SECT. m. OF COAL. 119 



the gas over hot lime. The water of the lime is de- 

 composed, and carbonic oxide and sulphuretted hydrogen 

 are produced ; but the latter may be absorbed by passing 

 the gas again over lime, or through a mixture of saw- 

 dust and the oxide of iron. The oxide of iron decom- 

 poses the sulphuretted hydrogen, forms water and 

 sulphide of iron, then the air restores the sulphide to 

 oxide, and the sulphur is deposited in the mixture. 

 After passing the gas through it till none of that im- 

 purity remains, the gas is fit for use. The test is the 

 nitro-prusside of sodium, which the gas stains purple if 

 any of the impurity remains. 



Paraffin, already mentioned as isomeric, is a pure hy- 

 drocarbon, colourless, transparent, and of crystalline tex- 

 ture. It melts at a heat of 120 or 130, burns like wax 

 without smell or smoke, and makes beautiful candles, 

 which give a brilliant light on account of the 86 per cent, 

 of carbon they contain. Paraffin oil is much used for 

 lamps ; the manufacture of these two substances at Bath- 

 gate is one of the largest chemical establishments in the 

 world. 



The black foetid gas water resulting from the distilla- 

 tion of coals, formerly thrown away, is so rich in the 

 salts of ammonia, that it has become the chief source 

 from which these materials so important in the arts are 

 obtained. 



Ammonia is well known to be a colourless gas, with 

 an acrid pungent smell, consisting of one equivalent of 

 nitrogen and three of hydrogen. It has an alkaline 

 character, combining with acids, and is extremely soluble 

 in water. 



Now the gas water contains carbonate of ammonia 

 and sulphide of ammonium, and when any acid strong 

 enough to decompose these substances is put into the 

 liquid, the carbonic acid and sulphuretted hydrogen being 

 volatile are driven off, and the acid combines with the 



