PURIFICATION.

191

parts with its sulphur, a small portion of which also absorbs oxygen and produces sulphuricacid, more especially if any ammonia or other alkaline matter be present in the material;and as no water is combined with the sulphuret prior to this oxidizing process, the resultingoxide of iron must be anhydrous, whatever form of the oxide may have been originally em-ployed. This accounts for' an otherwise inexplicable fact, that when the hydrated sesqui-oxide is used the first time, it purifies only about half as much gas as on the second and thirdtimes of being used, when it has become in a great measure anhydrous. It has been found,indeed, by experiment, that in the first trial of a given quantity of hydrated oxide of ironplaced in a dry-lime purifier, it purified only 1500 cubic feet of gas ; on its second trial, afterexposure to the air, it purified 2400 feet; that after a second exposure to the air it purified3050 feet; and after another exposure, the gas purified by the same quantity of oxide ofiron amounted to 4200 cubic feet, or nearly three times the original volume. This agreeswith the result of the daily experience of gas engineers. It may therefore be assumed notonly that the hydrated oxide becomes anhydrous by the process of revivification, but that theanhydrous form is the more active condition of the two, though the greater activity of thelatter does not depend on its hydrated or anhydrated condition, but is attributable to thepeculiar molecular arrangement, analogous to that of spongy platinum, cobalt, nickel, andsome other substances, in which state a larger extent of surface is exposed to act uponthe gas.

One of the greatest disadvantages connected with the oxide of iron process of purifica-tion is the tendency of the sulphuret of iron to ignite and become red-hot. When thishappens, and is not checked at the commencement, the fire spreads through the entire mass,volumes of sulphurous acid are given oft', the air becomes unfit for respiration to a consi-derable distance, and the oxide of iron is in a great measure spoilt. Nothing but great carecan prevent the occurrence of such an accident, for it arises apparently from the same causewhich leads to the ignition of metallic iron wdien in a state of minute division, such as re-sults from the reduction of oxide of iron at a low red-heat by hydrogen gas.

The chemical and molecular actions that take place wdien sesquioxide of iron is exposedto impure coal-gas, and afterwards to the action of atmospheric air, are not yet thoroughlyunderstood. According to theory, not more than 16 lbs. of oxide of iron should be requiredto remove the wdiole of the sulphuretted hydrogen produced from a ton of coal, but itusually takes about four times that quantity to produce the desired effect. The oxide maybe used over and over again, until the quantity of sulphur in it becomes so great that byfusing during the revivification it seals up the particles of metallic oxide so as to preventtheir further action on the foul gas, w’hen of course a fresh supply is required.

The use of lime subsequently to that of the oxide of iron for the purpose of removing thecarbonic acid ought never to be omitted.

The evidence of the numerous scientific witnesses examined at the trial respecting thevalidity of Mr. Hills’s patent confirmed in all essential points the foregoing account, con-densed for the most part from the ‘ Journal of Gas Lighting,’ of the process of purificationby the oxides of iron. The principal point on which Mr. Hills rested his claims was, thatprevious to the date of his patent no one had knowingly used the hydrated oxide of iron for