ed
with bones, and the combustion maintains itself without addition of
other fuel. A circular gallery, C, surrounds the bottom of the oven and
carries the products of combustion into the chimney, H. The calcined
bones are taken out at the lower opening, G, by removing the bars of
grid B." (Translation of the description from FIGUIER, _Merveilles de
l'industrie_, volume 3, 1874, page 537.)]

[Illustration: Figure 6.--AN ADVERTISEMENT with view of plant for
manufacturing superphosphate about 1867. (From E. T. FREEDLEY,
_Philadelphia and its Manufacturers in 1867_, page 288.)]

Graham had come to this understanding of the phosphoric acids through
his previous studies of "Alcoates, definite compounds of Salts and
Alcohol analogous to the Hydrates" (1831). Liebig started from analogies
he saw with certain organic acids when he formulated the phosphoric
acids with a constant proportion of water (aq.) and varying proportions
of "phosphoric acid" (P) as follows:

2 P 3 aq. phosphoric acid
3 P 3 aq. pyrophosphoric acid
6 P 3 aq. metaphosphoric acid.

[Illustration: Figure 7.--FLORIDA HARD-ROCK PHOSPHATE MINING. (From
Carroll D. Wright, _The Phosphate Industry of the United States_, sixth
special report of the Commissioner of Labor, Government Printing Office,
Washington, 1893, plate facing page 43.)]

Salts are formed when a "basis," i.e., a metal oxide, replaces water.
When potassium-acid sulfate is neutralized by sodium base, the acid-salt
divides into Glauber's salt and potassium sulfate, which proves the
acid-salt to be a mixture of the neutral salt with its acid. Sodium-acid
phosphate behaves quite differently. After neutralization by a potassium
"base" (hydroxide), the salt does not split up; a uniform
sodium-potassium phosphate is obtained. Therefore, phosphoric acid is
truly three-basic![23]

This result has later been confirmed, but the analogy by means of which
it had been obtained was very weak, in certain parts quite wrong.

The acids from the two lower oxides of phosphorus were also considered
as three-basic. Adolphe Wurtz (1817-1884) formulated them in 1846,
according to the theory of chemical types:

(PO)...
O^{3} phosphoric acid
H^{3}

(PHO)..
O^{2} phosphorus acid
H^{2}

(PH^{2}O).
O hypophosphorous acid.[24]
H

Further proof for these constitutions was sought in the study of the
esters formed when t