82 The Poetry of Physics and The Physics of Poetry
small. Hydrogen and oxygen also combine to form hydrogen peroxide
(H 2 O 2 ), with one gram of hydrogen combining with 16 grams of oxygen.
The ratio of the amount of oxygen combining with a fixed amount of
hydrogen for hydrogen peroxide and water is therefore two to one.
From these two laws of chemical combination referred to as the law
of definite proportions and the law of multiple proportions, John Dalton
in 1808, deduced that all matter is composed of elementary particles,
which he called atoms after Leucippus and Democritus. He concluded
like the early Greeks that these atoms retain their identity in chemical
reactions. He also deduced that all the atoms composing an element are
identical and that compounds consist of identical molecules. The
molecules are themselves aggregates of atoms, which have combined in
simple numerical proportions.
So, for example, a water molecule consists of two hydrogen atoms
and one oxygen atom, H 2 O (H and O are the symbols for hydrogen and
oxygen), whereas hydrogen peroxide molecules consist of two hydrogen
atoms and two oxygen atoms, H 2 O 2 , which explains why twice as much
oxygen combines with a fixed amount of hydrogen to form hydrogen
peroxide as opposed to water. Once it was known that for water there are
two hydrogen atoms for each oxygen atom the relative weight of the
oxygen atom and the hydrogen atom was determined to be 16 to 1. By
considering all of the various reactions the relative weight of all the
atoms were obtained. In the very first compilation made by Jons
Berzelius in 1828, a scale was chosen such that the atomic weight of
hydrogen was one. This scale has been refined so that today carbon has
the atomic weight of exactly 12.0.
From the fact that two volumes of hydrogen gas plus one volume of
oxygen gas combine to form two volumes of water vapor instead of one
volume, we learn that both hydrogen and oxygen gas consists of
molecules containing two hydrogen and two oxygen atoms respectively.
The other gaseous elements with the exception of the noble gases like
helium and neon are also diatomic.