assumption that the sal microcosmicum was an ammonia salt; instead, it is “a tertiary neutral salt, consisting of alkali minerali fixo (i.e., 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:

... ::   .. ...      . .

 H³  P    H² P   and  H P.[22]

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 filled 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.)

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.)

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.)

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.

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) · · ·
O³     phosphoric acid
H³

(PHO) · ·
O²     phosphorus acid
H²

(PH²O) ·
O      hypophosphorous acid.[24]
H

Further proof for these constitutions was sought in the study of the esters formed when the acids react with alcohols.

Among the analogies and generalizations by which the research on phosphoric acid was supported, and to the results of which it contributed a full share, was the new theory of acids. Not oxygen, Lavoisier’s general acidifier, but reactive hydrogen determines the character of acids. In this brief survey, it seems sufficient just to mention this connection without describing it in detail.

The study of phosphoric acids led to important new concepts in theoretical chemistry. The finding of polybasicity was extended to other acids and formed the model that helped to recognize the polyfunctionality in other compounds, like alcohols and amines. The