of the hydraulic main it will have deposited from one-half to one-third of its condensable constituents.

The temperature within the retorts may be taken at from 1800° to 2000° F., and yet in the hydraulic main the temperature will be found to be only 140° to 180° F. The reason of this difference, is due to the rapid absorption of heat by the volatile constituents of the coal in assuming the gaseous form; this heat becoming latent in the gas, in the same way as the latent heat of steam when water boils. On leaving the hydraulic main the gas has a temperature of from 110° to 150° F., and this has to be reduced to a temperature of from 50° to 60° F., and the remainder of the tar got rid of, before the gas is allowed to enter the purifying plant. These operations are performed in the apparatus known as the condensers, the operation itself being known as "condensation." The object of condensation is to take out on the works, and at an early stage, substances which cannot be retained in the gas, and which would be deposited in the purifying apparatus and distributing mains, care being taken to retain in the gas all substances which are useful in contributing to the illuminating power of the same. Water, and most of the compounds which the water holds in solution, are useless, and cannot be distributed with the gas; it is necessary therefore that these should be removed.

The heaviest hydrocarbons (tar) are useful as illuminants, but cannot be distributed with the gas as they become liquefied at ordinary temperatures; these should also be removed. The permanent gases and the lighter hydrocarbon vapours are however extremely useful as illuminating agents, and can be carried by the gas; it is essential therefore that no more of these substances be removed than is absolutely necessary to effect a proper removal of the compounds which must be got rid of on account of their uselessness as light givers, or incapability of being distri

buted. The most useful of all substances as light givers when burnt are the vapours of hydrocarbons, and therefore great pains should be taken to keep in the gas the largest possible quantity of these vapours; the method for effecting this is described a little later on.

The operation of condensation is in reality two-fold: consisting, firstly, in cooling the gas to the required temperature; and, secondly, in removing the tar. The orthodox methods in use do not completely effect the second object, as a considerable portion of the tar is present in the form of innumerable minute globules of tarry matter, each one of which contains a small volume of gas, and it is necessary to subject these tarry vesicles to some degree of friction in order to break them up, and thus effect their removal, the treatment which they receive in the ordinary condensing plant not affording sufficient friction. The apparatus for effecting this friction is generally some form of washer, such as Livesey's, or Walker's, usually fixed at the bottom of the scrubber, as described under that apparatus, or by the employment of the condenser of Pelouze and Audouin.

As the substances to be removed by condensation for the most part deposit themselves naturally as the crude gas gets cool, no complex apparatus is necessary. All that is required is a length of pipe in which the gas may cool down from about 150° F. (the temperature at which it leaves the hydraulic main) to about the normal temperature of the atmosphere; but before proceeding to describe the different forms of apparatus employed for this purpose, it will be necessary to revert to the means to be adopted to retain the illuminating hydrocarbons in the gas, and incidentally the treatment to which the gas should be subjected before entering the condensers.

It was previously remarked, that the tar thrown down in the hydraulic main consisted of impure hydrocarbons, and it has been shown by the late Rev. Mr. Bowditch and

others that the hydraulic main tar, which is of a very heavy character, if allowed to travel or come in contact with the gas, possesses the property of robbing the latter of its lighter and light-giving hydrocarbons; so, in order to avoid this, the tar which is deposited in the hydraulic main should be run off from the bottom of the latter, and deposited in the tar well, the gas travelling by a separate main at a higher level, the latter being joined to a pipe carried with a gradual inclination round the inside of the retort-house (known as the foul main), and from thence to the condenser; the gas would thus travel with the lighter tars, which are deposited after leaving the hydraulic main, and if arrangements are made for taking off the tar when the temperature falls to about 100° F., and the gas travels slowly, it is believed that the deposition of naphthalene is prevented, and the illuminating power increased by the abstraction of some of the illuminating matters from the tar. Further, in order to avoid any contact of the gas with the heavy tar, the hydraulic main should be supplied with ammoniacal liquor, all tar as deposited being at once got rid of as described; this will prevent the gas having to bubble through the column of tar as is ordinarily the case, with the consequent absorptive action of the thick tar.

After being deprived of the heavy tar, the process of condensation should be performed very slowly, and the gas be not allowed to cool below 50° F. If condensation is carried beyond this, there is a risk of some of the lighter hydrocarbons being thrown down, and the gas reduced in illuminating power accordingly; for this reason the condensing plant should be under complete control, by the provision of suitable bye-pass valves and connections, so as to regulate the amount of cooling according to the temperature prevailing. It is necessary to have thermometers inserted in the inlet and outlet of the condenser for the purpose of ascertaining the temperatures at any moment.

Condensers are divided into two classes: atmospheric condensers and water condensers, according to the cooling medium employed.

Atmospheric condensers comprise the ordinary vertical, the horizontal, the annular, and the battery condenser, the principal water condensers

being Livesey's, and Morris and Cutler's.

The vertical atmospheric condenser is shown in fig. 12, and consists of a series of vertical pipes attached to a cast-iron box or cistern at the bottom, and connected in pairs by semicircular bends at the top. The cast-iron cistern is provided with a series of midfeathers, which dip to а certain depth in liquid, thus forming a seal, so that the gas is forced to pass up one pipe and down the next right along the series; the condensed products are deposited in the cistern, from whence they flow to the tar well.

FIG. 12.

FIG. 13.

Condensers on the atmospheric plan act by transmitting heat from the gas and vapours within, to the external air in contact with its outer surface, so that the gases and vapours are cooled so as to separate the one from the other.

The horizontal atmospheric condenser is shown in fig. 13,

and consists of a series of pipes laid in a horizontal direction, and with a slight fall; the gas generally enters at the top, and leaves at the bottom, of the apparatus, whence the condensed products also flow away by reason of the slight amount of fall previously alluded to. outlet pipe is sealed in a seal-pot, so that the condensed products flow away to the tar well automatically without the gas escaping, this device being commonly applied when we wish to get rid of liquid products without allowing gas to escape.

The annular condenser (fig. 14) is an extremely efficient instrument, and consists of a series of large pipes placed in a vertical position, each pipe enclosing a smaller pipe, which is open to the atmosphere, the two pipes thus forming an annular space through which the gas is caused to pass. Other pipes, placed diagonally, connect the top and bottom of the condensing columns alternately. By this arrangement the gas passes through the annular space always in the downward direction, whilst the current of air which is produced by the heating agency of the hot gas, moves upwards through the inner air pipe.

FIG. 14.

The bottom of the inner or air pipes should be provided with butterfly valves, so that in cold weather the airdraught may be regulated so as not to reduce the temperature to too great an extent. A small pipe is connected to the bottom of each column to carry away the condensed products.

The battery condenser (fig. 15) consists of an oblong