the latter lowering it. The gas thus obtained burns with a blue flame, giving little light, and must be enriched with hydrocarbons to make it suitable for illumination. For power or fuel purposes, however, it has a value nearly half that of coal-gas or about twice that of producer-gas, and does not require the addition of hydrocarbons.

Producer-gas as made in the original Siemens producer is the result of the partial combustion of coal or other fuel by air. Its heating power depends upon the carbon monoxide, hydrogen, and hydrocarbon contained, but these are much diluted by nitrogen from the air and by carbon dioxide due to complete combustion. The later forms of producer employ a jet of steam, which with the coal forms carbon monoxide and hydrogen, also acting to force air through the fuel. Hence it is a combination of the water-gas apparatus with the Siemens producer, and has the advantage over the former of continuous action. A producer of this kind consists of a brickwork chamber, as represented in Fig. 63, into which coal is fed by hand or automatically through a hopper at the top. Sufficient fuel is introduced to form a mass 2 to 3 feet deep for anthracite and 3 to 4 feet deep for bituminous coal. Under this is a bed of ashes almost as thick, in order that the coal may be completely burned before it is dropped below, and to avoid too high a temperature in the lower part of the producer, where the air is introduced. The Taylor gas-producer here represented is provided with a circular iron plate upon which the fuel rests, that is revolved occasionally by hand with a crank and gearing, the object being to prevent the formation of channels in the fuel, and to discharge the ashes at the periphery. The small boiler (Fig. 63a) is to supply steam that is decomposed, and also forces air through the mass, as already stated. The gas from the producer passes through cast-iron tubes contained in the economizer, thus transferring its heat to the air which flows around these tubes on the way to the producer. The gas then enters the scrubber filled with coke that is sprayed with water to remove tar, sulphur, and ammonia. This operation is completed in the purifier and the gas enters the holder.

In the Loomis-Pettibone gas-plant the mass of fuel, about 8 feet deep, rests upon a grate. The draft is downward, being produced by a positive exhauster, and the resulting producer-gas

is stored in a holder.

When the fuel becomes incandescent the air is shut off and steam forced through the mass, producing

water-gas that is carried to another holder, these operations being performed alternately in periods of about five minutes.

In

The two kinds of gas may be used separately or mixed. passing downward through the highly heated fuel the gas is freed

from tar. The hot gas from the producers is carried through a vertical boiler which supplies the steam required.

A modification

of this plant for 5 to 50 H. P. operates by suction, the flow of gas being produced by the engine which draws in its supply. A blower worked by hand is used in starting, but a holder is not required, the gas being used as fast as it is generated.

Producer-gas consists of nitrogen, carbon monoxide, hydrogen, and small quantities of carbon dioxide and hydrocarbons. The reaction may be represented theoretically as follows:

3C + H2O + 02 + 4N2 = 3CO + H2 + 4N2;

practically as follows:

2

8C+3H,O+302+4N2 = 7CO+3H2+4N2+ CO2. Practically 2 to 5 per cent of carbon dioxide is formed, hence the proportion of carbon monoxide is somewhat less and that of nitrogen more than the equation indicates. Either anthracite or bituminous coal may be gasified in a producer, the latter giving the richer gas, but analyses do not show much difference in composition, because the heavy hydrocarbons are condensed before the analysis, but not in actual use.

One pound of anthracite (85 per cent carbon, 5 hydrocarbon, and 10 ash) makes 80 to 90 cu. ft. of producer-gas retaining about 80 per cent of the original energy in the fuel and having the composition and heating power given in the table on page 212.

Blast-Furnace Gas.-In order to maintain the reducing action in a blast-furnace less than one-third of the carbon can be allowed to become carbon dioxide, hence the discharged gas consists largely of carbon monoxide and is capable of giving 90 to 115 B.T.U. per cu. ft. In the production of a ton of pig iron about 130,000 cu. ft. of this gas are given off and 80 to 120 cu. ft. are required by a gas-engine per H. P.-hour. An ordinary furnace yields about 25 tons of pig iron per hour, hence the gas from it would give 25 x 130,000 100 32,500 H.P. as a by-product.

From 10 to 25 per cent of the gas is used for heating the blast, leaving 25,000 to 29,000 H.P., of which 30 to 40 per cent is needed. in the blowing-engines. Hence 15,000 to 20,000 H. P. is available for other purposes from each furnace in operation. The usual composition of blast-furnace gas (by volume) is: carbon monoxide 24 to 30 per cent, carbon dioxide 9 to 12 per cent, nitrogen 58 to 60 per cent, hydrogen and hydrocarbons 3 to 5 per cent.

For the other gases the following figures may be taken:

45.6

32.0

45.6

65.6

65.9

Pounds in 1000 cubic feet.....
B.T.U. in 1000 cubic feet..... 1,000,000 725,000 325,000 140,000 160,000

Oil-Engines. An internal-combustion engine using kerosene oil differs from the gas-engine proper only in having a device by which the liquid fuel is vaporized. In the case of a volatile liquid a true vapor will be formed by passing air through it or merely over its surface. In order to use a liquid not readily volatile, heat must be applied to produce vaporization. If the vaporizer is too hot, the fuel will be carbonized; if too cold, it will not vaporize; and for any one liquid fuel the range is very small.

A

Overcoming Unsteadiness of Gas- and Oil-Engines. Some types of gas-engines, especially those of small size and the half single-acting forms, are likely to be unsteady in speed. In incandescent lighting this is very objectionable, and some means should be employed to overcome the difficulty in such cases. storage-battery connected in parallel with the generator may be used, or the engine may be run part of the time to charge the battery from which the lamps are fed. These applications of the storage-battery are discussed in Chapter XXI.

A heavy fly-wheel on the gas-engine shaft tends to prevent variation in speed due to intermittent action, and is still more effectual if a fly-wheel is also applied to the generator-shaft, provided a spring or other elastic connection is interposed between