﻿Acetylene. 113 



The heat abstracted by the walls of the iron tube pre- 

 vents the mixture from obtaining its limit of explosive- 

 ness, and a still smaller percentage of either gas mixed 

 with the air of a room would explode. Lechatelier 

 (Comptes Kendus, 1895,11, 1145) gives 2.8°/^ of acetylene 

 mixed with air as the explosive limit, and it is to be 

 noticed that in a dwelling house the danger from explosion 

 is enhanced by the inequality of such mixtures. A flame 

 spreading from a spot rich in gas would propagate itself 

 explosively through a mixture very poor in gas. 



The danger is enhanced in the case of acetylene by the 

 low temperature at which it takes fire, 480° Cent. Most 

 other gases must be treated to about 600° to take fire 

 and marsh gas, the fire-damp of mines, fortunately re- 

 quires a much higher temperature to ignite, so that a 

 spark from flint and steel does not suffice to cause an 

 explosion. Acetylene burns with greatest increase of 

 volume when the products are carbonic oxide and hydro- 

 gen. The violence of combination of acetylene with 

 oxygen can be well shown by igniting equal volumes of 

 the two gases. A quantity equal to 3-4 grains makes a 

 far louder report than the same weight of powder or of 

 nitro-glycerine. 



The dangerous properties shown by acetylene need not. 

 condemn it, but particular care must be taken to prevent 

 leakage if acetylene gas comes into use; fortunately, small 

 pipes can be used and the gas contains no ammonia, which, 

 in common gas, destroys the grease on the stopcocks and 

 promotes leakage. 



If instead of igniting a mixture of air and acetylene, 

 the latter alone is passed through a glass tube heated to 

 dull redness, at first a slight change takes place, and liquid 

 benzene and other products condense in the colder parts 

 of the tube ; at a little higher temperature the change 

 goes further — carbon is deposited and hydrogen is set 

 free. If the interior of the tube is carefully watched it 



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