says that thunder may be imitated by folding some
of the powder in a cover and tying it up tightly: this is the cracker.
It thus appears that fireworks preceded fire-arms. To the same author we
are indebted for prescriptions for making the skin incombustible, so
that we may handle fire without being burnt. These, doubtless, were
received as explanations of the legends of the times, which related how
miracle-workers had washed their hands in melted copper, and sat at
their ease in flaming straw.

[Sidenote: Arabian chemists.]

Among the Saracen names that might be mentioned as cultivators of
alchemy, we may recall El-Rasi, Ebid Durr, Djafar or Geber, Toghrage,
who wrote an alchemical poem, and Dschildegi, one of whose works bears
the significant title of "The Lantern." The definition of alchemy by
some of these authors is very striking: the science of the balance, the
science of weight, the science of combustion.

[Sidenote: Djafar discovers nitric acid and aqua regia,]

[Sidenote: and that oxidation increases weight.]

[Sidenote: He solves the problem of potable gold.]

To one of these chemists, Djafar, our attention may for a moment be
drawn. He lived toward the end of the eighth century, and is honoured by
Rhazes, Avicenna, and Kalid, the great Arabic physicians, as their
master. His name is memorable in chemistry, since it marks an epoch in
that science of equal importance to that of Priestley and Lavoisier. He
is the first to describe nitric acid and aqua regia. Before him no
stronger acid was known than concentrated vinegar. We cannot conceive of
chemistry as not possessing acids. Roger Bacon speaks of him as the
magister magistrorum. He has perfectly just notions of the nature of
spirits or gases, as we call them; thus he says, "O son of the doctrine,
when spirits fix themselves in bodies, they lose their form; in their
nature they are no longer what they were. When you compel them to be
disengaged again, this is what happens: either the spirit alone escapes
with the air, and the body remains fixed in the alembic, or the spirit
and body escape together at the same time." His doctrine respecting the
nature of the metals, though erroneous, was not without a scientific
value. A metal he considers to be a compound of sulphur, mercury, and
arsenic, and hence he infers that transmutation is possible by varying
the proportion of those ingredients. He knows that a metal, when
calcined, increases in weight, a discove