Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/473

HISTORY.] CHEMISTRY 461 ( <Aoyttrrov, combustible) was applied by Stahl the matena aut principiurn ignis nonipse ignis. The phlogiston of Stahl answers in some measure to the souls and spirits assigned to metals and salts by the alchemists, or to what Geber called the " humidity," and Cardan the " celestial heat" of metals. When by means of charcoal a metallic calx was reduced, or a compound containing sulphur was obtained from fused sodium sulphate, phlogiston was supposed to be absorbed from the charcoal, which with tamp black and other reducing agents came in time to be regarded as nearly pure phlogiston. Bodies that would not burn were thought to have already parted with their phlogiston. From a consideration of the insolubility of most combustible substances arose the idea that phlogiston was a dry and earthy body, capable of receiving a motion of great velocity the motus verticillaris manifested when ignition or flame was produced. John Hey had in 1630 remarked that metals grow heavier when calcined by the absorption of " thickened air," but had given no general theory of combustion, or explanation why many substances become lighter or are lost sight of when heated. Boyle, too, had noticed the increase of weight caused by the calcination of metals, and had attributed it to the combina tion of the latter with heat particles ; and Stabel and others were not slow to object that this fact negatived the supposition that calcination consisted in a subtraction of phlogiston ; the Stahlians, however, met the difficulty by declaring that substance to be the principle of levity or negative weight. F. Hoffmann, who contributed greatly to the progress of auulytical chemistry in Germany, held with Stahl that sul phur consisted of acid and phlogiston, and that combustible bodies contained something which might be described as phlogiston, but thought it possible that the calces of metals were formed, not by the subtraction of phlogiston, but by the combination of the metals with an acid material. Boerhaave, without directly attacking the phlogistic theory, casts doubts upon the assumption of the existence of a combustible principle and of earthy substances in the metals. The view of Homberg (1652-1715) was that the principle of combustibility in inflammable minerals and in vegetable substances was sulphur ; and E. F. Geoffrey (1672-1731) regarded phlogiston as a sulphurous or oily principle. Amongst the most active supporters of the doctrines of Stahl were Neumann (1683-1737); J. H. Pott (1692-1777), distinguished for his researches on the behaviour of mineral substances at high temperatures ; Marggraf (1709-1782); and Macquer (1718-1784), the discoverer of arsenic acid. Other celebrated chemists who flourished during the phlogistic period were Re aumur (1683- 1757), Hellot (1685-1766), and Duhamel du Monoeau (1700-1782), who first proved the nature of the base of common salt. The phlogistic theory of Stahl, though incorrect, was of no small assistance towards a true understanding of chemical phenomena. It was based upon experimental data, the interpretation of which served for the correlation of facts of which but vague and enigmatic explanations had formerly been given. The supposed subtraction of phlogiston in the calcination of metals, though equivalent in reality to the addition of oxygen, was yet a loss of potential energy, by virtue of the combination of the metal with the gas ; and the gain of phlogiston was an increase of potential energy, attendant on the removal of oxygen. It was only in the latter part of the 18th century that the influence of the presence of air upon the formation of many chemical compounds was generally perceived, and that through the use of the balance the nature of gases began to be comprehended and such airy nothingness became commonly regarded as an intimate and necessary constituent of various solid and fluid bodies. The phlogistic theory gave to its adherents so plausible and moreover so circumstantial an account of the modes ot chemical action, that facts and observations which caused at a later time a complete revolution in the theory of chemistry, such, for instance, as regarded the existence and properties of oxygen, remained without explanation, and almost unheeded. Robert Hooke, so early as 1665, in his Microyraphia, foreshadowed the discoveries of Priestley and his contemporaries, when he advanced the opinion that in common air there existed a substance like, if not the same as, that fixed in saltpetre, and which at an elevated temperature dissolved combustibles such as sulphurous bodies with a rapidity sufficient to occasion the motion of fire, and to create light ; this solvent he considered to be far less for a given bulk of air than of saltpetre. The investigations of Mayow (1645-1679) are particularly interesting. In treatises published at Oxford in 1668 and 1674 oxygen is actually described by him under the name of fire-air, aerial spirit, and nitre-air ; all acids are said to contain it, and it is necessary for combustion and respira tion, processes which are therefore analogous ; it is the nitre-air of the atmosphere that causes fermentation and the souring of wines, that produces sulphuric acid from sulphur, and effects the calcination of metals. Early in the 18th century Newton in his Opticks indi cated the nature and the modes of formation of gases. " Dense bodies," he tells us, " by fermentation rarefy into several sorts of air ; and this air by fermentation, and sometimes without it, returns into dense bodies ; " and further on he remarks that the particles shaken off from bodies by heat or fermentation, so soon as they are beyond the reach of the attraction of the body, recede from it, and also from one another with great strength, so as sometimes to take up above a million of times more space than they did before in the form of a dense body. This vast con traction and expansion seem to him unintelligible by feigning the particles of air to be springy and ramous, or rolled up like hoops, or by any other means than a repulsive power ; the particles of fluids which do not cohere strongly are most easily rarefied into vapour ; but those which are grosser, or cohere by a stronger attraction, are not separated without a stronger heat, or perhaps not without fermentation ; being rarefied by fermentation they become true permanent air, those particles receding from one another with the greatest force, and being most difficultly brought together, which upon contact adhere most firmly. (Opticks, bk. iii., qu. 30 and 31, 1730.) In 1727 Dr Stephen Hales (1677-1761), who had fur some years been engaged in investigations similar to those of Mayow, gave to the world iu his Statical Essays the collective results of his observations. The atmosphere he describes in this work as a fine elastic fluid, with particles of very different natures floating in it, whereby it is fitted to be the breath of life of vegetables as well as of animals. The effect of respiration and of the burning of sulphur iu air is to deprive it of its elasticity ; and country air is cleaner and more elastic than that of towns. Elasticity, we read, is not an essential immutable property of air particles ; " they are easily changed from an elastic to a fixed state by the strong attraction of the acid, sulphureous, and saline particles, which abound in the air. Whence it is reasonable to conclude that our atmosphere is a chaos, consisting not only of elastic, but also of unelastic air particles, which in great plenty float in it." (Stat. JEss., vol. i. 4th ed., 1769.) Hales did not, however, attempt to determine the distinctive properties of the various gaseous constituents of the atmosphere, and of the substances on

which he experimented; all are indiscriminately designated