+ 8o THE POPULAR SCIENCE MONTHLY. 



on account of the foul odor evolved from the lime when it is taken 

 fro,ni 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 effective as the other, and has generally superseded it.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 pro- 

 cess consists in passing the gas through some form of the hydrated 

 sesquioxide of iron mixed with other substances. The great advan- 

 tage of this process is its economy, it being practicable to use the 

 same mixtures over and over almost indefinitely. The New York Mu- 

 tual 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 dire,ctly 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 con- 

 denser in the same way as coal-gas. One great advantage of the 

 naphtha-gas is that, containing neither sulphur compounds nor am- 

 monia, it needs no purification, and therefore saves one item in the 

 expense of manufacture. Moreover, a loss of some of the luminifer- 

 ous hydrocarbons is avoided, a certain amount of them being neces- 

 sarily 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 car- 

 bon, which, having a very powerful attraction for oxygen, abstracts 

 it from the steam (water being a compound of hydrogen and oxy- 

 gen), 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 ox- 

 ide. The resultant is a mixture of hydrogen and carbonic oxide, 

 which gases are both combustible but non-illuminating. In some pro- 

 cesses for the manufacture of this gas, the petroleum or naphtha gas 



