Page:The American Cyclopædia (1879) Volume XII.djvu/783

 OXYDENDRUM in connection with questions of Russian pos- sessions in central Asia. (See AFGHANISTAN, BOKHARA, and KHIVA.) Of the numerous books and papers written upon the Oxus, see espe- cially Sir H. 0. Rawlinson's "Monograph on the Oxus " (" Journal of the Royal Geographi- cal Society," 1872), and for description Mac- Gahan's " Campaigning on the Oxus, and the Fall of Khiva" (London and New York, 1874). OXYDENDROI. See TREE SORREL. OXTGEN (Gr. 6?6f, acid, and yewae/v, to gen- erate), the most abundant of all elementary substances, having when free a gaseous form, which has never been reduced to a liquid. Its symbol is O, its atomic weight 16. It com- poses eight ninths of the water on the globe, nearly one fourth of the atmosphere, and a large part of the earth's crust, principally in the form of oxides of the various metallic ele- ments. It was discovered by Priestley in Eng- land in 1774, and almost simultaneously by Scheele in Sweden. It was called by Priest- ley dephlogisticated air, and by Scheele em- pyreal air. Condorcet proposed the name vital air, in reference to its agency in supporting life. Lavoisier, whose claim to its subsequent discovery is disputed, made a series of careful experiments in which he proved that the com- bustion of bodies in the air consisted in their union with this gas, to which he gave the name of oxygen, because he thought it essen- tial to the constitution of an acid ; it has since been shown that this view was erroneous. Preparation. Oxygen may be obtained by several different methods, which depend upon different physical as well as chemical princi- ples. It may be mechanically separated from the nitrogen of the atmosphere by employing the principle of osmose in dialysis (see DIAL- YSIS), a process due to Graham. It may be obtained from water by electrolysis (see GAL- VANISM, vol. vii., p. 596) ; from several of its compounds by the dissociating action of heat, as from peroxide of mercury in the original exper- iment of Priestley ; from other higher oxides of metals by heat alone, or by the joint action of heat and some substance which will unite with a lower oxide, thus leaving a part of the oxygen free. Until recently, the usual mode of preparing it in large quantities was to sub- ject binoxide of manganese to a red heat in a furnace, as shown in fig. 1. The cylindrical cast-iron retort a contains the binoxide, which is placed in the furnace &. One pound of good oxide will yield six or seven gallons of oxy- gen, with some carbonic acid, which may be removed by means of the Woulff's wash bottle c, containing a solution of potash or soda ; MnO 2 becomes by the action of heat MnO + O. It may be obtained from binoxide of manganese by employing strong sulphuric acid and a mod- erate degree of heat, a glass flask or retort being used in place of iron. The reaction in this case may be represented by the following equation : H a SO 4 + Mn0 2 =MnSO 4 + H a O + O. It may be procured by the dissociating action OXYGEX 769 of heat on sulphate of zinc, which at a red heat gives off a mixture of sulphurous acid and oxygen, from which the sulphurous acid may FIG. 1. be removed by washing, either with an alka- line solution or with water. Oxygen may also be obtained in decomposing sulphuric acid by passing its vapor over red-hot platinum foil or sponge, the products being, as in the last case, sulphurous acid and oxygen, H 2 S04= H 2 O + SO 2 + O. This process is recommended by Deville and Debray as the cheapest meth- od for procuring it in large quantities. For class-room experiments and lectures oxygen is usually obtained by decomposing with heat chlorate of potash, KC10 3, which becomes K01+3O. This may be effected with the salt alone, but the high temperature required to produce perfect decomposition, and the vio- lence with which it takes place, make it prefer- able to mix the salt with about its own bulk of some oxide, as binoxide of manganese. The operation may be conducted in apparatus shown in fig. 2. Boussingault proposed a' method of obtaining oxygen by alternately oxidizing and deoxidizing a metal or metallic base. The sub- stance used by him was the oxide of barium or barytes, and it is raised to the peroxide by passing a current of slightly moistened air, deprived of carbonic acid, over the protoxide heated to redness in a porcelain tube. When peroxidation has taken place, the current of air is cut off, and the tube is heated to full redness, which drives off the surplus oxygen, FIG. 2. and reduces the barytes again to a protoxide. Mar6chal and Tessie du Motay prepare oxy- gen in large quantities by heating manganatea,