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  Blue colour was first manufactured at Alexandria, and afterwards by Ves-
torius, at Puzzuoli.   Vitruvius has explained the manner in which it was
made in his own day.     With the flower of sulphur a quantity of sand was
ground until reduced to a fine powder, when filings of Cyprian copper were
added, and the whole brought to a paste by being mixed with water.      
   In
this state the composition was moulded into balls, and exposed to intense
heat in a furnace, by which means the copper and sand " imparted to
each
other their different qualities and their blue colours."
   In examining the methods of manufacture adopted by the ancients, from
the few scanty notices which are left for us, chiefly by Vitruvius, from
whose
writings nearly all the facts here stated are obtained, we are particularly
struck with the extreme simplicity of the contrivances employed, and the
accuracy of the information, although unconnected with any sound scientific
principle. The facts known to the Romans and other ancient nations, were
the result of fortuitous circumstances, and not of inductive reasoning, as
in
the present d&y. Still the reader must be interested by the inventions
already
mentioned; and the ancient process of making white lead is not less worthy
of notice.
   " The Rbodians," says Vitruvius, " place in the bottoms
of large vessels a
 layer of twigs, over which they pour vinegar, and on the twigs they lay
 masses of lead.   The vessels are covered to prevent evaporation; and when,
 after a certain time, they are opened, the masses are found to be changed
into
 white lead.  In the same way they make verdigris, which is called wruca,
 by means of plates of copper. The white lead is roasted in a furnace, and
 by the action of the fire becomes red lead. This invention was the result
of
 observation in the case of an accidental fire; and by the process, a much
 better material is obtained than that which is procured from mines."