnished or oiled paper and allowing it to dry. The colour is mixed with the gall and then applied, and cannot afterwards be removed.

It takes out spots of grease and oil, and is useful to the laundress in washing dresses, the colours of which would run or be removed by the ordinary process of washing. A small portion dropped into ink renders it fluid.

BINNACLE, or BITTACLE (from the French, habitacle, a small habitation). The box in which the compass is placed for steering a ship. The form is not of much importance by day, provided the man at the helm can get a good view of the compass-card; but at night it is of great consequence to throw the strong light of a lamp on the card, to preserve the lamp from the action of wind and weather, and to prevent its light from being seen out of the ship.

Sir Home Popham's binnacle is in general use in the navy. It consists of a square box about 2 feet high and 18 inches in the side, with a top in the form of the frustum of a square pyramid. The four sides are glazed with plate-glass, and the compass-card, which is about level with the upper part of the square box, is distinctly seen. By night thin copper screens shut up those sides which are not necessary for the steersman's purpose, and the inner surfaces of these screens assist in reflecting the light of the lamp strongly upon the card. The lamp is attached to one of these screens, a hole being cut in it to admit the necessary light. To prevent the fracture of the glass

the light downwards. By these means the card is well illuminated, and if the shutters a a be put down, no light escapes except at the aperture 6, and this directs the light upwards in a direction in which it will not fall upon any part of the ship.

BIRD-LIME. This substance is now in great part imported into England from Turkey, but was formerly manufactured here to a considerable extent, and exported to India for the purpose of destroying insects. It is a green substance, viscid and tenacious, made from the bark of various shrubs, but chiefly from that of the common holly. It may also be made from mistletoe and other parasites. It contains a resin (which has been called viscine), mucilage, colouring and extractive matter, and a little free acid. The middle bark of the holly contains a large amount of viscid matter, and from this, and the bark stripped from the young shoots, the best birdlime is made. The bark is boiled six or eight hours in water, until tender; it is then drained from the water, and laid in pits, for two or three weeks, to ferment, being occasionally moistened with water, if necessary. After this it is taken out, and reduced to a paste by pounding in mortars, then washed and kneaded, and packed in earthen vessels. After three or four days, during which it ferments and purifies itself, it is fit for use.

BISCUIT (Latin, bis coctus, twice baked). An unfermented bread, which, if properly prepared, can be kept for a great length of time, and hence its use as a common form of bread at sea. At present the ships of the Royal Navy are supplied with biscuits made by machinery, before the introduction of which they were made by hand in the following manner. A gang of five men were employed, who were severally named, the furner, the mate, the driver, the breakman, and the idleman. The driver made the dough, by mixing the flour and water together in a trough with his naked arms, until, by a laborious operation, a proper consistency was obtained. The rough dough was then deposited on a break, or wooden platform, to be worked by the breakman, who kneaded it by riding or jumping upon it by means of a breakstaff. When sufficiently kneaded, it was taken to a strong table, called the moulding-board, where it was first cut into slips, then divided into lumps of the size of the biscuit, then moulded by hand into the circular shape, and lastly docked, or pierced full of holes, by means of a docker, the whole of the gang assisting in this operation. The oven being properly heated, the biscuits were pitched in by the joint labours of the furner, the mate, and the idleman. The gang thus produced 100 lbs. weight of biscuit in 36 minutes, including 15 minutes, the time allowed for baking. Hence the nine ovens in the Royal Clarence Victualling Yard at Gosport required the labour of 45 men to keep them in full operation, and the produce was about 14 cwt. of biscuit per hour, at a cost for labour and utensils of 19d. per cwt.

The good quality of biscuit chiefly depends on the thorough kneading of the dough, and its subsequent division into portions of equal size and thickness. If

VOL. I.

the meal be not equally mixed with the water, some portions of the biscuit will be wetter than others, and will require more baking than the dry parts: these will be overbaked or burnt up, or the moist parts will remain unbaked; the biscuit thus becomes hard or flinty. So also if the biscuits are of unequal thickness, if the thin ones are not overbaked, the thicker must be underbaked; and the thin parts of a badly moulded biscuit will perish from the action of the fire, while its thick part is converted into flint.

About the year 1833 this method was superseded by machinery contrived for the purpose by Mr. Grant, of Gosport. In the first process, the meal is conveyed into a cylinder 44 feet long, 3 feet 2 inches in diameter, and the water is let in from a cistern at the back of the cylinder, regulated by a gauge to the exact quantity required for mixing the meal. Through the centre of the cylinder is fitted a shaft armed with knives, and working horizontally. The shaft being set in motion, the knives revolve through the meal and water. During the first half-minute the meal and water do not appear to unite; but after this the dough begins to assume a consistency, and in two minutes 5 cwt. of well mixed dough is produced. The cylinder is formed so that its lower half is easily separated by means of a wheel and pinion from the upper sides, thereby forming a trough containing the dough, from which it is removed, and placed under the breakingrollers to be kneaded. These rollers, two in number, weigh 1,500 pounds each, and are propelled from off a two-throw crank-shaft by means of connecting rods and pendulums; they pass backwards and forwards over the dough during five minutes, when the 5 cwt. of dough is brought into a solid, perfect, and equal consistency. From the breaking-rollers the dough is cut into pieces 18 inches square, and placed on boards 6 feet long by 3 feet wide, which are conveyed by means of a line of friction-rollers connected by an endless chain under a second set of rollers, to be rolled to the required thickness of the biscuit. The square of dough being thus pressed out, so as to cover the surface of the board, on which it is transferred under the cutting and stamping-plate, is at the same moment cut and stamped, or docked, into 42 hexagonal biscuits, which, being now complete, are at once conveyed to the oven on carriages constructed for the purpose. The hexagonal shape is preferred in order that there may be no waste, the sides of each biscuit by this contrivance fitting accurately into those of the adjoining biscuits, as seen in Fig. 134. Each biscuit is stamped with the broad arrow, the number of the oven, and also docked by the same movement which cuts it out of the piece of dough. The hexagonal cutters do not completely separate the biscuits, so that a whole sheet of them

Fig. 134.

can be put into the oven at once, and after being withdrawn, they are broken asunder by hand.

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