Graphite, also, burns slowly,
but at a lower temperature. The other bodies (coals, shales, &c.) differ
considerably among themselves in the temperature at which they commence
to burn. Some, such as anthracite, burn with little or no flame, but
most give off gases, which burn with a luminous flame. They deflagrate
when sprinkled on fused nitre, forming carbonate of potash. In making
this test the student must remember that sulphur and, in fact, all
oxidisable bodies similarly deflagrate, but it is only in the case of
carbon compounds that carbonate of potash is formed. Carbon unites with
iron and some of the metals to form carbides; combined carbon of this
kind is detected by the odour of the carburetted hydrogen evolved when
the metal is treated with hydrochloric acid; for example, on dissolving
steel in acid.
The natural carbon compounds, although, speaking generally, insoluble in
hydrochloric or nitric acids, are more or less attacked by aqua regia.
The assayer seldom requires these compounds to be in solution. The
presence of "organic matter"[117] interferes with most of the reactions
which are used for the determination of the metals. Consequently, in
such cases, it should be removed by calcination unless it is known that
its presence will not interfere. When calcination is not admissible it
may be destroyed by heating with strong sulphuric acid and bichromate or
permanganate of potash or by fusion with nitre.
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