[Illustration: Figure 4.--APPARATUS FOR CONVERTING WHITE PHOSPHORUS into the red allotropic form, 1851. Redistilled phosphorus is heated in the glass or porcelain vessel (g) which is surrounded by a sandbath (e) and a metal bath (b). Vessel (j) is filled with mercury and water; together with valve (k), it serves as a safety device. The alcohol lamp (l) keeps the tube warm against clogging by solidified vapors. Because of hydrogen phosphides, the operation, carried out at 260 deg. C., had to be watched very carefully. (According to Arthur Albright, 1851; reproduced from HUGO FLECK, _Die Fabrikation chemischer Produkte ..._, page 112.)] Chemical Constitution of Phosphoric Acids In a long article on phosphorus, Edmond Willm wrote in 1876: "For a century, urine was the only source from which phosphorus was obtained. After Gahn, in 1769, recognized the presence of phosphoric acid in bones, Scheele indicated the procedure for making phosphorus from them."[19] Actually, Gahn used at first hartshorn (_Cornu cervi ustum_), and Scheele doubted, until he checked it himself, that his esteemed friend was right. A few years later, Scheele corrected Gahn's 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.