When lead is kept nearly at a red-heat in the open air for some time, it is converted into a pigment called red lead; this is a calx of lead. To restore this calx again to metallic lead, it is only necessary to heat it in contact with almost any combustible matter; all these bodies therefore must contain one common principle, which they communicated to the red lead, and by so doing reconverted it to the state of metal. Metals then were regarded as compounds of calces and phlogiston. Thus far the theory works glibly enough; but now comes a startling fact, which was long unnoticed by the blind adherents of Stahl, or, if noticed, intentionally overlooked. It was observed very early, that when a metal was converted into a calx, its weight was increased. When this difficulty first forced itself upon the attention of the Phlogistians, it was necessary that they should either explain it, or at once abandon their theory. They accordingly endeavoured to evade the difficulty, not only by asserting that phlogiston had no weight, but that it was actually endowed with a principle of levity.
It was not possible, however, that any rational notions should have been entertained upon the subject of combustion, at a period when the composition of the atmosphere even was unknown. Let us therefore follow the stream of discovery, skimming the surface merely, as it flowed onward towards quite a new field of science—Pneumatic Chemistry.
Boyle and Hooke, who had improved the air-pump invented by Otto de Guericke, of Madenburgh, first used this apparatus for investigating the properties of air; and they concluded from their experiments that air was absolutely necessary to combustion and respiration, and that one part of it only was employed in these processes; and Hooke formed the sagacious conclusion, that this principle is the same as the substance fixed in nitre, and that combustion is a chemical process, the solution of the burning body in elastic fluid, or its union with this matter.
Mayow, of Oxford, in 1674, published his treatises on the Nitro-aërial spirit, in which he advanced opinions similar to those of Boyle and Hooke, and supported them by a number of original and curious experiments.
Dr. Hales, about 1724, resumed the investigations commenced with so much success by Boyle, Hooke, and Mayow; and endeavoured to ascertain the chemical relations of air to other substances, and to ascertain by statistical experiments the cases in nature, in which it is absorbed or emitted. He obtained a number of curious and important results; he disengaged elastic fluids from various substances, and drew the conclusion, that air was a chemical element in many compound bodies, and that flame resulted from the action and reaction of aërial and sulphurous particles; but all his reasonings were contaminated with the notion of one elementary principle constituting elastic matter, and modified in its properties by the effluvia of solid or fluid bodies.
The light of Pneumatic science which had dawned under Hooke, Mayow, and Hales, burst forth in splendour under the ascendency of that constellation of British science, Black, Cavendish, and Priestley.
In 1756, Dr. Black published his researches on calcareous, magnesian, and alkaline substances, by which he proved the existence of a gaseous body, perfectly distinct from the air of the atmosphere. He showed, that quick-lime differed from marble and chalk by not containing this substance, which he proved to be a weak acid, capable of being expelled from alkaline and earthy bodies by stronger acids.
As nothing is more instructive than to enquire into the circumstances which have led to a great discovery, I quote with pleasure the following passage from Dr. Thomson's History of Chemistry.
"It was the good fortune of chemical science that, at this time (1751), the opinions of professors were divided concerning the manner in which certain lithonthriptic medicines, particularly lime-water, acted in alleviating the excruciating pains of the stone and gravel. The students usually partake of such differences of opinion: they are thereby animated to more serious study, and science gains by their emulation.
"All the medicines which were then in vogue as solvents of calculi had a greater or less resemblance to caustic potash or soda; substances so acrid, when in a concentrated state, that in a short time they reduce the fleshy parts of the animal body to a mere pulp. They all seemed to derive their efficacy from quick-lime, which again derived its power from the fire. It was therefore very natural for them to ascribe its power to igneous matter imbibed from the fire, retained by the lime, and communicated by it to alkalies which it renders powerfully acrid. It appears from Dr. Black's note-books, that he originally entertained the opinion, that caustic alkalies acquired igneous matter from quick-lime. In one of them, he hints at some way of catching this matter as it escapes from lime, while it becomes mild by exposure to the air; but on the opposite blank page is written, 'Nothing escapes—the cup rises considerably by absorbing air.' A few pages further on, he compares the loss of weight sustained by an ounce of chalk when calcined, with its loss while dissolved in muriatic acid.