Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/493

OZOXE.J CHEMISTRY No method has yet been devised of directly separating pure oxygen frein air, but Graham has shown that it is possible to obtain an "air" containing about 41 6 per cent, by volume of oxygen instead of about 20 8 per cent., which is the amount present in ordinary air, by dialyzing air through india-rubber. For this purpose a bag com posed of the thinnest india-rubber supported on cloth is connected with a Sprengel air-pump ; if the pump is kept in action, after the air is exhausted from the bag, it is found that it continues slowly to deliver "air" which, however, is richer in oxygen than ordinary air in about the proportions above-mentioned. The gases do not pass through actual pores, but apparently they are dissolved by tha rubber, which is thus wetted through by the liquefied gases, and evaporate into the vacuum on the other side of the membrane, the increased amount of oxygen being doubtless due to the greater solubility of oxygen in rubber. Many othsr methods of obtaining oxygen are known, but the above given are sufficient to illustrate the nature of the changes by which it is produced. Oxygen is a colourless, odourless, and tasteless gas, which has hitherto resisted all attempts to liquefy it ; it is only slightly soluble in water, 100 volumes of which at ordi nary atmospheric temperatures dissolve about three volumes of oxygen. All bodies which burn in the air burn with greatly increased brilliancy and rapidity in oxygen ; thus, a glowing splinter of wood bursts into flame when plunged into oxygen, and burns with great brilliancy; and even iron, if heated to redness before it is introduced into the gas, readily burns in it. In all cases in which bodies burn in oxygen the substances burnt combine with the oxygen to form new substances, and the heat and light developed are a consequence of their union. The weight of the pro ducts corresponds exactly with the weight of the body burned plus the weight of the oxygen consumed. The combination of two or more bodies when thus accom panied by the development of heat and light is termed combustion, the body burnt being the combustible, and the body in which the burning takes place the supporter of combustion. Thus, hydrogen is a combustible since it burns in oxygen, but oxygen is a supporter of combus tion. These terms are merely relative, however, since oxygen may just as readily be burnt in hydrogen as hydrogen in oxygen ; and, similarly, air may be burnt in coal gas. The combination of oxygen with other elements is termed oxidation. In all cases of oxidation heat is developed, but it depends entirely upon the rapidity with which the oxidation is effected whether light is also pro duced, that is to say, whether w uat is ordinarily termed combustion takes place. Thus, when iron is burnt in oxygen, the combination of the two elements is effected with great rapidity, and a large amount of heat is developed within a very short space of time, and hence the product of com bustion is intensely heated and becomes incandescent ; when, however, iron slowly oxidizes or rusts, no light is produced, although actually more heat is developed than when ths same weight of iron is burnt in oxygen, the oxide Fe 3 O 4 being formed in the latter and the oxide Fe. 2 O 3 in the former case. The various elements enter into reaction with oxygen with very various degrees of readiness, but as in the case of hydrogen and oxygen it is mostly necessary at least to start the reaction by the application of heat ; and if the com bination of the two elements can give rise to the develop ment of only a moderate amount of heat, it is usually necessary to continue the application of heat until the oxidation is complete. Phosphorus slowly absorbs oxygen, but the remaining non-metallic elements are not affected by it at ordinary temperatures. The highly positive metala readily absorb it ; the majority of the metals, however, when in the massive state are unacted upon in dry oxygen, but undergo oxidation more or less readily in moist oxygen or air. The coating of oxide first formed frequently protects the metal from more than a superficial oxidation. Some of the metals when in a state of very fine division, for instance, lead as obtained by the ignition of its tartrate, undergo oxidation so readily, however, that spontaneous combustion results from their mere exposure to air or oxygen. The spontaneous combustion of substances such as woollen rsfuse, greasy tow, and hay is a precisely similar phenomenon, the heat developed by their slow oxidation being to a great extent retained, as they are bad conductors of heat, until finally it becomes sufficient to inflame them. Most animal and vegetable substances when exposed to the air decay, and it is generally supposed that they simply undergo slow oxidation. Pasteur has shown, however, that the oxidizing pow r er of atmospheric oxygen is much exaggerated, and that the decay of animal and vegetable substances exposed to air is not simply the result of the action of the atmospheric oxygen, but of the action of oxygen assisted by microscopic organisms, the decay taking place at an extremely slow and almost imperceptible rate when these organisms are entirely excluded. When the colourless gas nitric oxide is mixed with oxygen, red fumes of higher oxides of nitrogen are formed, and by this reaction oxygen may with facility be detected and distinguished from other gases. A solution of potas sium hydroxide and pyrogallol, or pyrogallic acid, or an am- moniacal solution of cuprous chloride, absorbs oxygen with avidity and may be employed to remove it from a mixt iro of gases Symbol, 3 ; Molecular wt, 47 83. When exposed to the action of electricity, and especially under the influence of w r hat is termed the silent discharge, oxygen undergoes a contraction of volume and acquires remarkably different properties, its chemical activity being greatly enhanced. It has been shown that the change consists in the conversion of the oxygen into an allotropic modification w ? hich has received the name of ozone (ow) in allusion to its peculiar odour ; and from Sir Benjamin Brodie s experiments (If. Soc. Trans., 1872, p. 435) there can be no doubt that ozone differs from ordinary oxygen merely in that its molecule contains three atoms ; this difference is expressed in the following graphic formulae : 0=0 Oxygen. A O O Ozone. Various forms of apparatus are employed in ozonizing oxygen, but all are constructed on the principle of the Siemens s induction tube. This consists of two wide tubes of nearly equal diameter placed one within the other and coated on their exterior surfaces with tinfoil; the coat ings of tinfoil are connected with the terminals of a power ful induction coil, and a current of oxygen is passed through the narrow space between the two tubes, and is thus submitted to the action of the electric discharge. In the apparatus employed by Brodie the coatings of tinfoil are dispensed with, but the inner tube is filled with water in which is placed one of the terminal wires of the induc tion coil, and the outer tube is immersed in a vessel of water in connection with the other terminal wire of the coil. It is essential that the oxygen submitted to the electric action be pure and in a very dry condition ; and it is espe cially desirable to prevent the elevation of temperature

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