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CHEMISTRY 861 alchemists deserve mention. It had been stated that mercury could be converted into an infu- sible metal when subjected to the continued action of heat. Boerhaave maintained a quan- tity of mercury at a moderate heat during 15 years without effecting such transformation. Another portion of mercury, being strongly heated in a closed vessel for six months, re- mained unaltered. It had also been stated by the alchemists that mercury could be converted by repeated distillation into a more volatile body. Boerhaave distilled pure mercury 500 times without perceiving any alteration in its boiling point. From these experiments he in- ferred that the given statements were errone- ous. Geoffroy (1672-1731) published system- atic tables of affinity, which exerted a long- continued influence on the science. Marggraf "of Berlin (1709-'82) was distinguished as an analyst and technical chemist ; he first called attention to the existence of sugar in the beet root and other plants indigenous to Europe. Macquer of Paris (1718-'84) pointed out the existence of arsenic acid, and made other new observations. His idea of four material ele- ments, earth, air, fire, and water, illustrates the tendency of the period gradually to do away with the vague expressions of the alchemists. Equally characteristic is the fact that although Macquer had himself often used the quantita- tive method of analysis in investigating minei-al waters and metallurgical matters, and consid- ered it very necessary in such connection, he utterly neglected it when studying theoretical questions, and viewed with indifference the anti- phlogistic theory, founded on such unimpor- tant data, as he thought, as mere differences of weight. He regarded phlogiston as identical with light, and capable of passing through trans- parent media, thus explaining the reduction of peroxide of mercury by heat, which had been observed in glass vessels. The English chemists of this period were, meanwhile, observing facts which afterward constituted the most effective weapons of the opponents of the phlogiston theory. The discoveries of Black, Cavendish, and Priestley led naturally to its overthrow. The investigation of the alkalies by Black of Edinburgh (1728 '99) was of great impor- tance. It had previously been supposed that when limestone was burned it united with fire and thus obtained caustic properties. From caustic lime this fire could be transferred to the mild (carbonated) alkalies, rendering them caustic, while the lime having given up its fire became again mild. Black disproved this view by showing that the mild alkalies are compound bodies and become caustic, not by combining with fire, but from losing one of their constitu- ents, a gas, which he called fixed air. He proved that limestone loses weight when burned to caustic lime, from which he inferred that the latter is contained in the former. He observed that the mild alkalies effervesce with acids, a gas similar to that given off by burning lime- stone being evolved. This gas (carbonic acid) he regarded as the second constituent of the limestone or other mild alkali, drawing the con- clusion that the true caustic alkalies must be simple bodies which become mild only by uni- ting with fixed air. This view, after some oppo- sition, was soon universally received by chem- ists. Black also distinguished more clearly than had been done before the difference be- tween lime and magnesia. The discovery of' latent heat is also due to him. The celebrated physicist Cavendish (1731-1810) added greatly to chemical knowledge by his accurate experi- ments upon gases. He investigated hydrogen, and was led to consider it identical with phlo- giston. According to him, when dilute sul- phuric acid is added to metallic iron or zinc, inflammable air or phlogiston separates un- changed from its combination with the calx, while 'the latter unites with the acid. If con- centrated sulphuric acid be used, the phlogiston is no longer set free as inflammable air, but combines with a portion of the acid, forming another gas which is not inflammable (sulphur- ous acid, already called phlogisticated sulphuric acid by Stahl). Cavendish's view of the iden- tity of hydrogen and phlogiston was soon ad- mitted by the supporters of the phlogiston theory, especially after it was found that calxes could be transformed to metals when heated in an atmosphere of hydrogen. The original idea of phlogiston was thus somewhat modified. "With Stahl a dephlogisticated substance meant one which had been oxidized : thus, sulphuric acid was dephlogisticated sulphur ; while at the end of the phlogiston period it meant as well a body which had been deprived of hydrogen : thus Cavendish and Priestley call oxygen dephlo- gisticated water. Cavendish also investigated carbonic acid and the quantitative composition of atmospheric air. The latter he proved to contain oxygen and nitrogen in the same pro- portions at all seasons of the year and in differ- ent localities. He investigated the changes caused in air by combustion, showing that car- bonic acid is formed only when the combustible is of animal or vegetable origin. Synthetically, he ascertained the composition of nitric acid by passing a series of electric sparks through air ; also that of water. This last was specially important, having been used with great effect by the opponents of the phlogiston theory. Although few men contributed more to the over- throw of this theory than Cavendish himself, yet he to the last remained true to its tenets, having explained all his discoveries in accord- ance with them. His reputation has suffered greatly from this, for the chemists who sub- sequently correctly explained his observations have shared with him the merit of them. One of the last and firmest defenders of phlo- giston was Priestley (1733-1804). Having in- vented suitable apparatus, similar to the pneu- matic trough of the present day, he was enabled to collect and observe the properties of all the more important gases, the number of his dis- coveries in this field being truly remarkable.