The Element From Animals and Plants
It was a little late to search for the philosophers’ stone in 1669, yet it was in such a search that phosphorus was discovered. Wilhelm Homberg (1652-1715) described it in the following manner: Brand, “a man little known, of low birth, with a bizarre and mysterious nature in all he did, found this luminous matter while searching for something else. He was a glassmaker by profession, but he had abandoned it in order to be free for the pursuit of the philosophical stone with which he was engrossed. Having put it into his mind that the secret of the philosophical stone consisted in the preparation of urine, this man worked in all kinds of manners and for a very long time without finding anything. Finally, in the year 1669, after a strong distillation of urine, he found in the recipient a luminant matter that has since been called phosphorus. He showed it to some of his friends, among them Mister Kunkel [sic].”1
Neither the name nor the phenomenon were really new. Organic phosphorescent materials were known to Aristotle, and a lithophosphorus was the subject of a book published in 1640, based on a discovery made by a shoemaker, Vicenzo Casciarolo, on a mountain-side near Bologna in 1630.2 Was the substance new which Brand showed to his friends? Johann Gottfried Leonhardi quotes a book of 1689 in which the author, Kletwich, claims that this phosphorus had already been known to Fernelius, the court physician of King Henri II of France (1154-1189).3 To the same period belongs the “Ordinatio Alchid Bechil Saraceni philosophi,” in which Ferdinand Hoefer found a distillation of urine with clay and carbonaceous material described, and the resulting product named escarbuncle.4 It would be worth looking for this source; although Bechil would still remain an entirely unsuccessful predecessor, it does seem strange that in all the distillations of arbitrary mixtures, the conditions should never before 1669 have been right for the formation and the observation of phosphorus.
For Brand’s contemporaries at least, the discovery was new and exciting. The philosopher Gottfried Wilhelm von Leibniz (1646-1716) considered it important enough to devote some of his time (between his work as librarian in Hanover and Wolfenbüttel, his efforts to reunite the Protestant and the Catholic churches, and his duties as Privy Councellor in what we would call a Department of Justice) to a history of phosphorus. This friend of Huygens and Boyle tried to prove that Kunckel was not justified in claiming the discovery for himself.5 Since then, it has been shown that Johann Kunckel (1630-1703) actually worked out the method which neither Brand nor his friend Kraft wanted to disclose. Boyle also developed a method independently, published it, and instructed Gottfried Hankwitz in the technique. Later on, Jean Hellot (1685-1765) gave a meticulous description of the details and a long survey of the literature.6
To obtain phosphorus, a good proportion of coal (regarded as a type of phlogiston) was added to urine, previously thickened by evaporation and preferably after putrefaction, and the mixture was heated to the highest attainable temperature. It was obvious that phlogiston entered into the composition of the distillation product. The question remained whether this product was generated de novo. In his research of 1743 to 1746, Andreas Sigismund Marggraf (1709-1782) provided the answer. He found the new substance in edible plant seeds, and he concluded that it enters the human system through the plant food, to be excreted later in the urine. He did not convince all the chemists with his reasoning. In 1789, Macquer wrote: “There are some who, even at this time, hold that the phosphorical (‘phosphorische’) acid generates itself in the animals and who consider this to be the ‘animalistic acid.’”7
Although Marggraf was more advanced in his arguments than these chemists, yet he was a child of his time. The luminescent and combustible, almost wax-like substance impressed him greatly. “My thoughts about the unexpected generation of light and fire out of water, fine earth, and phlogiston I reserve to describe at a later time.” These thoughts went so far as to connect the new marvel with alchemical wonder tales. When Marggraf used the “essential salt of urine,” also called sal microcosmicum, and admixed silver chloride (“horny silver”) to it for the distillation of phosphorus, he expected “a partial conversion of silver by phlogiston and the added fine vitrifiable earth, but no trace of a more noble metal appeared.”8
Robert Boyle had already found that the burning of phosphorus produced an acid. He identified it by taste and by its influence on colored plant extracts serving as “indicators.” Hankwitz9 described methods for obtaining this acid, and Marggraf showed its chemical peculiarities. They did not necessarily establish phosphorus as a new element. To do that was not as important, at that time, as to conjecture on analogies with known substances. Underlying all its unique characteristics was the analogy of phosphorus with sulfur. Like sulfur, phosphorus can burn in two different ways, either slowly or more violently, and form two different acids. The analogy can, therefore, be extended to explain the results in both groups in the same way. In the process of burning, the combustible component is removed, and the acid originally combined with the combustible is set free. Whether the analogy should be pursued even further remained doubtful, although some suspicion lingered on for a while that phosphoric acid might actually be a modified sulfuric acid. Analogies and suspicions like these were needed to formulate new questions and stimulate new experiments. They are cited here for their important positive value in the historical development, and not for the purpose of showing how wrong these chemists were from our point of view, a point of view which they helped to create.
The widespread interest in the burning of sulfur and of phosphorus, naturally, caught Lavoisier’s attention. In his first volume of Opuscules Physiques et Chimiques (1774), he devoted 20 pages to his experiments on phosphorus. He amplified them a few years later10 when he attributed the combustion to a combination of phosphorus with the “eminently respirable” part of air. In the Méthode de Nomenclature Chimique of 1787, the column of “undecomposed substances” lists sulfur as the “radical sulfurique,” and phosphorus, correspondingly, as the “radical phosphorique.” The acids are now shown to be compounds of the “undecomposed” radicals, the complete reversion of the previous concept of this relationship. A part of the old analogy remained as far as the acids are concerned: sulfuric acid corresponds to phosphoric; sulfurous acid to phosphorous acid with less oxygen than in the former.11