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 _Methode 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]

Early Uses

In the 18th century, phosphorus was a costly material. It was produced
mostly for display and to satisfy curiosity. Guillaume Francois Rouelle
(1703-1770) demonstrated the process in his lectures, and, as Macquer
reports, he "very often" succeeded in making it.[12] Robert Boyle had
the idea of using phosphorus as a light for underwater divers.[13] A
century later, "instant lights" were sold, with molten phosphorus as the
"igniter," but they proved cumbersome and unreliable.[14] Because white
phosphorus is highly poisonous, an active development of the use in
matches occurred only after the conversion of the white modification
into the red had been studied by Emile Kopp (1844), by Wilhelm Hittorf
(1824-1914) and, in its practical application, by Anton Schroetter
(1802-1875).[15]

[Illustration: Figure 3.--DISTILLATION APPARATUS (1849) for refining
crude phosphorus. The crude phosphorus is mixed