of the lamp, the salt will melt, and the gas, which is oxygen gas, will be given out in abundance, and may be collected in the receiver B.

Oxygen gas may also be thus obtained from the red oxyde of mercury; from red lead, and from nitre.

When the gas to be procured is absorbable by water, quicksilver is used instead of water: but as this fluid is very expensive, a smaller vessel is necessary, and it may be made of wood or stone, as neither of these substances are acted upon by the mercury.

A small glass vessel, capable of containing an punce measure, is used for measuring gases; for if it be successively filled, and inverted under a large jar, we may by this means throw into that jar any given or required quantity of an elastic fluid, or a certain number of measures of one elastic fluid, and any number of another, as we please.

G, in fig. 3, represents a tube for receiving a mixture of gases that are to be exploded by the electric spark. It is a strong glass tube closed at one end, having a scale upon it cut with a diamond. Near the, closed end two wires pass through the glass, and almost touch one another: these are cemented in, so as to make the holes air-tight. When this graduated tube is filled with the fluid in the trough, and inverted upon the shelf, the gases to be exploded are introduced in the usual way. If now the interval between the two wires be made a part of the electrical circuit, by fastening chains connected with a Leyden phial to the rings of the

wires, the spark will pass through the interrupted space between the two wires, and explode the gases.

In compound distillations, or when a decompo-. sition of the materials subjected to this process takes place, and gases are formed, some of which are absorbable, and others are not capable of being ab sorbed, it is often required to preserve separate the several new substances so procured. To do this the following apparatus was invented by M. Lavoisier, and soon came into general use.

A, fig. 4, is a glass tubulated retort, the neck of which is adjusted to the balloon or receiver X, which is double tubulated at B and C; by the former it admits the gas from the retort A, and by the latter it is conveyed into the liquor contained in the glass vessel GH. With this vessel, which has three tubulures, may be connected two or more similar vessels, by means of other glass tubes proceeding from cz, and fitted into their tubulures; and from the last tubulure of the range, is adapted a glass tube F, which is conveyed under a receiver placed upon the shelf of the pneumatic cistern, as B in fig. 3. Water is put into the first of these vessels, a solution of caustic potash in the second, and such other substances in the rest as are necessary for absorbing the gases that pass over. To prevent the danger that might occur from a reabsorption of the gas, which sometimes takes place, and would occasion the water from the pneumatic tub to enter rapidly into the vessels through the

tube F, a capillary tube, as H, is adapted to the middle tubulure of each receiving vessel, which goes into the liquid contained in it: so that if absorption takes place either in the retort or other vessels, the external air enters through these tubes to fill the vacuum which is occasioned by the absorption, and prevents the water coming into the vessel.

Having shown the apparatus necessary for obtaining the several kinds of gas, we may observe that in chemistry there are operations which require very different kinds of vessels: such for instance is fusion, or the melting of a solid body, as a metal, and causing it to pass from the solid to a liquid state by the action of heat. These substances require, according to their several natures, vessels of different kinds strong enough to resist the fire; they are made of earthenware, of porcelain, or a mixture of clay and powder of black-lead, or of black-lead altogether. These vessels are called crucibles, and are represented by fig. 5. Sometimes they have covers made of earthenware, but in other cases the fused metal must be exposed to a current of air; for this purpose the crucibles are broad and shallow. These, as at fig. 6, are called cupels, and they are generally made of calcined bones, mixed with a small quantity of clay, or of a mixture of clay and black-lead powder. As the cupels do not admit of being put in a close furnace, or of being surrounded by coals, they are placed under a sort of oven of earthenware, called a muffle,

one of which is represented in fig. 7, which, with the included cupel, is exposed to the heat of a furnace.

Blow-pipes are used for directing the flame of a candle or lamp against any piece of ore or other substance required to be examined. They ought to have a bulb upon the middle of their stem, in which the moisture from the breath may be collected. See fig. 8.

The various degrees of heat which are required for the performance of chemical operations, render it necessary that the chemist should be possessed. of a furnace. A description of a very useful one is as follows: it consists of a cylindrical body of sheet iron, coated within with a mixture of loam and clay (fig. 9). The aperture A at the top is closed oc casionally with an iron saucer full of sand, which forms a sand-bath. B is the door of the fire-place, and C is the ash-pit register which slides so as to admit more or less air. D is an iron tube which goes into the chimney of the room to carry off the smoke.

As a further illustration of the operative part of chemistry, we will suppose that an unknown liquid is offered for chemical examination; and that it is first required to obtain a knowledge of the gaseous matters which it will yield. In this case a portion of the fluid is put into the retort C (fig. 3), placed either over the naked burning coals, or the flame of a lamp, or in heated sand. The other end of the retort is immersed in water, or mercury, contained in the trough

A, under the receiver B. Heat being applied, and gaseous matter extricated, it rises in bubbles through the fluid, fills the, now, upper part of the receiver, and gradually assumes the place of the water. The gas, confined in the receiver, may then be submitted to any examination. If it is required to separate the fluid part from the substance in the retort, a receiver D is fixed on to the retort A, fig. 1, and heat being applied, the distillation of the aqueous part will take place, and may be preserved for future examination. Distillation is sometimes performed through a series of receivers, both for the sake of greater safety, and for a more certain and correct mode of obtaining the several products; but, in the more ordinary operations, the distillation is performed by a common still or alembic, The substance to be distilled being put into the vessel, generally made of copper, the head of the alembic, with a crane-neck, is closely fitted on and luted; and its beak is inverted into the worm, or serpentine metallic tube, which, passing through water, kept cold, condenses the heated vapour into a fluid, which is then collected, as it runs from the spout of the worm.

If the separation of the aqueous part be only wanted, then the matter remaining in the retort is placed in an open dish; and, by the continued action of heat, the liquid part will be dispersed by evaporation, leaving the solid remains. On fresh water being added to this, a solution will be obtained, which may be com pletely separated from the undissolved parts by filtra