torts may be injured, as well as the illuminating quality of the gas be so materially diminished as to render it of less value to the consumer. The degree of heat usually employed is that of a cherry redness, and when the coal has remained in the retorts a sufficient length of time for all its gaseous matter to be expelled, the covers being removed from their mouths, the residue consisting of the coke, is raked out, and falls into a receptacle below them in order to become cold. At large establishments, whenever the coke has been taken out of them, the retorts are not suffered to lose their heat, but are immediately replenished with fresh coal to renew the operation; and this practice is continued till the retorts are either damaged by wear, or some other circumstance may require their removal.*

* Mr. Ibbetson, of Chelsea, has recently obtained a patent for some improvements in the process of making gas; and he states his supposition that more than half the usual cost might be saved by the adoption of his plan. The principal feature of it is, to cause the volatile products to pass through red-hot coke as they are disengaged from the coal, so as to effect their more complete conversion into gas, and particularly that part which forms the tar. Another part of it is, to obtain carburetted hydrogen by passing the steam of water through red-hot coke, inferring that the hydrogen of

OF THE USE OF THE HYDRAULIC MAIN.

After the retorts have been properly heated and supplied with coal, the next object of attention is the mode by which the different volatile substances are collected and condensed, as they are gradually produced. To effect this purpose, the first vessel employed is called the hydraulic main, (Plate I. Fig. B,) which is a strong castiron pipe of large dimensions. Its position is required to be perfectly horizontal; it is placed at the front of the brick-work, where the retorts are

*

the water will dissolve, and combine with, the carbon of the coke, so as to produce carburetted hydrogen. He also supposes that the reason why coke and brick act upon oil is their moisture, and that their action ceases when that is exhausted: he also supposes that brick and coke do not act mechanically only, but chemically. His pamphlet contains some useful suggestions, but it is to be regretted that facts are not given to prove that his scheme has been realized.

The length of the Hydraulic Main is regulated by the number of the retorts to which it is adapted; its diameter is usually about twelve inches. The number of upright and dip-pipes is equal to that of the retorts, each requiring one, and the latter are generally about two feet long, with a diameter of about three inches. The invention of the Hydraulic Main is claimed by Mr. Clegg, and was a most important improvement in the apparatus for making gas.

set, and is supported by appropriate iron columns. The length and diameter of vessels of this kind depend upon the number of the retorts connected with them; and they are usually fixed so as to be parallel to the upper parts of the brick-work which contains the retorts, at the distance of about two feet from it, in order that they may be kept cool.

As the principal purpose of the hydraulic main is the condensation of the several substances when the heat first expels them from the coal, it is constructed so as to contain a certain quantity of cold water. One end of it is perfectly closed by means of a flanche, the other end has also a similar flanche; and, at a little distance above its horizontal diameter, a pipe is attached to it through which the gas passes. In the inside of that end of the main to which the pipe is connected, a semicircular piece of iron is placed, so that the water is preserved at a certain height, and cannot sink below the proper level required for the operation.

By a reference to Plate I., it will be perceived that some upright pipes (2, 2) which are fixed to the retorts ascend above the Hydraulic Main, and by means of proper bent pipes they are connected with others called dip-pipes. These latter are firmly attached to the Hydraulic Main, in which proper apertures are made to receive them; and

the ends that are fixed to the Main descend so far into it as to be immersed in the water two inches or more below its surface; by this means the gas is forced to pass through a certain quantity of the water before it can enter the upper part of the Main to escape. The gas, as it passes through the water contained in the lower part of the Hydraulic Main, is partially condensed, and deposits a portion of the tar* and ammonia that are mixed with it in its first production; but from the heat which it retains after this first operation, some of the tar still remains, and it requires farther condensation. In order to cause a complete deposition of the tar, and to bring the gas into such a state that it may more easily be purified, it passes, by means of the pipe affixed to the end of the Hydraulic Main, into a very large vessel, denominated the Condensing Apparatus.

OF THE CONDENSING APPARATUS.

When coal gas is first produced, the quantity of heat which it contains is so large as to occasion

* Sometimes the tar accumulates so as to render it necessary to convey it from the Hydraulic Main, and this is effected by means of a pipe adapted for the purpose; the ammonia has such a strong affinity for water, that the greater portion of it combines with it in its passing through

some other substances to remain in a state of combination with it. These require its temperature to be reduced, in order to separate them from it; and, indeed, in proportion as the gas is deprived of its excess of heat, is the ease and readiness of its purification from those substances which are combined with it and contribute to render it offensive.

The vessel which is used for the more effectually cooling and condensing of the gas is called the Refrigerator or Condenser; and it consists of a very large cistern or tank, the interior of which is fitted up with pipes, that are kept constantly immersed in cold water.* Its form is usually

* The condensers, or purifiers, first employed for coal gas, consisted of an oblong-square cistern, divided by several upright partitions, and these were perforated alternately at one end. For instance: the first partition had the holes at the lower end; the second at the upper; the next at the lower; thus changing through the whole series; so that, as the gas entered at the top of the vessel, it had to force its way through the whole body of water before it could escape to the burners. Another mode of construction was to have the partitions shorter, and fix them alternately to the top and bottom of the vessel, so that they projected between each other; thus the first, being attached to the top of the vessel, extended nearly to its bottom, and the next, being fixed to the bottom, ascended nearly to the top, going on in the same manner through the whole range. Mr. Malam, of the Chartered Gas Works, afterwards adopted the plan of placing the shelves horizontally, instead of vertically; but the principle