,
sodium), _alkali volatili_, and _acido phosphori_."[20]

In the years after 1770, phosphorus was discovered in bones and many
other parts of various animals. Treatment with sulfuric acid decomposed
these materials into a solid residue and dissolved phosphoric acid. Many
salts of this acid were produced in crystalline form. Heat resistance
had been considered one of the outstanding characteristics of phosphoric
acid. Now, however, in the processes of drying and heating certain
phosphates, it became clear that three kinds of phosphoric acids could
be produced: _ortho_, _pyro_, and _meta_.

Berzelius cited these acids as examples of compounds which are ISOMERIC.
This word was intended to designate compounds which contain the same
number of atoms of the same elements but combined in different manners,
thereby explaining their different chemical properties and crystal
forms. It was in 1830 that Berzelius propounded this companion of the
concept, ISOMORPHISM, which was to collect all cases of equal crystal
form in compounds in which equal numbers of atoms of different elements
are put together in the same manner. Together, the two concepts of
isomerism and isomorphism seemed to cover all the known exceptions from
the simplest assumption as to specificity and chemical composition.

However, only a few years later Thomas Graham (1805-1869) proved that
the three phosphoric acids are not isomeric. He used the proportion of 2
P to 5 O in the oxide which Berzelius had thought justified at least
until "an example of the contrary could be sufficiently
established."[21] Refining the techniques of Gay-Lussac (1816) and
several other investigators, Graham characterized the three phosphoric
acids as "a terphosphate, a biphosphate, and phosphate of water."
Actually, this was the wrong terminology for what he meant and
formulated as trihydrate, bihydrate, and monohydrate of phosphorus
oxide. In his manner of writing the formulas, each dot over the symbol
for the element was to indicate an atom of oxygen; thus, he wrote:

... :: .. ... . .
H^{3} P H^{2} P and H P.[22]

[Illustration: Figure 5.--OVEN FOR THE CALCINATION OF BONES, about 1870.
"The operation is carried out in a rather high oven, such as shown....
The fresh bones are thrown in at the top of the oven, B. First, fuel in
chamber F is lighted, and a certain quantity of bones is burnt on the
grid D. When these bones are burning well, the oven is gradually fill