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177
The Story of Alchemy and the Beginnings of Chemistry
·
M. M. Pattison Muir
145
Dalton
used
the
word
atom
to
denote
the
smallest
particle
of
an
element,
or
a
compound,
which
exhibits
the
properties
characteristic
of
that
element
or
compound.
He
supposed
that
the
atoms
of
an
element
are
never
divided
in
any
of
the
reactions
of
that
element,
but
the
atoms
of
a
compound
are
often
separated
into
the
atoms
of
the
elements
whereof
the
compound
is
composed.
Apparently
without
knowing
that
the
supposition
had
been
made
more
than
two
thousand
years
before
his
time,
Dalton
was
led
by
his
study
of
the
composition
and
properties
of
the
atmosphere
to
assume
that
the
atoms
of
different
substances,
whether
elements
or
compounds,
are
of
different
sizes
and
have
different
weights.
He
assumed
that
when
two
elements
unite
to
form
only
one
compound,
the
atom
of
that
compound
has
the
simplest
possible
composition,
is
formed
by
the
union
of
a
single
atom
of
each
element.
Dalton
knew
only
one
compound
of
hydrogen
and
nitrogen,
namely,
ammonia.
Analyses
of
this
compound
show
that
it
is
composed
of
one
part
by
weight
of
hydrogen
and
4.66
parts
by
weight
of
nitrogen.
Dalton
said
one
atom
of
hydrogen
combines
with
one
atom
of
nitrogen
to
form
an
atom
of
ammonia;
hence
an
atom
of
nitrogen
is
4.66
times
heavier
than
an
atom
of
hydrogen;
in
other
words,
if
the
atomic
weight
of
hydrogen
is
taken
as
unity,
the
atomic
weight
of
nitrogen
is
expressed
by
the
number
4.66.
Dalton
referred
the
atomic
weights
of
the
elements
to
the
atomic
weight
of
hydrogen
as
unity,
because
hydrogen
is
lighter
than
any
other
substance;
hence
the
numbers
which
tell
how
much
heavier
the
atoms
of
the
elements
are
than
an
atom
of
hydrogen
are
always
greater
than
one,
are
always
positive
numbers.
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