When the hydrogen of coal-gas combines with the oxygen of the air, the carbon is set free in a solid state. As the hydrogen consumes all the oxygen in the nearest layer of air, the carbon must delay a moment in the flame before it can get at oxygen to unite with. This finely divided solid thus becomes heated white-hot, and gives out light. This condition lasts but an instant. The next instant the carbon combines with atmospheric oxygen, and passes off as a colorless gas. As fast, however, as the carbon is consumed, a fresh supply is set free, and thus the light is constantly kept up.

The value, then, of coal-gas for illumination depends upon a most delicate adjustment of affinities.

Were the affinity of carbon for oxygen a little stronger than it now is, the carbon would burn at the same time with the hydrogen, and there would be no light in the flame. On the other hand, were the affinity of carbon for oxygen less than it is, the carbon would not burn at all, but, after developing light, would pass away from the flame in the form of soot.

194. Bunsen's Lamp. The fact that carbon must

Fig. 29.

remain an instant in the flame in the solid state to develop light, is illustrated by Bunsen's burner. It is shown in Figure 29, and consists of a brass tube, near the bottom of which are four round holes, through which the air can pass into the tube. A second tube opens into the inside of the brass tube from the bottom, just on a level with these holes. The coal-gas passes into the larger tube, through this

second tube. The air passes in through the holes at the same time, and the two gases become intimately mixed before they reach the top of the tube. Upon lighting the mixture as it escapes from the tube, it burns

with scarcely any light, but a good deal of heat. If we close the holes at the bottom of the tube, the gas burns with the ordinary luminous flame. In the first case, there is an excess of oxygen mixed with the coal-gas, so that there is enough to combine with the carbon as soon as it is set free, before it becomes sufficiently heated to develop light. In the second case, the carbon cannot get at the oxygen to combine with it, until it has passed through the burning layer of hydrogen, and thus become intensely heated.

195. The Best Shape for a Gas-Flame. From what has been said, it is evident that the light of the flame is developed only at the surface, in the burning layer of hydrogen. The ordinary gas-burner is so made that the flame will be flat, in order that it may have as much surface as possible. The greater the surface, the greater the light developed.

196. The Argand Burner.-A sheet of paper will evidently present to the air just as much surface when it is bent round till the two edges meet, as when it is flat; while in the second case it is in a more compact form. So the ordinary gas-flame will present just as much surface, if its edges are bent round till they meet. In the Argand burner, this form is given to the flame, by supplying the gas through a circle of small holes in a hollow brass ring. A current of air passes up through this ring, furnishing oxygen to the inner surface of the flame. 197. The Bude Light. - If the interior of the ring of an Argand burner be closed, the flame becomes of a dull red color. Since no oxygen is supplied to the inner surface of the flame, the combustion is imperfect, and develops but a low degree of heat.

If, on the other hand, a stream of pure oxygen be supplied to the interior of the cylindrical flame, the flame diminishes in size; but the light becomes very intense,

and of a pure white color. The increased supply of oxygen increases the energy of the combustion of the hydrogen, and the intensity of the heat developed by it. In the first case, the carbon is heated only to a dull redness, while in the second case it is raised to a full white heat. 198. A Burning Candle is a miniature Gas-Factory. We have seen that wax or tallow, when heated in a flask, is converted into an inflammable gas. This gas, in burning, produces water and carbonic acid; hence it must contain hydrogen and carbon, which are the main ingredients of coal-gas. This gas is in fact identical with coal-gas. The heat of the burning match converts some of the wax of the candle into gas, which takes fire, and in burning develops sufficient heat to convert more of the wax into gas; and thus the supply is kept up.

199. The Flame of a Candle consists of three distinct Parts. In the flame of a candle there are three

Fig. 30.

distinct parts: (1) the dark central zone, or supply of unburnt gas surrounding the wick; (2) the luminous zone, or area of incomplete combustion; and (3) the nonluminous zone, or area of complete combustion. If we put one end of a small glass tube (see Figure 30) into the dark central zone, the unburnt gas will pass up the tube, and may be ignited at the other end. In the luminous part of the flame, as in the case of the gas-flame (193), the gas is not completely burnt, and carbon is separated in the solid state; and it is this carbon heated white-hot which renders the flame luminous. In the outer zone the supply of oxygen is greater; all the carbon is at once burnt to carbonic acid, and the flame here becomes non-luminous.

SUMMARY.

Vegetable compounds may be broken up by destructive distillation.

In the preparation of charcoal, wood is decomposed into carbon and a number of volatile compounds.

When these volatile products are cooled and condensed, we obtain: (1) permanent gases; (2) a watery fluid; (3) a dark resinous fluid.

(1) The permanent gases are chiefly marsh-gas, HC, and olefiant gas, H,C.

(2) The chief ingredient of the watery fluid is pyroligneous acid, or wood vinegar.

(3) The dark liquid is wood tar.

The most important ingredients of wood tar are wood naphtha, creosote, paraffine, and asphalt.

Mineral coal has probably been formed by the gradual decomposition of vegetable matter buried in the earth, and thus excluded from the air; the decomposition in many cases being hastened by the internal heat of the earth.

When the volatile products were allowed to escape freely, anthracite coal was formed; when they could not escape, bituminous coal was formed.

The volatile products often escaped into large cavities in the rocks, and on condensing gave rise to the celebrated rock-oil, or petroleum.

In some cases, this oil may have been formed by the decomposition of the vegetable matter at a temperature too low for its conversion into vapor, and may afterwards have been distilled by the internal heat of the earth.

The products of the destructive distillation of bituminous coal are more numerous than those of the distillation of wood. They are less rich in compounds containing oxygen, but richer in those containing nitrogen.

When the coal is distilled at a high temperature, a large amount of gas is obtained, and the liquid products are then called tars. When the coal is distilled at a low temperature, only a small amount of gas is obtained, and the liquid products are called oils.

When the gas obtained by the distillation of coal is purified, it forms the well-known illuminating gas.

The most important ingredients of coal-tar are carbolic acid, ammonia, aniline, benzole, toluole, cumole, paraffine, and naphthaline.

The aniline colors are prepared chiefly from benzole. Only gases burn with flame. Solids that appear to burn with flame are first converted into a gas by heat, and this gas burns with flame.

The two conditions essential to illumination are the presence of a solid and intense heat.

The illuminating gases are all hydrocarbons, or compounds of hydrogen and carbon.

The hydrogen has the stronger affinity for oxygen, and burns first. The carbon is set free in a solid state, and delaying for an instant in the intense heat of the flame, before combining with oxygen, becomes luminous.

The light of the flame is increased (1) by increasing the surface of the flame, as in the ordinary gas-flame, and that of the Argand burner; and (2) by increasing the intensity of the combustion of the hydrogen, as in the Bude light.

The heat of the flame is increased by mixing oxygen with the gas before it burns, so that the carbon and hydrogen may burn together, as in the Bunsen burner.

The burning candle or oil-lamp is a miniature gasfactory.

The candle flame consists of three parts: (1) an inner zone of unburnt gases, surrounded by (2) a luminous zone of incomplete combustion, outside of which is (3) a non-luminous zone of complete combustion.