_{2}O.

Both the theory and the practice of substitution enable us to further
prove the presence of two hydrogen atoms in a water molecule.
Decomposing water by sodium, only one-half of the hydrogen contained
is eliminated, the other half, together with all of the oxygen,
uniting with the metal to form sodium hydroxide, H_{2}O + Na = H +
NaHO. Doubling the amount of sodium does not alter the result, for
decomposition according to the equation H_{2}O + 2Na = H_{2} + Na_{2}O
never happens. Introducing the ethyl group into the water molecule and
reacting under appropriate conditions with ethyl iodide upon water,
the ethyl group displaces one atom of hydrogen, and, uniting with the
hydroxyl residue, forms ethyl alcohol, thus: H_{2}O + C_{2}H_{5}I =
C_{2}H_{5}OH + HI. Halogens do not act directly on water, hence we may
not properly speak of halogen substitution products. By the action,
however, of phosphorus haloids on water an analogous splitting of the
water molecule is again observed, one-half of the hydrogen uniting
with the halogen to form an acid, the hydroxyl residue then forming a
phosphorus compound, thus: PCl_{3} + 3H_{2}O = 3HCl + P(OH)_{3}.

Now these examples, which might readily be multiplied, prove not only
the presence of _two_ hydrogen atoms in the water molecule, but they
further demonstrate that these two atoms _differ from each other_ in
respect to their form of combination and power of substitution. The
two hydrogen atoms are certainly not of equal value, whence it follows
that the accepted formula for water:

H
> O
H

or as preferred by some: H-O-H, is not in conformity with established
facts. Expressed as here shown, both hydrogen atoms are assigned equal
values, when in fact only _one of the atoms is united to oxygen in
form of hydroxyl_, while the second is loosely attached to the
univalent hydroxyl group. Viewed in this light, water then is
decomposed according to the equation: H_{2}O = H + (OH), never in this
manner: H_{2}O = 2H + O. Hence, water must be considered as a
combination of one hydrogen atom with one molecule of hydroxyl,
expressed by the formula H(OH), and it is this atom of hydrogen _not_
united to oxygen which is eliminated in the generation of oxygen or
substituted by metals and alkyl groups. The hydrogen in the hydroxyl
group cannot be substituted, excepting it be the entire group as such;
this is proved by the action of the halogens