When bone ash or any other substance containing phosphate of lime is
treated with sulphuric acid, the products formed are superphosphate of
lime and hydrated sulphate of lime; this mixture is known as
superphosphate of lime, in commerce, and is the substance used in this
process. This substance is capable of absorbing carbonic acid and
ammonia from foul gas. The complete action can only take place in the
presence of a certain proportion of carbonic acid, so that the process
is not so successful with "well-scrubbed illuminating gas." The
superphosphate is converted into carbonate of lime, while the ammonia
combines with the phosphoric acid to form phosphate of ammonia; the
hydrated sulphate of lime is also acted upon, and forms carbonate of
lime and sulphate of ammonia; so that, presuming the action to be
complete, and the material to be thoroughly saturated with carbonic
acid and ammonia from the foul gas, the result is a mixture of
carbonate of lime and phosphate and sulphate of ammonia.

Under these circumstances, the mixture absorbs one equivalent of
carbonic acid for every four equivalents of ammonia; therefore, if the
superphosphate process be substituted for the ordinary washers and
scrubbers, a large proportion of the carbonic acid and also the whole
of the sulphureted hydrogen is left in the gas, and must be dealt with
in other ways.

This superphosphate process has been at work at the South Metropolitan
Gas Works, Old Kent Road, for nearly two years. In practice it is
usual to water the superphosphate before use with ammoniacal liquor,
and it is used in dry purifiers, in layers about eight inches thick.

This process has been thoroughly investigated at the Munich Gas Works,
by Drs. Bunte and Schilling, and the report made by these gentlemen