In the making of coal if this distillation is complete, a substance called graphite is obtained. Graphite is the black lead used in lead pencils and in stove polish. It is a shiny, black mineral with a slippery feeling and is nearly 100 per cent carbon. If the distillation is less complete, hard coal, called anthracite containing about 90 per cent carbon, results. If still less perfect, soft or bituminous coal, having varying percentages of carbon, is formed.

Coal

Graphite

Where the process goes on under water, peat is found. This is partially formed coal, but little distilled and contains only about 40 per cent carbon.

Besides carbon, these substances are made up of gases composed of carbon and hydrogen, called hydrocarbons. These gases give the yellowish and orange flames in a coal fire. Pure carbon does not burn with flame - it merely glows. Anthracite coal contains only from 3 to 4 per cent of volatile matter, but bituminous coal may have 30 to 40 per cent of these hydro-carbon gases.

Coke is made by the destructive distillation of soft coal. Like charcoal, it is chiefly carbon, but contains more mineral matter (ash). The coke obtained as a bi-product in the manufacture of coal gas is rather soft, but when coke is made as the principal product, it is hard and brittle. Coke makes a very hot fire without flame, but does not last as well as hard coal. The ash should be allowed to accumulate in the grate when burning it. Many consider it an improvement over soft coal for household use and it might be used to advantage more than it is.

Graphite is so hard and compact that it cannot be burned. Anthracite ignites with some difficulty and then burns slowly with intense heat.

Bituminous coal ignites readily and burns well when there is sufficient draft. The "coking" variety cakes over on top and the fire must be broken up to allow the air to penetrate the fire. Soft coal should be put on the fire in small amounts as otherwise the hydrocarbon gases escape unburned and thus much heat value is lost. Smoke is made up of finely divided particles of carbon and is always an indication of incomplete combustion and, therefore, loss.

Oil from tank (not shown) is forced up O, is vaporized in passing through the straight tube, mixes with air at A,. and burns with a blue flame at the top.

Kerosene and gasoline are also important fuels. Gas will be taken up under the subject of light. Petroleum is an oily liquid found in many places in large quantities, particularly in Pennsylvania and Ohio. It is made up almost entirely of compounds of carbon and hydrogen (hydro-carbons ).

When the crude petroleum from the Pennsylvania district is purified by distillation and other processes, the main product is kerosene. The lighter and more volatile products are gasoline, naphtha, and benzine - all three having much the same composition. Gasoline is the most volatile. Among the heavier products are various lubricating oils, vaseline, and paraffin.

In order to burn, kerosene must be vaporized. In the new blue flame oil stoves, various devices are employed to vaporize the oil. In Fig. 12 the oil passes through a tube heated by the flame, where it is changed to vapor which is mixed automatically with air and is then burned. Sometimes an alcohol flame is used to start this process, but the flame of the burning oil itself continues it. A slight pressure of air is maintained in the oil reservoir to give a constant small jet of oil to be vaporized. In other styles of stoves, the oil is fed automatically by gravity to a hollow ring, when it becomes heated to the point that it gives vapor. The vapor mixes with air and burns with a blue flame.

Fig. 13.

Blue Flame Oil Stoves

Fig. 11. Charcoal Kiln

Gasoline is burned on much the same principle as kerosene. It vaporizes much more easily and the pressure for the flow of the gasoline is furnished usually by having the tank a few feet above the burner.

Gasoline

The measure of safety of kerosene is the temperature at which it will give off an inflammable gas. This is called the flash point and is determined by heating the oil slowly and observing the temperature at which a flash can be produced by applying a lighted taper to the surface of the oil. Below the flash point, there is no danger of explosion from oil. Most states in the United States have a legal flash point, or a fire test, below which standard kerosene cannot be sold. The flash point of good kerosene is 1200 F. The fire test is the temperature at which the oil will take fire and burn when a light is applied. This is about 300 F higher than the flash point. The ordinary temperature of the room is above the flash point of gasoline, naphtha, benzine, etc. In other words, these substances are constantly giving out an inflammable vapor.

A comparison of the heating value of the various fuels will be of interest. Practical tests of the amount of steam produced in a steam boiler have shown that one cord of ordinary wood is approximately equal to one-half ton of coal; a gallon of oil (or gasoline) is equal to about twelve pounds of coal; 1,000 cubic feet of coal gas is equal to 50 or 60 pounds of coal, or about four and one-half gallons of oil. Hard coal has a little higher fuel value than soft coal, because the combustion is commonly more perfect. Coke is nearly equal to hard coal by weight, but is much more bulky. It is usually sold by measure. A bushel of coke weighs 40 pounds, of anthracite 67 pounds, and of soft coal 76 pounds. Damp wood is a much poorer fuel than dry wood, because so much heat is absorbed and wasted in changing the water into steam.

The heat given off by a fuel is not the only point to be considered. In the cook stove, but a small portion of the heat given off by the solid fuel can be used for cooking, as most of it is radiated into the room or carried up the chimney. In the gas or oil stove, the flame may be applied exactly where it is wanted, so that the proportion of heat which can be used is much greater. Moreover, the flame can be shut off instantly when wanted no longer and all expense stopped. On the other hand, the range usually serves to heat the water of the hot water system, incinerate garbage, and in winter helps to heat the house.

Fig. 12. Burner of a Blue Flame Oil Stove