ANTIPHLOGISTIC EXPERIMENTS Black and Cavendish.—The first blow to the phlogistic theory was dealt by Black, himself an adherent. It had long been known that limestone was turned into quicklime by heating, and that quicklime brought into contact with the mild alkalis rendered them caustic, returning itself to chalk. This was ex plained by supposing that the fire gave a "burning principle" to the limestone, which handed it on in turn to the mild alkali. Black, using the balance, found that both chalk and magnesia alba lost weight when they were heated, the loss being due to the escape of a gas previously fixed in them. The caustic lime and magnesia, freed from the gas, no longer effervesced with acids: moreover, the chalk and magnesia alba lost the same weight of gas whether they were strongly heated or were dissolved in an acid. The effervescence was obviously due to the escape of the air. Then an aqueous solution of the caustic lime or magnesia when exposed to the atmosphere gradually formed a white precipitate and lost its causticity as the "Fixed Air" from the atmosphere dissolved in the surface layer of the liquid. Caustic lime and magnesia act on the mild alkalis by combining with their fixed air, and not by any exchange of phlogiston. Black had proved by quantitative experiments the difference between the mild and caustic alkalis, and had shown that fixed air was a normal constituent of our atmosphere (1756).
Twenty years later Cavendish published his researches on "Fac titious Air," in which he described the preparation and properties of carbon dioxide and hydrogen. His work on hydrogen is of the first importance. "Inflammable Air," he found, could be pro duced readily by the solution of zinc, iron and tin in dilute sul phuric or in hydrochloric acid. He collected and measured the gas over water, and found that a given weight of one of the metals gave the same measure of gas whichever acid was used as the solvent and however diluted the acid. He concluded therefore that the gas came from the metal and was either phlogiston or a compound of phlogiston with water.
Informed of Priestley's observations that in the explosion of inflammable air with common air a dew was deposited on the vessel and a loss of weight ensued, Cavendish carefully repeated the experiments and showed there was no loss of weight, while "almost all the inflammable air, and about one-fifth part of the common air, lose their elasticity and are condensed into a dew which lines the glass." He then burnt a quantity of hydrogen in air and, collecting the dew, found it had the properties of water; the two airs, he said, "are turned into pure water." Then Caven dish fired a mixture of hydrogen and oxygen, and was puzzled for some time by finding the condensed water to be acid. He proved this acid was due to the presence of adventitious nitrogen (from the air) which was acted on in the explosion if the oxygen was in excess. Afterwards he passed electric sparks through a mixture of air and oxygen in a bent tube over mercury in presence of potash. In this remarkable experiment he not only proved the composition of nitre, but proved that the nitrogen of the air could be completely removed with the exception of a small inert bubble of gas about one io part of the whole. He had isolated argon (q.v.) from the air—by the same method used by Rayleigh and Ramsay more than ioo years later. In spite of the accuracy and the definite plan of his work, Cavendish was not convinced that water was a compound substance; James Watt saw the true significance of his work, and so did Lavoisier when he repeated it.
Scheele found that various substances—especially bodies like sulphur and the volume of air in which they were burnt ; and the residual gas he found, against his ex pectation, to be lighter than ordinary air, and this lighter portion was no longer able to support combustion. The portion of the air concerned in the burning—his "fire-air" had disappeared. He burnt a jet of hydrogen in a measured volume of air over water, noticed how the flame died out, and then how the water rose in the vessel. Finding no evidence of any change in the water, he concluded that the missing air being neither condensed in the residual "foul air," nor dissolved in the water, must have escaped through the glass. The phlogiston (hydrogen) must have com bined with the fire-air to form "caloric," and this had passed through the glass walls. He set himself to reverse the process. If caloric could be passed into a substance avid for phlogiston, the caloric might be broken up, the phlogiston retained and the fire-air liberated. Several bodies, he thought, would strongly attract phlogiston—such as nitric acid, pyrolusite (which he had himself investigated) and the red calx of mercury. When heat was applied to nitric acid, red fumes were evolved, and when these were absorbed by lime, a colourless gas remained, which sup ported combustion brilliantly. He had liberated the sought-for "fire-air." Similarly the calx of mercury, when heated in a vessel over a charcoal furnace, combined with the phlogiston of the caloric to form metallic quicksilver and the pure fire-air was set free. He added his fire-air to the residual "foul-air" in due pro portion and found the mixture behaved as common air.
Scheele interpreted all his results in terms of the phlogistic theory. Thus when he first isolated chlorine by heating pyro lusite with hydrochloric acid, he imagined the pyrolusite had absorbed phlogiston from the acid, and he named the new gas "dephlogisticated marine acid." We owe to Scheele hydrofluoric, hydrocyanic, arsenic, tungstic and molybdic acids, and a long list of organic substances including glycerine and milk-sugar, with oxalic, tartaric, citric, malic and other acids.
Scheele's discovery of fire-air, probably made in 1773, was not published until 1777. Meanwhile, on Aug. 1, 1774, Priestley had concentrated the sun's rays through his new burning-glass on the red calx of mercury contained in a glass vessel over mercury, and found an air was expelled from it very readily. This air he found was not soluble in water, "but," he writes, "what sur prised me more than I can well express was that a candle burned in this air with a remarkably vigorous flame." Now Priestley had previously shown that the respiration of animals acted like burning bodies on common air, diminishing its volume and render ing it noxious. This new air, he found, differed from common air not only in supporting combustion more vividly but in prolonging the life of animals breathing it. His explanation was that while common air is always partially saturated with phlogiston and cannot make room for much more, the new air is uncontaminated and phlogiston can rapidly escape into it—as into a vacuum. He named the new gas therefore "dephlogisticated air," and the residual inert nitrogen he named "phlogisticated air." Priestley was the first to prepare and describe the gases nitric oxide and nitrous oxide, which he collected over water, and the gaseous hydrogen chloride, ammonia, sulphur dioxide and silicon fluoride, which he collected over mercury.
outstanding feature of Lavoisier's work is the clearness with which he interpreted quantitative results, a clear ness founded on his conviction (partly gained from Black) that no ponderable matter disappears in any chemical change. He heated water in a sealed glass vessel for many days, and found the weight of the whole unaltered. On pouring out the water he found the glass had lost in weight, but on evaporating the water he re covered the lost weight in the alkaline silicates which had been dissolved from the glass.
In 1772 Lavoisier began his researches on combustion. When sulphur and phosphorus were burnt in air a portion of the air was fixed. The same thing was observed when lead and tin were calcined in air sealed in glass vessels: there was no change in weight until the vessels were opened and the air rushed in. In 1775, after hearing from Priestley of his "dephlogisticated air," and after repeating the experiment with the red calx of mercury, Lavoisier grasped the true explanation of combustion. The "pure" or "vital" air was combined with the metal in the calx, and this pure air formed that portion of common air which produced calcination by combining with the metal. Then followed the quantitative demonstration that mercury heated in common air slowly combined with the "vital" part and formed the red powder—mercurius calcinatus: from this powder, more strongly heated, the vital air was recovered, and this mixed with the inert "azote" (left in the first experiment) exactly reformed the original common air.
Finding that non-metallic elements such as sulphur and phos phorus gave acids when combined with his vital air, and having shown that carbon yielded Black's Fixed Air (Bergman's Aereal Acid), Lavoisier adopted the name oxygen (acid-maker) for the substance which, combined with caloric, formed "oxygen gas." While the non-metallic elements gave acids by their combination with oxygen, the metals gave calces—the oxides of the metals; and the union of the two kinds of oxides gave the various metallic salts. But, the phlogistonists could still object, this did not ex plain the formation of inflammable air when metals were dissolved in acids. Just the same salt was formed by the union of a calx with an acid as by the solution of the metal in the acid; but in the latter case the metal gave up its phlogiston as inflammable air. Moreover, unless the calx were originally present in the metal, there was no obvious source from which the metal could obtain the oxygen necessary to form the calx. It was a difficulty only to be solved by Cavendish's proof that water was formed when inflammable air was burnt. Informed of Cavendish's work, Lavoisier and Laplace repeated the experiment and by their cor rect interpretation supplied the missing data. In the solution of a metal by an acid, the oxygen of the water combines with the metal, liberating the inflammable air—now named hydrogen— whilst the two oxides (the calx and the acid) combine to form the salt. This was the death-blow to the Phlogistic Theory.
Founded on Lavoisier's oxygen theory, a new nomenclature was evolved, and this has largely survived to our day. It was perhaps natural that the new school should consider oxygen to be an essential constituent of all acids, and that the acid from sea-salt, muriatic acid, should be regarded as formed by the union of an oxide of an unknown non-metallic element Murion with water, and that chlorine—Scheele's "Dephlogisticated Marine Acid"—should be an oxygenated compound of this, or "Oxy muriatic Acid."