356 SMITH'S INTERMEDIATE CHEMISTRY 



lampblack, mainly very finely divided carbon, is used in making 

 printers' ink, India ink, and black varnish. 



Carburetted Water Gas. To fit water gas, essentially 

 H 2 + CO (p. 337), which burns with a pale blue flame, for public 

 service as an illuminating gas, unsaturated hydrocarbons, and 

 particularly ethylene C2H4, which burn with a highly luminous 

 flame, must be added. The water gas is passed through a tower, 

 filled with strongly heated brick on which oil is continually sprayed. 

 Mixed with the vapor of the oil, the gas goes into the " super- 

 heater " where, at a higher temperature, the decomposition into 

 unsaturated hydrocarbons (cracking) takes place. The gas is 

 then cooled and washed to remove the condensible hydrocarbons, 

 which would otherwise collect in the service pipes with resulting 

 waste of combustible material as well as obstruction in the delivery 

 of the gas. A typical carburetted water gas has the composi- 

 tion: Illuminants (largely ethylene) 16.6 per cent; heating 

 gases methane 19.8 per cent, hydrogen 32.1 

 per cent, carbon monoxide 26.1 per cent; im- 

 purities (nitrogen and carbon dioxide) 5.4 per 

 cent. 



Carburetted water gas has now largely sup- 

 planted coal gas (p. 423) for lighting and heating 

 purposes. 



Non-luminous Gas Flames. When gas is to 

 be used for heating, the complete combustion of 

 the gas, without any intermediate liberation of free 

 carbon, is desirable. This is achieved in the 

 Bunsen burner (Fig. 90) by admitting air at the 

 bottom of the burner, in such a way that the air 

 mixes with the gas before the latter reaches the 

 flame. The air cools the middle zone of the flame, so that at 

 this point the temperature required for dissociating the ethy- 



