Rape oil, according to Lefort, has the formula C10H18O2.; hemp oil, C15H28O2. All the fatty oils yield by dry distillation principally defiant gas, a small quantity of carbonic acid, and no sulphuretted hydrogen; and if pure oil were used, it would require no purification, and apparatus only of the simplest kind. Under such circumstances perhaps a lamp is the most economical; but the gas may be made of impure oils and fats, in which case some purification is required. On the continent of Europe gas is made from suint, or the fatty materials contained in the soap suds after washing wool and yarns. The liquid is mixed in cisterns with milk of lime and left to stand 12 hours, when a kind of lime soap is formed, which is made into bricks and dried. These are subjected to dry distillation, and yield a gas of high illuminating power. The wash water of a woollen mill of 20,000 spindles will yield annually enough of this substance to produce over 1,100,000 cubic feet of gas; and if the time of burning is 1,200 hours, this quantity will supply 500 burners, each consuming nearly two cubic feet per hour, and giving a light more than sufficient for the mill.- Water Gas. When steam is forced' through retorts containing red-hot coke, charcoal, or anthracite, there are produced hydrogen, carbonic oxide, carbonic acid, and a small amount of light carburetted and of sulphuretted hydrogen gases.

The carbonic acid and sulphuretted hydrogen may be removed by lime, or lime and oxide of iron. The remaining gases, principally hydrogen and carbonic oxide, may be used for heating purposes, or may be made available for lighting in two ways: 1, by heating coils of platinum wire in the flame; 2, by impregnating it with the vapors of various hydrocarbons, as benzole or naphtha, or mingling it with permanent hydrocarbon gases, the latter being preferable, as it has been found very difficult to convert the lighter hydrocarbon oils into products which will not cause a deposit in the distributing pipes. Many hundred patents have been issued to inventors for making this kind of gas. That of Selligue, a French gas engineer, proposed to him by Jobard of Brussels, consisted of a furnace and three vertical cylindrical retorts, the first two filled with charcoal or coke. Steam was passed into the first, causing evolution of the gases above mentioned, which were passed into the second retort, where the red-hot coal or coke converted the carbonic acid into carbonic oxide. The gases were then passed into the third retort, which was two thirds filled with red-hot iron chains, upon which a stream of oil from bituminous shale was made to flow.

Mr. White of Manchester patented several years ago a process by which the water gas was passed into another retort, in which illuminating gases were being generated, in such a manner as to sweep the latter out of the retort as quickly as possible, to remove them from the decomposing action of the intense heat. The retorts and settings were similar to those in use for ordinary coal gas, except that the retorts had a horizontal partition, dividing them into two chambers, extending to within a foot of the back. White's method is known as the English hydrocarbon process. Experiments which have been made in the United States show that anthracite can be used with greater advantage in producing water gas than coke or charcoal. The citizens' gas light company of Brooklyn are now (March, 1874) making gas according to what is known as the Gwynne-Harris or American hydrocarbon process, one of the patents of which was issued to W. H. Gwynne in 1863. Steam from a boiler is first passed through a superheater which is raised to a temperature of about 600°; thence through false bottoms into retorts containing incandescent anthracite coal, where it is completely decomposed, forming hydrogen and carbonic oxide.

This is conducted into the hydraulic main, where it mingles with the gas generated from bituminous coal or naphtha, and which, being rich in heavy hydrocarbons, produces sufficient illuminating power.