HISTORY.] CHEMISTRY 463 nature of heat and fire, he found that measured quantities of common air, when kept in contact with certain sub stances, e.g., solution of potassium sulphide or moist iron-filings, contracted in volume, and became incapable of supporting combustion. As the specific gravity of the air had not augmented, the decrease of bulk, Scheele con cluded, could not be due, as he had at first conjectured, to the absorption of phlogiston ; the atmosphere must, there fore, consist of two distinct bodies. One of these, the residual air, he assumed to be incapable of combining with phlogiston ; the other, having a strong attraction for that substance, had united with it, forming heat, which had penetrated through the walls of the vessel containing it hence the diminution of the original volume of air. Heat, Scheele considered, was decomposed by means of bodies which had a strong attraction for its phlogiston, such as the calces of gold, silver, and mercury, and oil of vitriol mixed with black manganese ore, and consequently the other constituent of heat, empyreal or fire-air (oxygen), became isolated. Heat could be synthesized, for it was produced by the union of the phlogiston of coals with fire- air. Light, like heat, was a compound of fire-air and phlogiston, but was richer in the latter constituent, to the varying proportions of which it owed its differences of colour. Subsequently, when it became impossible for Scheele to ignore the consideration of the increase observ able in many substances after burning or calcination, he so far modified his views as to regard fire-air as a compound containing, with a very little phlogiston, a saline principle (principium salinum) and water, which last gave to fire-air the greater part of its weight. When fire-air formed heat oy combining with phlogiston, it gave up its water to the materials it dephlogisticated, and thus it was that they were rendered heavier by ignition. Such, in brief, were the theoretical conceptions of Scheele ; it is upon his work as a practical chemist that his fame must rest. Tartaric acid was isolated by him in 1769, and he made the dis covery in 1774 of baryta and of dephlogisticated muriatic acid (chlorine), and in 1779 of glycerine, the properties of which he pointed out in 1784 ; in 1781 he demonstrated the nature of hydrofluoric acid, first obtained by him in 1771, and prepared tungstic acid, before unknown; and between the years 1776 and 1786 he discovered benzoic, molybdic, lactic, mucic, oxalic, malic, and gallic acids, and made important observations on the compounds of arsenious acid. The advance made during the last part of the 18th century in analytical chemistry is attributable in great measure to the labours of Torbern Bergman of Upsala (1735-1784), who devised systematic methods of examining compounds by the wet way, and by means of the blow-pipe, and first rendered it possible to analyze minerals insoluble in acids by fusing them with an alkali or alkaline carbonate. In 1718 E. F. Geoffroy had published tables in which he exhibited the reciprocal chemical affinities of various sub stances, and these tables had been improved upon by Gellert in his Metallurgic Chemistry, 1751, and by Limbourg, 1761. Bergman, in 1775, gave in a disserta tion on elective attractions, as he named affinity, an arrangement in 59 columns of all the chemical substances known at the time, in which was shown the order of their decomposition when in solution, and when exposed to a strong heat. The nature of the compounds formed by the mixture of reagents depended, in Bergman s estimation, on the sum of their attractions. Bergman contributed also in some measure to the determination of the constitution of neutral salts a subject treated of by Homberg in 1699, and, after Bergman and Kirwan, investigated by Wenzel in his Vorlesungun uber die chemische Verwandtschaft der Korper, published in 1777. From Wenzel s observations the idea of equivalency took its rise. He showed that the products of the mutual decomposition of two neutral salts were themselves neutral, or, in other words, that the same weight of base satisfies definite quantities of two different acids. Thus, according to his experiments, 123 parts of lime and 222 of potash must be considered equivalent to each other, being both competent to neutralize 181 5 parts of sulphuric, or 240 parts of nitric acid. In England, ten years previously to the publication of Wenzel s treatise, Cavendish described certain quantities of fixed alkali and marble as " equivalent ; " and in 1788 he stated that a quantity of oil of vitriol sufficient to produce 100 parts of plumbum ponderosum with sugar of lead would dissolve 33 of marble, since he found by experiment that so much oil of vitriol would saturate as much fixed, alkali as a quantity of nitrous acid sufficient to dissolve 33 of marble. (Phil. Trans, 1767, p. 102; 1788, p. 178.) In 1792, J. B. Richter (1762-1807) published a work on Stochiometrie, or the Art of Measuring Chemical Elements, in which he gave in two series of tables the weights of different bodies which neutralized 1000 parts of various acids, and the weights of acids that similarly cor responded to 1000 parts in the case of the bases, and pointed out the proportionality that existed between the weights of the bases, as also of the acids, in each series. There was, he remarked, a constant ratio between the quantity of an acid and the quantity of oxygen in the weights of the bases needed for its complete saturation, a fact afterwards restated by Berzelius, who showed that a simple and uniform relation was observable between the amounts of oxygen in the acid and basic portions of salts of the same class. Although neither the science of chemistry nor yet a change in its objects can be said to have originated with Lavoisier (1743-1794), the means he introduced of attain ing those objects, the ideas he put forth concerning the constitution of bodies, and the explanations he gave of various phenomena were new, and gave to the science in the twenty years preceding his death a completely altered aspect. The mine of chemistry had yielded rich returns long before Lavoisier came to labour in it ; he availed himself of the old workings and, extending them, opened the main lode. "He discovered," says Liebig, "no new body, no new property, no natural phenomenon previously unknown; but all the facts established by him were the necessary consequences of the labours of those who preceded him. His merit, his immortal glory consisted in this that he infused into the body of the science a new spirit ; but all the members of that body were already in existence, and rightly joined together." (Letters on Chemistry, ii.) In the first of his papers, in 1765, Lavoisier indicates no doubt of the existence of phlogiston. In 1775 he still spoke of it, but in the following year he expressed his conviction that, for the elucidation of certain phenomena, one must ascribe to phlogiston other qualities than those assigned to it by Stahl. It was in 1775 that he presented to the French Academy a memoir On the Nature of the Principle which combines with the Metals during their Calcination, and which augments their Weight, in which he describes the formation of fixed air from charcoal in the reduction of calces, and the preparation and properties of oxygen; but he makes no allusion to the fact that Priestley had in November of the foregoing year made him acquainted with that gas, or to the observations of Bayen in the Journal de Physique, February and April 1774, to the effect that the change of a metal into a calx is attributable to the fixation of an aerial fluid, and that the red oxide of mercury is reduced by heat without the addition of anything, and loses in
weight by reduction.