A Manual of the Chemistry of the Carbon Compounds

THE CARBON COMPOUNDS. 19
most be obtained in the free state To effect this the compound is
burned with copper oxide, or, better, a mixture of copper oxide and
mercuric oxide, in a tube, the fore part of which is filled with red*
hot metallic copper. Previous to the combustion all the air must be
driven out of the apparatus, which is easily done by placing some
magnesite in the closed end of the tube, and heating it until all the
gas given off is completely absorbed by caustic potash. The mixture
of nitrogen and carbon dioxide given off during the combustion is
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collected over mercury in a wide graduated glass tube, and the nitrogen
which remains in the conibitstiou tube when the combustion is finished
also driven in the graduated tube by heating the magnesite again.
After absorbing the carbon dioxide by caustic potash, the volume of
the remaining nitrogen is read off, and from it, measured under given
circumstances of temperature and pressure, its weight can be easily
calculated.
Determination of oilier Elements.—-Compounds containing cMorim,
bromine, or iodine, are heated in a narrow combustion tube with pure
caustic lime. After cooling, the contents of the tube are dissolved
in dilute nitric acid, and the quantity of chlorine, &c, determined by
precipitation with silver nitrate. Chlorine may also be oonverted
into silver ohloride by heating the substance to be analysed in a
sealed glass tube with nitric acid and silver nitrate. To ensure
complete oxidation it is advisable to add some potassium diohromate.
There exist a few chlorine compounds, which are decomposed by
water, with the formation of hydrochloric acid; others yield the
whole of the chlorine by bringing them in contact with water and
sodium amalgam. From these solutions all the chlorine is precipitated
by silver nitrate.
Sulphur and 'phosphorus are determined by heating the substance
with pure nitre and sodium carbonate, or by oxidizing it with nitric
acid HI sealed tubes, sulphuric or phosphoric acid being formed,
which are estimated by known methods.
In a similar way boron, silicon, arsenic, &c, are determined.
Compounds containing metals leave on ignition the metal either in the
free state as silver, gold, platinum, or as an oxide or carbonate. Sometimes
it i3 preferable to oxidize the compound with nitric acid or with
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