vapour, which is peculiar. When pure, the acid as well as its vapour is
colourless; when mixed with nitrous acid it is of various tints, and
generally yellow. The acid of commerce is at times rendered impure by
sulphuric acid, a circumstance which must be attended to in applying the
subsequent tests.—The simplest test for nitric or nitrous acid is the
action of copper, lead, or tin. If any of these metals in small
fragments, or powder, be thrown into either acid previously diluted with
an equal volume of water, an effervescence takes place, which in the
case of lead or copper is much accelerated by heat; nitric oxide gas is
disengaged; and ruddy fumes of nitrous acid gas are formed when the gas
comes in contact with the oxygen of the air. Another characteristic
test, which has the advantage of being applicable on an extremely small
scale, is morphia, the alkaloid of opium. This substance is turned in a
few seconds to a beautiful orange-red colour by nitric acid, and after
longer contact forms with it a bright yellow solution. No other acid has
this effect. Muriatic acid, as Dr. O’Shaughnessey has remarked,[304]
does not act at all on morphia, and sulphuric acid chars and blackens
it. When nitric acid is added to a solution of narcotin in sulphuric
acid, the colour of the solution is changed from yellow to
blood-red.[305] When it is added to a solution of proto-sulphate of
iron, the solution becomes brown, and the addition of sulphuric acid
then alters the colour to violet.[306] When it is added even in the most
minute proportion to sulphuric acid, the addition of a few particles of
the alkaloid brucia will render the whole fluid red, passing gradually
to yellow.[307]—Many other characteristic tests might be mentioned; but
those now specified are more than enough.