216 



COMMON SUBSTANCES 



stoves (see Fig. 141). Bituminous coal is a softer vari- 

 ety. It contains more gases, burns at a lower tempera- 

 ture, and shows much more flame while burning than 

 does hard coal. 



254. Illuminating Gas. If soft coal (bituminous) be 

 heated to a high degree without any supply of air, 

 the coal will be decomposed, its elements 

 combining and mixing with each other to 

 form new substances. The solid substance 



that remains 



IB is nearly pure 



C ; it is called 

 coke. The liq- 

 uids unite in 

 a mixed mass 

 called coal 

 tar. The gases 

 that are given 

 off ar e first 

 passed through 

 water, which 

 dissolves the 



ammonia (NH 3 ) and thus removes it. The gases which 

 remain in the mixture form illuminating gas. This 

 contains some free hydrogen and some compounds of 

 hydrogen and carbon. So we see that illuminating gas 

 contains largely the elements C and H, both of which 

 burn in air. 



The process is carried on in gas works. Coal is put 

 into large iron retorts (a, Fig. 142) and heated by a 



FIG ' 142 



