OF THE MANUFACTURE OF GAS. 



75 



class, are those wliich possess the highest illuminating power. 

 All the compounds belonging to this class are, as before 

 stated, decomposed more or less rapidly at a red heat, and in 

 the ordinary process of gas-making, the interior walls of the 

 retorts soon become coated with a stratum of carbon derived 

 from this source. Now the extent of this decomposition 

 must depend, first, upon the length of time during which they 

 are exposed to the heated materials, and secondly, upon the 

 number of particles which are in contact with the red hot 

 surface, consequently it will be diminished, first, by removing 

 the gases rapidly from the retort, and secondly, by the mixture 

 of the illuminating constituents with the non-illuminating 

 ones ; for it is evident that the number of particles of olefiant 

 gas in contact with a, given surface, would only be half so great 

 if this gas were diluted with an equal volume of hydrogen, as 

 it would be without such an admixture. 



Besides the use that has already been mentioned of the 

 second class or non-illuminating gases, they are of value as 

 forming a medium for the solution of the vapours of such 

 hydrocarbons as exist in the liquid or even solid state at the 

 ordinary temperature of the atmosphere, and they thus ena- 

 ble us to convert an additional quantity of illuminating 

 materials into the gaseous form, which they retain perma- 

 nently unless the temperature fall below the point of satura- 

 tion. The gain in illuminating power which is thus obtained 

 will be perhaps better seen from the following example : — 

 Suppose 100 cubic inches of olefiant gas, allowed to saturate 

 itself with the vapour of a volatile hydrocarbon, containing 

 three times as much carbon in a given volume of its vapour as 

 that contained in an equal volume of olefiant gas, took up or 

 dissolved three cubic inches of this vapour, then, if we 

 express the value of 1 cubic inch of olefiant gas by unity, the 

 illuminating power of the 103 cubic inches of the mixture of 

 olefiant gas and hydrocarbon vapour will be 109. Now if we 

 mix these 103 cubic inches with 100 cubic inches of hydrogen, 



