82 
By a consideration of these tables, together with the one 
showing the composition of coal (previously given), we may 
reasonably suppose the following to be the way in which temper- 
ature affects this distillation :—The effect of the first application 
of heat to coal is, no doubt, to cause, primarily, the production 
of carbon di-oxide and water, through the union of the oxygen 
with a portion of the carbon and hydrogen. The next increase 
of temperature will, most probably, produce hydrocarbons of the 
fatty series, especially marsh gas (CH,), hydrides (C,H), . . ), 
and the olefines (C, H,,). Some of these latter are liquid, and 
would appear in the tar; while others are gaseous, and possessed 
of moderate luminosity. At a still higher temperature these 
compounds of the “ fatty or paraffin series”’ are partly decom- 
posed into others of the ‘aromatic series,’ with the separation 
of free hydrogen. Some of these are gaseous at ordinary tem- 
peratures, and add great luminosity to the gas; others are liquid, 
and considerably enhance the value of the tar. These are 
especially benzene and its homologues. If the temperature be 
still further increased, these last-named valuable compounds are 
decomposed, with the formation of free hydrogen, free carbon, 
naphthalene, anthracene, &c. 
The temperature of the gas, as taken from the retorts, is 
usually about 130° Fahr. It is cooled by passing through appar- 
atus termed condensers, where it is freed from the portions 
which are liquid at the ordinary temperature of the atmosphere. 
From this the gas is passed through a series of vessels, termed 
washers and scrubbers, where the impurities of sulphuretted 
hydrogen, ammonia, and carbonic acid are largely abstracted. 
From these the gas is passed through purifiers, suitably charged, 
and worked with lime or oxide of iron, which further eliminate 
the carbonic acid and sulphur compounds contained in it. The 
gas is now ready for use. 
The following may be taken as an average specimen of gas 
made in England :— 
Chartered Gas Company’s Gas, 1876 ( Dr. Humpidge ). 
INTLROPEDIS ccc sie. teen eles 3'48—Inert constituent. 
ELV OV OREN, ci isc cits vquires shove 6 nie 50°59—Heat-giving constituent. 
Carbon monoxide............ SL PN et 3 
Marshes: (CuEl))  goerreieciee es 38°39—Light-giving. 
Heavy hydrocarbons (C,Hm).. 4:41— ,, * = 7°85 of 
ethylene (C,H,). 
Carbon density, 3°56 ........ 100-00 
The liquid distillate in gas manufacture consists—(1) of 
water, charged with ammonia and its compounds ; and (2) of tar. 
The first is now the principal source of the ammonia used in 
the arts and in agriculture. It is usual to first convert it into 
sulphate of ammonia by treatment with sulphuric acid, This 
