on account of the foul odor evolved from the lime when it is taken from the purifier. The dry-lime process consists in passing the gas through moistened slacked lime placed upon trays. This is about as effective as the other, and has generally superseded it. The iron process consists in passing the gas through some form of the hydrated sesquioxide of iron mixed with other substances. The great advantage of this process is its economy, it being practicable to use the same mixtures over and over almost indefinitely. The New York Mutual Company, for instance, have used one mixture satisfactorily for three years.
Either petroleum or some of the products of its distillation at a low temperature, as naphtha, rhigolene, gasolene, etc., may be used in the manufacture of gas. These products are of little commercial value as compared with those, like kerosene, which are produced at a higher temperature, but for this reason they are of especial value for the manufacture of gas. The principles on which the manufacture of petroleum-gas depends do not differ much from those involved in the making of coal-gas. In both cases, as already stated, the material is subjected to destructive distillation in a retort; but in this the material may either be introduced directly into the retort, or first converted into a vapor, and conducted into it in that state. The first step, however, is to vaporize the liquid either in the retort or before it reaches there; and the second, to decompose the vapor, and convert it into a fixed gas, which is carried into an hydraulic main and condenser in the same way as coal-gas. One great advantage of the naphtha-gas is that, containing neither sulphur compounds nor ammonia, it needs no purification, and therefore saves one item in the expense of manufacture. Moreover, a loss of some of the luminiferous hydrocarbons is avoided, a certain amount of them being necessarily condensed in the passage through the washers, scrubbers, and purifiers.
The manufacture of water-gas differs entirely from that of coal or naphtha gas. It involves the production, first, of a non-illuminating gas from steam, and, second, of petroleum, naphtha, or cannel gas, to furnish the illuminating power. The great advantage of it is, that very large volumes of non-luminous combustible gas can be made very cheaply. This is done by passing steam over incandescent carbon, which, having a very powerful attraction for oxygen, abstracts it from the steam (water being a compound of hydrogen and oxygen), and unites with it, forming, at first, carbonic acid. This, in passing over another bed of coal, is deprived in turn of one-half its oxygen, and converted into carbonic oxide. Hydrogen, the other constituent of the steam, being set free, mixes with the carbonic oxide. The resultant is a mixture of hydrogen and carbonic oxide, which gases are both combustible but non-illuminating. In some processes for the manufacture of this gas, the petroleum or naphtha gas
