362 CHEMISTRY Most important among them was the prepara- tion of oxygen from the red oxide of mercury by means of heat, although Priestley himself failed to appreciate the true value of his dis- covery. He, however, observed that this gas is given off by growing plants, whence he concluded that the latter thus replace the oxygen which has been removed from the air by combustion, &c., and that the two processes are in equilibrium with each other, the con- stant composition of air being thus maintained. Of equal importance for the development of the present system, though exerted in a differ- ent direction, were the labors of the Swedish chemists Bergman and Scheele. The method of analysis by the wet way introduced by Boyle had been but little followed till Bergman (1735- '84), carrying out the idea, established a com- S'ete series of reagents, and taught their use. e thus laid the foundation of the present sys- tem of inorganic analysis. He saw the advan- tage to be gained by cansing each ingredient of a compound to unite with some other body with which it formed a combination of known constitution, capable of easy separation, rather than seek to isolate and determine it as such. Bergman analyzed a great number of sub- stances, and investigated the composition of many salts. But although possessing such correct views, he was an indifferent analyst, not equal even to some of his contempo- raries. His reputation was so great, however, that many years elapsed before any correc- tion of his results was allowed, though many of them have been since proved erroneous. He made numerous important discoveries ; correctly explained the difference between cast iron, wrought iron, and steel, as well as the composition of many salts previously mis- understood. His most important work was upon chemical attraction (affinity). Some idea of its magnitude may be given by stating the fact that he drew up tables of the affinity of 50 different substances for each other, arranging them in two series according to their behavior when treated respectively in the dry or wet way. An enumeration of the original observa- tions and discoveries of Scheele (l742-'86) would fill a volume. His investigations in organic chemistry alone are sufficient to prove him one of the best analysts that ever lived. He separated all the more common organic acids from plants, and knew how to distinguish them when mixed with each other, having de- vised processes for their separation. Several acids of the animal economy did not escape him. He detected the presence of glycerine in the fats, separating it by means of oxide of lead, and prepared prussic acid from prussian blue. He also discovered molybdic and tungstic acids. Most fruitful was his research on the minerals of manganese, which, when the means at his disposal are considered, must be regarded as standing without parallel in the annals of chemistry. He discovered, first, the presence of a new metal (manganese) ; second, on add- ing muriatic acid to the black oxide of man- ganese, on which he was experimenting, a pe- culiar gas (chlorine) was evolved, the prop- erties of which he accurately described. It is worthy of remark that he named it dephlo- gisticated muriatic acid, and at that time phlo- giston was synonymous with hydrogen, show- ing that he regarded it in the same light as chemists do now. Thirdly, the ore of man- ganese on which he operated happened to con- tain a quantity of baryta, which he separated, and, having studied its properties, recognized as a new and peculiar substance. In solutions of its salts he found a test for sulphuric acid, which has since been universally used. Equally able was his investigation of fluor spar ; he found it to be a compound of lime with a peculiar acid, which destroyed his vessels so rapidly that he was unable to collect it. He investigated at- mospheric air in its relations to combustion, finding that it contains two different gases, one of which, "empyreal air" (oxygen), is capable of supporting combustion and respiration, while the other, "vitiated air " (nitrogen), cannot maintain these processes. He proved that the metals, when burned to calxes, absorb empyreal air, while the calxes give it off when reduced to metals. He prepared oxygen at about the same time as Priestley, entirely independently, as admitted by Priestley himself, having obtained it from peroxide of manganese and from salt- petre as well as from the oxides of silver and mercury. The fact that the phlogiston theory had become inadequate to explain many points already known in chemistry, is more fully ex- hibited by Scheele than by any other chemist. He ostensibly believed in this theory, yet ob- servation had taught him that oxygen is ab- sorbed by metals during calcination and by com- bustibles when burning. To explain this, he conceived that oxygen is a compound of water with a certain light saline matter, in which com- pound but little phlogiston is contained. Dur- ing combustion the phlogiston of the combustible unites with the saline matter of the oxygen, pro- ducing light and heat, while the residual product is a compound of the matter which had origi- nally been combined with phlogiston in the com- bustible, with the water of the oxygen. This theory, it will be seen, differs most essentially from that of Stahl, who saw in combustion nothing but a separation of phlogiston, while Scheele regarded it as a mutual decomposition of the combustible and the substance supporting combustion, new compounds being produced. These views clearly show the transition state of chemistry at that period. In general, the ideas which the several chemists attached to phlogis- ton toward the end of the 18th century were far from exhibiting the accordance which had previously existed. The main feature of the phlogiston theory consisted in regarding com- bustion as depending upon decomposition, while chemists have since learned that it arises from the formation of compounds. It how- ever explained the analogies and mutual rela-
