Figure 1.—The alchemist discovers phosphorus. A painting by Joseph Wright (1734-1779) of Derby, England.
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]
Figure 2.—Galley-oven, 1869. The picture is a cross section through the front of the oven showing one of the 36 retorts, the receivers for the distillate, and the space in the upper story used for evaporating the mixture of acid solution of calcium phosphate and coal. (According to Anselme Payen, Précis de Chimie industrielle, Paris, 1849; reproduced from Hugo Fleck, Die Fabrikation chemischer Produkte aus thierischen Abfällen, Vieweg, Braunschweig, 1862, page 80 of volume 2, 2nd group, of P. Bolley’s Handbuch der chemischen Technologie.)
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.” Hankwitz [9] 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 later [10] 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]