It is generally associated with caesium.
It is detected by the spectroscope, which shows two violet lines and two
dark red ones. Like caesium, it is precipitated with platinic chloride,
and in the ordinary course of work would be weighed as potassium. It is
separated from potassium by fractional precipitation with platinic
chloride. Rubidium platino-chloride is much less soluble than the
potassium salt.
AMMONIUM.
It is usual to look upon the salts of ammonia as containing a compound
radical (NH_{4} = Am), which resembles in many respects the metals of
the alkalies. Ammonium occurs in nature as chloride in sal ammoniac
(AmCl), as sulphate in mascagnine (Am_{2}SO_{4}), as phosphate in
struvite (AmMgPO_{4}.12H_{2}O). Minerals containing ammonium are rare,
and are chiefly found either in volcanic districts or associated with
guano. Ammonia and ammonium sulphide occur in the waters of certain
Tuscan lagoons, which are largely worked for the boracic acid they
contain. The crude boracic acid from this source contains from 5 to 10
per cent. of ammonium salts. It is from these that the purer forms of
ammonium compounds of commerce known as "from volcanic ammonia" are
derived. But the bulk of the ammonia of commerce is prepared from the
ammoniacal liquors obtained as bye-products in the working of certain
forms of blast furnaces and coke ovens, and more especially in
gas-making.
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