Water being a colourless fluid, ought, one would imagine, when mixed
with other substances of no decided colour, to produce a colourless
compound. Nevertheless, it is to water only that blue vitriol, or
sulphate of copper, owes its vivid blueness; as will be plainly evinced
by the following simple experiment. Heat a few crystals of the vitriol
in a fire shovel, pulverize them, and the powder will be of a dull and
dirty white appearance. Pour a little water upon this, when a slight
hissing noise will be heard, and at the same moment, the blue colour
will instantly re-appear.
Under the microscope, the beauty of this experiment will be increased,
for the instant that a drop of water is placed in contact with the
vitriol, the powder may be seen to shoot into blue prisms. If a crystal
of prussiate of potash be similarly heated, its yellow colour will
vanish, but re-appear on being dropped into water.