ATTESPniG THE FORMATION OF COAL, &c. 273 



hundred-weight and a half, into retorts heated into a bright 

 redness as is now done, it is exposed to two very different 

 conditions; the surface of the mass, the exterior, in contact 

 with the intensely hot retort is instantly decomposed, and 

 charred ; hydro-carbons, as olefiant gas, &c., are eliminated, 

 which also, at this high temperature, are partly decomposed, 

 and resolved into light carburetted hydrogen and pure hy- 

 drogen, with deposition of carbon, which, with some un- 

 decomposed olefiant gas and volatile hydro-carbons, pass oj0f 

 from the retort — ^the interior of the mass, on the contrary, is 

 for some time exposed to a very moderate temperature, and 

 a simple distillation is accomplished; those compounds which 

 are formed at a comparatively low temperature, the heavy 

 hydro-carbons, which would ordinarily be in a liquid 

 state, are given off; a portion, rising into vapour as it reaches 

 the hotter surface, passes off with the gases formed, and 

 condenses again when it has left the retort in the form of 

 tar; but that portion of the vapour which, in its passage, 

 comes into contact with the red-hot surface of the exterior 

 of the mass and of the sides of the retort, deposits a portion 

 of its carbon, and is resolved into simple compounds, olefiant 

 gas and volatile hydro-carbons, which themselves partly un- 

 dergo the change already described. As the heat penetrates 

 to the centre, and a red-hot mass of charred material of 

 considerable thickness comes to surround the decomposing 

 coal within, as happens towards the end of the distillation, 

 the whole of the hydro-carbons, viz., light oils, volatile hydro- 

 carbons, olefiant gas, and even light carburetted hydrogen 

 itself, that are eliminated, are decomposed in passing over 

 such an extent of red -hot surface, and pure hydrogen is almost 

 alone evolved. The carbonic oxide, which is formed from 

 the union of the oxygen oi the coal and of the air admitted 

 with the carbon of the coal, is also partially decomposed 

 during the whole of the process, but in an opposite direction ; 

 not by depositing carbon, but by taking up more, and being 



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