(C) Peter Meiers - http://www.fluoride-history.de
Fluoride Research in the 19th and early 20th century
Before it was announced for the first time that fluoric acid appeared to contain a
(C) Peter Meiers - http://www.fluoride-history.de
Fluoride Research in the 19th and early 20th century
Before it was announced for the first time that fluoric acid appeared to contain a new chemical element, "fluorine", some researchers already speculated about any possible biological significance of fluoride. They had good reason to have their focus on bones and teeth, for of all animal and human tissues teeth and bones stand out by their hardness and by their resistance to putrefaction which the soft tissues usually succumb soon after the death of their owner. In primeval times the hard tissues have been used as building materials, as tools and weapons, while there was still no knowledge about what determines their special properties.
Around 1680 the French researcher Papin developed a "digesteur" in which he subjected bones to water boiling at elevated pressure. The resulting gelatinous concoctions later were given soldiers to drink in order to transmit hardness and resistance (1).
In 1758, Hérissant contributed to the knowledge of the chemistry of bones when he treated bones with nitric acid which dissolved lime-like matter (i.e. the minerals) and left a flexible organic substance ("gelatin") that still had the form of the bones. Somewhat later, Hérissant showed a lime-like material to be responsible also for the hardness of shells and corals (1).
(1) Chevreul M.: "Résumé historique des travaux dont la gélatine a été l´objet"; C. R. 71 (1870) 855-872 and 912-927;
During his work on the chemical composition of fluorspar the apothecary Carl Wilhelm Scheele delivered ways to identify fluorspar, i.e. (a): the etching of glass by fumes developed from samples on treatment with sulfuric acid; (b) the release of silica when the gas produced during the etching of glass gets in contact with water; (c) precipitation of fluorspar when the acid released by reaction (a) is led into a tube containing lime water.
Upon treatment of fluorspar with phosphoric acid (to show that fluoric acid is not only formed upon the use of sulfuric acid), Scheele remarked that the residue in the retort (calcium from the fluorspar and phosphate from the added phosphoric acid) behaves like the mineral part of bones which "recently has been shown to be made of phosphoric acid and lime" (1). In consequence of this notice, it was for some time believed that Scheele was the discoverer of the constitution of "bone earth", and Assessor Gahn of Fahlun, the actual discoverer, was so indifferent about his reputation as a discoverer, that he never tried to correct the mistake (2).
(1) Scheele C.W.: "Chemische Untersuchung des Flusspats und dessen Säure", in "Carl Wilhelm Scheele, Sämmtliche physische und chemische Werke, nach dem Tode des Verfassers gesammelt und in deutscher Sprache herausgegeben von D. Sigismund Friedrich Hermbstädt", 1793, Reprint: Dr. Martin Sändig oHG, Niederwalluf 1971, Vol. 2, pp.5-32 (original article published in 1771); Thomson T.: "Chemistry of animal bodies", Edinburgh 1843, p. 235
Phosphate of lime also occurs in nature as "rock phosphate". On treating in a test tube a sample of rock phosphate from Estremadura (Spain) with sulfuric acid, Bertrand Pelletier and Louis Donadei found that -besides muriatic and phosphoric acid- it also contains (bound) fluoric acid which etched the tube (1). Fluorspar ("fluate" of lime) was then the only known solid compound of fluoric acid. As both phosphoric and fluoric acid were bound to one base, calcium, and still nothing was known about the chemical nature of fluoric acid, the authors speculated that fluoric acid might be the product of some modification ("transmutation") of phosphoric acid.
(1) Pelletier B., Donadei L.: "Mémoire sur le phosphate calcaire", Ann. Chim. 7 (1790) 79
Josse (1) summarized the analyses of bones from the viewpoint of a chemist: bones are nothing but an accumulation of a mineral, phosphate of lime, mixed with an organic substance made of gelatin. He then addressed the composition of teeth, more specifically of tooth enamel, as the underlying dentin is essentially bone. Dental enamel, he writes, is white, very fragile, of extreme hardness. Along a fractured area it presents a regular, pronounced crystallization, formed by well-ordered assembling of small brillant crystal n