attracts soda more strongly than the muriatic acid does. But if the lime and muriatic acid previously combined be mixed with the sulphate of soda a double decomposition is effected. The lime leaving the muriatic acid, unites with the sulphuric acid; and the soda, being separated from the sulphuric acid, combines with the muriatic. These decompositions are rendered more intelligible by the following diagram.

On the outside of the vertical brackets are placed the original compounds, sulphate of soda and muriate of lime; and above and below the diagram, the new compounds. The upper line being strait, indicates that the muriate of soda remains in solution, and the middle of the lower line being directed downwards, shows that the sulphate of lime is precipitated.

Obs. The turbid, or milky appearance which is seen on adding the clear solutions of muriate of lime and sulphate of soda together, is owing to the fact that one of the newly formed salts, sulphate of lime, is insoluble in water, and consequently is precipitated, or falls to the bottom of the vessel, occasioning in its descent the turbidness. The same appearance is seen in every case where an insoluble precipitate is formed by chemical decomposition.

82. EXPERIMENTS illustrating chemical affinity.

Exp. Mix together some olive oil and water in a vial. The oil will rise to the top of the water, and cannot be made to unite with it, there being no affinity between them. But if some pieces of potash be added, and the vial shaken, there takes place a chemical union between the three substances.

Obs. The potash having an affinity both for the oil and the water, it attracts each with a force sufficient to effect their union. In this manner soap is formed.

83. Some bodies do not act on each other unless either one or both are in a state of solution, or at least contain water.

Illus. Spread thinly, on a piece of tin-foil three or four inches square, some dry nitrate of copper, and wrap it up. No effect will be produced. Unfold the tin-foil, and having sprinkled the nitrate of copper with the smallest possible quantity of water, wrap the tin-foil up again as quickly as possible, pressing down the edges closely. Considerable heat, attended with copious red fumes, will now be excited; and if the experiment has been dexterously made, sparks of fire or even flame will be emitted. Obs. This shows that nitrate of copper has no action on tin, except it is in a state of solution.

84. Experiments illustrating single elective affinity.

Exp. 1. Add to a solution of soap in water a few drops of sulphuric, or any other acid. The acid will combine with the potash; the oil will be set free, and will rise to the top.

Obs. The same effect is produced by those waters which naturally contain a small quantity of any acid, as the carbonic. These waters are called hard, and will not wash, because the acid which they contain decomposes the soap.

2. To a solution of camphor in spirits or alcohol, add a quantity of water. The mixture will instantly become white and turbid, and the camphor will rise to the top in flakes.

Obs. The alcohol has a stronger attraction for the water than it has for the camphor, it therefore leaves the latter and unites to the former, while the camphor, being insoluble in water, takes the solid form, and rises to the top, because it is of less specific gravity than the fluid.

85. The properties characterizing bodies when separate, are destroyed by chemical combination,, and new properties appear in the compound.

Illus. Thus muriatic acid and lime, which in a separate state have a most corrosive taste, lose this entirely when mutually saturated; the compound is extremely soluble, though lime. itself is very difficult of solution: the acid no longer reddens syrup of violet, nor does the lime change it as before to green. The resulting compound, muriate of lime, also exhibits new properties. It has an intensely bitter taste; is susceptible of crystalization, and the crystals, when mixed with snow or ice, generate a degree of cold sufficient to freeze mercury.

OF THE GASES.

Def. The name gas is given to all the permanently elastic fluids, except the atmosphere, which is called air.

86. The gases are a numerous set of bodies, obtained from various substances, chiefly by chemical decompositions. They differ greatly from each other in some of their properties, in others, they are nearly alike. Thus some are acid, some are alkaline; some extinguish flame, and some support combustion, &c. But they all agree in respect to that ærial form, and that elasticity which constitute them gases.

87. The gases are supposed to consist of solid ponderable particles united chemically to caloric, and perhaps to electricity also. The weight of the gases depends on their solid particles; their elasticity on the repulsive power of caloric to which these particles are united.

Obs. 1. All the gases agree in the ratio of their expansibility by equal increments of heat. This is one part in 480 for every degree of Farenheit's scale between the freezing and boiling points.

2. The bulk of a gas is inversely as the pressure which it sustains. Thus the air (which is composed of two gases) at the earth's surface, sustains a pressure equal to the weight of the incumbent atmosphere, which is nearly equivalent to a column of mercury 30 inches high. If we double this pressure we reduce the air to one half its bulk; if we triple it, to one third, and so on in this proportion to any known extent. On the contrary, if we remove one half the pressure, we double the bulk of air, and if we remove nine-tenths of the pressure, we increase the bulk ten times, and so in this proportion, to any imaginable

extent.

All the gases are absorbed, in greater or less quantities, by porous substances. Charcoal possesses the highest absorbent power, but all substances possessing a certain degree of porosity, after being exposed to the action of the air-pump, absorb certain quantities. The quantity absorbed by charcoal, depends on the gas to which it is exposed. Thus a piece of red hot charcoal,

quenched by plunging it under mercury, absorbs 90 times its bulk of ammoniacal gas, but hardly twice its bulk of hydrogen gas. The other gases are absorbed in proportion between these.

All the gases are also more or less absorbable by water. Thus water absorbs several hundred times its bulk of sulphurous acid gas, but only a very minute quantity of nitrogen.

The num

Remark. The gases are probably very numerous. ber known at present is 43. The following table contains the most important and curious among them. The right hand column shows the composition of the gases on the left 'The cross + signifies "combined with ;' as ammonia is composed of 3 parts of hydrogen combined with 1 part of nitrogen.

+2 hydrogen. +1 hydrogen.

4 Sulphuretted hydrogen 1 hydrogen + 1 sulphur. 5 Phosphuretted hydrogen 1 phosphorus +1 hydrogen.

Obs. The method of making experiments on the gases is described under the head of " Chemical Utensils," article " gas apparatus."

OXYGEN GAS.

88. We have no knowledge of the properties of oxygen, except in a state of combination. It sometimes exists in a solid state, as when it combines with the metals to form oxides, and sometimes in an aeriform state, as when it is united to caloric and light, forming oxygen gas. No process has yet separated the oxygen from the solid, and the aeriform state at the same time; the consequence is, that the base, or ponderable part of oxygen has never been examined in a separate

state.

89. Oxygen is an invisible fluid, like air. It has neither taste nor smell. Its specific gravity is 1103, air being 1000. It is slightly absorbed by water. It is procured from a great number of substances; and forms about one-fifth of the atmosphere.

Exp. Take a quantity of black oxide of manganese, put it into a retort, or Florence flask, furnished with a tube, (see chemical utensils) and pour on one half its weight of sulphuric acid : mix them together, and place the retort, or flask over an Argand lamp. The gas comes over in abundance, and may be collected over water in the usual way, by plunging the beak of the retort, under a vessel filled with it. See gas apparatus.

Caution. When bubbles of oxygen cease to ascend up the water in the receiver, the retort must be removed, otherwise it will be broken by the water which will be forced over to fill the vacuum caused by the heat.

Obs. The above, is the most convenient method of collecting oxygen gas in the small way, and for ordinary experiments; where larger quantities are wanted, other methods are used.