HISTORY OF CHEMISTRY: Carl Wilhelm Scheele (1742-86)

Among the most eminent of the contemporaries of Priestley and Cavendish that cherished a belief in the existence of phlogiston was the Swedish chemist, Scheele (1742-1786). In experiments made to ascertain the nature of heat and fire, he found that measured quantities of common air, when kept in contact with certain substances, e.g., solution of potassium sulphide or moist iron-fillings, contracted in volume, and became incapable of supporting combustion. As the specific gravity of the air had not augmented, the decrease of bulk, Scheele concluded, could not be due, as he had at first conjectured, to the absorption of phlogiston; the atmosphere must, therefore, 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 observation in many substances after burning or calcinations, 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 by 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 discovery in 1774 of baryta and of dephologisticated muriatic acid (chlorine), and in 1779 of glycerine, the 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.






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