146 CHEMISTRY FOR AGRICULTURAL STUDENTS 



Among the by-products of coal gas manufacture, the most important 

 are: (i) Coke. (2) Gas carbon, formed as a deposit within the retorts, 

 and used for the "carbons" of galvanic batteries, and the poles of the 

 electric arc light. (3) Coal tar. From this, coal tar oil is separated by 

 distillation, pitch being left as a residue. From the coal tar oil are 

 obtained the hydrocarbons benzene or benzole, CgHg ; naphthalene, CmHg ; 

 and anthracene, Cj^Hk, ; from which the aniline, naphthalene, and 

 alizarine dyes are respectively derived. (4) Ammonia in the form of 

 sulphate. (5) Gas lime, a mixture containing chalk and unchanged slaked 

 lime, together with the sulphide and oxysulphide of calcium in cases 

 where the sulphur has not been removed by hydrated ferric oxide. 



Hydrocarbons burn with flames which are (i) non-luminous, 

 (2) luminous, and (3) smoky, depending upon the proportion 

 of carbon they contain. The luminosity of flame, in fact, 

 appears to be chiefly due to the incandescence of particles of 

 carbon set free owing to the decomposition of the hydro- 

 carbons. Place a clean white dish in any luminous hydro- 

 carbon flame. Note that it becomes covered with soot. 

 Observe that the flame essentially consists of three parts — (i) 

 a dark interior, consisting of unburnt but decomposing gases ; 

 (2) a luminous zone, consisting of burning but incompletely 

 burnt gases ; and (3) a non-luminous envelope, in which com- 

 plete combustion to carbonic acid gas and water is taking 

 place. (This envelope may be rendered visible by volatilising 

 a little common salt on a piece of platinum wire 

 in the flame.) Depress a sheet of paper into the 

 flame of a paraffin candle. Note that a ring of 

 charred paper is produced round an uncharred 

 interior, showing that the flame is hollow. When 

 air is admitted into the interior of a hydrocarbon 

 flame, carbon is no longer set free, and the flame 

 is thus rendered non-luminous. In the Bunsen 

 burner (Fig. 48) air is admitted by holes at the 

 Fig. 48. bottom of the tube. , If these be closed, the 

 flame becomes luminous. 



Fuels. — The combustibles used for heating must yield gaseous and non- 

 injurious products of combustion. These conditions are alone fulfilled by 



