MANUFACTURE OF COAL-GAS. 31 



resin, and also of coal when the process is well conducted. It is colourless, 

 tasteless, and without smell when pure. Water dissolves about one-eighth of 

 its bulk of this gas. It is formed of two volumes of hydrogen, and two of the 

 vapour of carbon, condensed into one volume. It burns with an intense white 

 light, and requires a large portion of oxygen for its combustion ; one volume 

 of the gas requiring not less than three volumes of pure oxygen, or fifteen 

 volumes of atmospheric air, for decomposition. The products of the com- 

 bustion are water and carbonic acid. It is decomposed by being passed over 

 bodies at a red heat, carbon being deposited, and hydrogen set at liberty. 

 This fact is of the greatest importance to the gas engineer ; it should teach 

 him the necessity of operating upon the coal in thin layers, and allowing a 

 ready escape for the gas when formed, or the decomposition spoken of will 

 take place ; and although the bulk of the gas so formed is increased (each 

 volume of olefiant gas containing two of hydrogen), yet it is possessed of 

 weak illuminating power, and cannot but be designated as bad gas. The 

 more olefiant gas we can form, the richer is the product for the purposes of 

 illumination. 



SULPHUR exists in coal as an impurity, under the form of the sulphuret of 

 iron, or " martial pyrites," as it is sometimes called. During the distillation 

 it is decomposed, the sulphur combining with a portion of hydrogen, and 

 escaping under the form of sulphuretted hydrogen gas, part of which unites 

 with the ammonia, and is condensed in the aqueous fluid which floats on the 

 surface of the tar ; while another portion escapes uncombined, and would mix 

 with and deteriorate the gas, were it not intercepted by the lime purifiers. 



SULPHURETTED HYDROGEN is a colourless gas, with an offensive taste and 

 odour, resembling that of putrefied eggs. It dissolves in its own bulk of water, to 

 which it communicates its taste, odour and characteristic properties. It is com- 

 bustible, burning with a blue flame, and emitting a suffocating smell similar 

 to that of a burning match. During the combustion its hydrogen unites with 

 oxygen to form water, while its sulphur unites with another portion of oxygen 

 to form sulphurous acid. It is to the presence of the vapour of this substance 

 that the disagreeable property of tarnishing metals which characterizes the 

 combustion of impure gas is owing. 



The solution of sulphuretted hydrogen has the properties of an acid. It 



