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kind. It is more often found in fissures in sand rocks; rocks in which no oil could ever have been generated, since whatever organic matter they might have contained was too much exposed to atmospheric oxygen to admit of its being bitumenized, or made bituminous. It is not only impossible that the oil could have originated in these sand rocks, or in the sandy shales which underlie them in the oil region in Western Pennsylvania ; but it is most probable that the oil ascended from still lower rocks in the form of vapor, which condensed in the cavities above. Since, then, petroleum is seldom found where it originated, but, ordinarily, in cavities of rocks higher up, it seems probable that it is mainly the product of distillation.

The chemical conditions essential to the generation of oil have evidently existed over a very wide area; but the oil is found only where fissures exist in the rocks. These fissures serve two purposes: one, to give space for the formation and expansion of the hydrocarbon vapor; the other, to furnish receptacles for the condensed oils. These fissures must connect with the sources of the oil. If they have any outlets at the surface of the earth, by which the more volatile portions of the oil may escape as gas, the oil within them becomes thicker and heavier. Hence, as a general rule, the oil found near the surface is heavy, the cavities containing it being likely to have outlets. It may, of course, happen that a deeply-seated fissure has such an outlet.

189. How Petroleum is obtained. The oil is obtained by piercing one of these cavities by a well. It often happens that the upper part of the cavity is filled with pent-up uncondensible gases. In this case, if the well happens to pierce the lower part of the cavity, the expansive force of the confined gases will drive the oil from the well in a continuous stream. Oil is often forced

from a new well with such velocity, that it rises in a jet a hundred feet high.

It sometimes happens that the lower part of the cavity is filled with brine, upon which the oil floats. If, in this case, the well pierces the lower part of the cavity, brine is the first product. After a time the salt-well may change to an oil-well.

COAL-GAS.

190. The Manufacture of Coal-Gas. — The idea of turning hydrocarbon gases to the practical purposes of illumination occurred at about the same time to Murdock, in England, and Lebon, in France. This was in 1790; but for a long time there was a strong popular feeling against the new light. In England, it was not until 1812 that Parliament consented to charter a company for its manufacture. Even then the enterprise was looked upon as so visionary, that the act of incorporation was said to be granted in order to make a great experiment of a plan of such extraordinary novelty. In December, 1813, Westminster Bridge was first lighted with gas. From this time gas-lighting made the most rapid progress in England; and now the consumption of gas in London alone amounts to more than seven billions of cubic feet annually. To make this gas, eight hundred thousand tons of coal are required; while the length of the main pipes in the streets of the city is more than two thousand miles.

Paris was first lighted with gas in 1820. There, as in England, strong prejudices had to be overcome.

The most essential parts of the apparatus used in the making of coal-gas are represented in Figure 28.

Of course, it is only soft or bituminous coal that can be used for making gas. This coal is distilled in long

iron retorts, seen at the left of the figure. When charged with coal, these retorts are closed air-tight. They are

[graphic]

Fig. 28.

then heated to a very high temperature by the furnaces beneath.

The gaseous and volatile compounds, formed by the distillation of the coal, pass up through pipes (one of which may be seen leading from each retort) into a long horizontal pipe, M, called the hydraulic main. This is half full of water, into which the pipes from the retorts dip. The gas readily escapes from the pipes by bubbling up through the water; but, of course, it cannot return through the water.

The gas is carried from the hydraulic main through a pipe into a tank, T, called the tar cistern. By this time the more condensible gases have become liquid, and collect in the smaller vessel into which the pipe passes, and from which they overflow into the larger tank. From the latter they are drawn off at intervals. These condensed products are coal-tar and a liquid highly charged with ammonia, called ammoniacal liquor.

The uncondensed gases pass on through the series of upright pipes at C, called the condenser. Here they become still further cooled, and all the remaining condensible gases are reduced to the liquid state.

After leaving the condenser, the gas still contains, besides the compounds fit for illuminating purposes, the noxious compounds carbonic acid and hydric sulphide (49). These are removed in the purifier, P. It consists of a chamber with several perforated, shelves, which are covered with slaked lime. In passing over this lime, the carbonic acid and hydric sulphide are absorbed, while the purified gas passes along into the gasometer, G.

This gasometer consists of a large sheet-iron bell-jar, which dips into a cistern of water. The latter is deep enough to allow the bell to be completely submerged, and filled with water. The bell is counterpoised with weights, and rises as the gas enters it.

From the gasometer the gas passes out, by the pipe at the right, into the streets and houses of the city.

ILLUMINATION.

191. Nature of Flame. — We know that coal-gas burns with flame, while the solid carbon burns without flame. All combustible gases burn with flame.

We have seen that wood, when heated in a closed tube, gives off an inflammable gas; and the same is true of wax or tallow heated in the same way. Every solid which appears to burn with flame can thus be converted into an inflammable gas by means of heat. When these solids begin to burn, the heat developed is sufficient to generate this inflammable gas. It is this gas, and not the solid, which burns with flame.

Flame, then, is gas burning.

192. A Solid and Heat are necessary to Illumination.— The oxy-hydrogen flame (22) develops intense heat, but scarcely any light. If, however, a cylinder of lime be held in this flame, we get the brilliant calcium, or Drummond light.

We see, then, that a solid and an intense heat are necessary to illumination.

193. These two Conditions are fulfilled in the Burning of Coal-Gas. — Illuminating gas, as we have seen, is a compound of carbon and hydrogen. If we hold a cold glass rod in a gas-flame, it becomes blackened with soot, showing that there is free carbon in the flame. We therefore conclude that the hydrogen of the gas combines with the atmospheric oxygen first. In doing so, it develops great heat.

In the manufacture of charcoal, the delicate cells of the wood are preserved intact; showing that the carbon, at the high temperature to which it is exposed in the process, has no disposition to melt. Its infusibility is its most marked characteristic (41).

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