the retorts have been heated to a red heat, the oil is introduced from the holder A through the inlet 7. The gases and tar vapors ascend in corresponding pipes to the vessel D, which contains a layer of tar to prevent the gas from passing back

into the retorts. From here the gas enters the condensers E, in which it is freed by coke from the condensable portions of tar. The gas then passes through F and from there into the gasometer. The manometer H serves for measuring the pressure when the gas leaves the retort. The test cocks K serve for judging of uniform production, to control the color of

the gas and to discharge the tar precipitated in the pipe G. The principal point in the operation is to attain a uniform brick-red heat, which is readily effected by manipulating the draughts, etc. Irregular heating may cause the retorts to crack, which would occasion loss of gas, etc. When the retort is but slightly heated, much tar is formed in the condensers, the oil not having yielded all substances suitable for production. Superheating the retort causes the separation of too large a quantity of coke.

The yield with the same material depends solely on the temperature at which the operation is carried on. The lower the temperature, the smaller the yield of gas, which possesses, however, greater illuminating power; and the higher the temperature the more gas, but of less illuminating power, is obtained.

The following points serve as indications of the correct and uniform production of gas:

I. A uniform cherry red heat of the retorts.

2. A pale brown color of the gas coming from the testcocks.

3. Proportional production of tar; which may vary from 26 to 30 per cent., but should not exceed 33 per cent.

4. Slight heating of the tar-condenser.

Hübner, in 1870, constructed a vertical gas retort, which has been improved by Schumann. It is, as a rule, lined partially or entirely with chamotte. The process of manufacturing the gas is the same as with horizontal retorts.

The advantages claimed for vertical retorts are as follows: With the same wall surface the vertical retort permits within a certain period of time a much larger charge of oil than the horizontal retort, because the oil can be very well conveyed to various portions of the red-hot walls and the oil vapors formed be readily brought into more intimate contact with the latter. For this reason the vertical retort possesses greater capacity than the horizontal form.

The yield from a vertical retort is more uniform and larger, because gasification is more complete, and tarry and asphaltlike residues cannot collect in it.

A smaller quantity of fuel is required for the production of a certain quantity of gas.

It is

Of the more modern gas producers, Hirzel's apparatus with spherical retort and gas augmenter may be mentioned. shown in Figs. 281 and 282.

A is the furnace for the retort B. The retort is fed from the oil-holder L by means of the siphon-like contrivance a in such a manner that only as much oil as can at one time be decomposed to gas reaches the heated retort, so that the latter is always empty and the manufacture of gas may be interrupted at any time by closing the cock on the pipe leading from the oil-holder L. RR are manometers which indicate the pressure in the retort and in the condenser.

The gas generated in the retort passes through the pipe D into the tar cylinder, the so-called hydraulic box E placed upon

the furnace, in which it has to pass first through a layer of tar. This layer of tar closes the mouth of the pipe D so that it is impossible for any of the gas to flow back from the gas-holder

into the retort. It never decreases, but to prevent, on the other hand, its increase, a pipe F is provided on the side of the hydraulic box E, through which an excess of tar passes together with the gas into the condenser G filled with coke. From the condenser the gas passes into the special purifiying apparatus I, filled with a suitable purifying mass, in which it is freed from all

impurities. From this purifying apparatus it passes through K into the gasometer.

To increase the yield of gas Hirzel's apparatus is provided with a so-called gas-augmenter. It consists of a vertical U-shaped retort S, which is filled with small pieces of coke and heated by waste gases of the principal retort. From a vessel M water drips through the siphon pipe Q into one of the legs of the retort S. Water-gas is formed which by mixing with the oil vapors increases the yield.

The furnace of R. Schwarz is furnished with a retort which can be revolved by an endless chain. The oil entering the hot revolving retort describes in it a spiral course, so that it constantly meets fresh gasifying surfaces.

D. H. Knapp uses two retorts, one inside the other; the oil is vaporized in the inner retort and gasified in the space between the two retorts.

Pintsch's gas producer consists of two retorts, one above the other; the oil is gasified in the upper retort, and the gas produced again heated in the lower, whereby condensation is reduced to a minimum and the yield of gas consequently increased to a maximum. Pintsch's retort is especially suitable for the manufacture of permanent gas for lighting railroad carriages.

All other systems with simple retorts 234 to 5 feet long, vaporize, as a rule, the higher oils only, while the heavier oils pass over into the hydraulic box and condenser, and run off from the latter together with the tar. Pintsch was led to the construction of the double retort by the desire to produce an oil-gas for lighting railroad carriages, which could be compressed without again liquefying and losing any of its illuminating power.

Pintsch's apparatus is shown in Figs. 283 and 284. From the oil-holder B placed on top of the furnace, the oil intended for gasification passes through an inverted siphon, over sheetiron trays C, into the upper retort A, and descends through pipes to the lower retort D. It has now become a thick yel