Properties.-Oxygen is a colorless, odorless, and tasteless gas. It is slightly soluble in water, 100 volumes of the liquid at 0° dissolving 4.1 volumes of the gas. The specific gravity of oxygen is 1.10563 (air = 1), and its density 15.96. Under a pressure of 22.5 atmospheres and at a temperature of -136°, it liquefies, and in that condition it has a specific gravity of o.899 (water 1). This liquid is colorless and boils at -184° under a pressure of one atmosphere. It is also distinctly magnetic; taking the magnetism of iron at 1,000,000, that of oxygen becomes 377.

In chemical properties oxygen is very active, combining either directly or indirectly with all the known elements except fluorine. It is not a combustible gas under ordinary circumstances, but it supports the combustion of many elements and compounds. The term combustibility is considered to refer to the power of a substance to continue burning in the air after being once ignited; that is, to combine with the oxygen of the atmosphere. When mixed with hydrogen the two explode violently on the application of flame or the electric spark, the result of the combination being water. With many other of the elements oxygen unites at ordinary temperatures, sodium, potassium, and phosphorus being examples; with others again, as sulphur, iron, carbon, and copper, an elevated temperature is required, while with platinum, gold, and silver, combination does not take place at any temperature. The products of these combinations are called oxides, those with the non-metals being acid-forming oxides; that is, capable of forming acids by combination with water, while most of those with metals are basic oxides, many of which form basic hydrates with water.

A great number of the compounds with which we will deal later are combinations of oxygen with some other element. As examples of the formation of oxides we may take the following:

All of these elements burn brilliantly in oxygen when once ignited; this is accomplished in the case of zinc and iron by attaching the glowing end of a match, and with phosphorus and carbon by bringing them in contact with flame a moment before introducing into the gas. In the act of respiration a chemical

process is involved, whereby certain organic constituents of the blood are consumed, with formation of carbon dioxide and water.

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Uses. The low cost at which oxygen can be produced by the Brin process has admitted of its application in a number of operations from which it has heretofore been excluded. Except in the nascent state, oxygen alone appears to have no bleaching action, but it has been found that a small proportion added to a solution of bleaching powder greatly increases the efficiency of the latter. In aging spirits, the gas has been found effective when forced into the liquid under a pressure of one or two atmospheres, and to accomplish in a short time what usually requires years by the ordinary method. It has also been found to assist in the removal of sulphur from illuminating gas when mixed with the latter previous to its passage over the lime. Oxygen gas when properly associated with hydrogen, or illuminating gas, is used in certain metallurgical operations where a high temperature is required, and for furnishing a brilliant light, as in the oxyhydrogen lantern. Although frequently tried, the use of oxygen in medicine has not been attended with very encouraging results.

PRACTICAL EXERCISES.

(1) Prepare oxygen by placing a few crystals of potassium chlorate in a clean dry test-tube, adapt a delivery-tube long enough to reach under the surface in a vessel near by. Apply a steady flame, taking care not to allow any water to be drawn back into the tube; this may be accomplished by taking the end of the delivery-tube from the water before the flame is withdrawn. As soon as bubbles of gas escape freely, and the air has been expelled, bring a test-tube filled with water over the escaping gas and collect it.

The evolution of oxygen is effected more easily by mixing with the potassium chlorate, before heat is applied, about one-fourth its weight of manganese dioxide.

(2) Apply a lighted taper to a tube full of oxygen, the gas will not burn. Plunge the taper into the gas and it will burn with greatly increased energy. Extinguish the flame, leaving a glowing spark, and again plunge into the gas, the taper will be re-ignited. These experiments indicate the non-combustibility of oxygen under ordinary circumstances, but show that it is an active supporter of combustion.

(3) A piece of glowing charcoal is lowered into the gas; a rapid combustion ensues and the charcoal disappears. Pour some lime water into the tube, agitate well; a white precipitate of calcium carbonate is produced. If this be tried with oxygen, previous to the burning of the charcoal, no precipitate will be formed. A number of other substances, as phosphorus, sulphur, and iron, will, when once ignited, burn in oxygen with great brilliancy.

(4) Take two test-tubes, one about twice the capacity of the other. Fill the larger with hydrogen and the smaller with oxygen, bring their mouths together, and, after turning once or twice to thoroughly mix their contents, open their mouths to a flame. A sharp report is caused by the combination to form water. If more than two volumes of hydrogen to one of oxygen be present, the surplus remains uncombined; if oxygen be present in greater proportion, the excess of it remains.

Formula, 03.

OZONE.

Molecular Weight, 47.88.

History. This peculiar substance was first noticed by Van Marum, in 1785, in passing electric sparks through oxygen, of which it appears to be only a modification. Schönbein, in 1840, gave it the name Ozone, from Greek w, signifying "I smell."

Occurrence.-Ozone is found in minute quantities in the atmosphere which is remote from large cities, and is especially noticeable after a thunder-storm. It is also supposed to be produced by the growth of plants.

Preparation.-(1) Ozone is evolved at the positive pole in the electrolysis of water acidified with sulphuric and chromic acids. (2) It is also formed by the silent discharge of electricity through air or oxygen. For this purpose a Siemens' ozonizing tube is employed. (3) When phosphorus is allowed to slowly oxidize in the air. This is best accomplished by standing two or three sticks of phosphorus on end in a dish containing a shallow layer of water, so that they are about one-half immersed in the liquid. A stoppered bell-jar is then placed over the whole, and very soon ozone may be detected by the appropriate tests.

It is difficult to obtain a definite volume of ozone, since it is always mixed with air or oxygen, but it may be accomplished by passing the mixture of ozone and oxygen from a Siemens' apparatus through a tube cooled with liquid oxygen to —181°, when the ozone separates as a steel-blue liquid, boiling at -106°.

Properties.-Ozone is a colorless gas with a peculiar, irritating odor. It possesses very active oxidizing powers, attacking most organic matter, and oxidizing many of the metals, like mercury and silver. It liberates iodine from potassium iodide, and this property is turned to account in the following test: Paper is saturated with solutions of potassium iodide and starch, and then dried. This paper becomes blue in the presence of a trace of

ozone.

Of course, chlorine and the higher oxides of nitrogen must be avoided.

At a temperature of 237° ozone is converted into ordinary oxygen; this change takes place, though slowly, at much lower temperatures.

Composition. It has been shown in several ways that ozone is an allotropic form of oxygen, in which three atoms combire to form a molecule instead of two; consequently, in its formation three volumes of oxygen condense and form two volumes of ozone.

The preparations known as "ozonized water," "pyrozone,' are mainly solutions of hydrogen peroxide (see p. 174).

Uses. Various uses have been suggested for ozone, chiefly in the industrial process of bleaching. It has also been used as a disinfectant and somewhat as an oxidizing agent in industrial processes.

OXYGEN AND HYDROGEN.

These elements combine in two proportions:
Hydrogen monoxide, or Water, H2O.

History. When Cavendish, in 1766, discovered hydrogen, it opened the way for breaking down the ancient belief in the elementary nature of water. This he followed up by exploding a mixture of hydrogen and air with the electric spark, and suggesting that hydrogen and oxygen, in the proportions of two volumes of the former and one of the latter, combine to form water. This was confirmed by Lavoisier, in 1783, who decomposed water into its elements and decided that it was composed of one part, by weight, of hydrogen, and eight parts, by weight, of oxygen. Gay-Lussac and Humboldt, in 1805, confirmed these experiments and those of Cavendish.

Occurrence. In addition to the abundant natural occurrence, as we are accustomed to see it, water is very widely distributed in such a way as to escape casual observation. All plants and animals contain it in large amount, and considerable quantities exist in the atmosphere.

Formation. The simplest method of forming this compound from its elements, hydrogen and oxygen, and proving the product

to be water, is to burn a jet of hydrogen in air, and hold over the flame a cool bell-jar, which will immediately become coated with a film of moisture. When a mixture of the two gases is brought in contact with flame, a violent explosion results. If, however, they are brought together at the moment of their combination, so as to prevent explosion, the mixture will burn with a very intense heat. The apparatus by which this is accomplished is illustrated in Fig. 48, and it is known as the oxyhydrogen blowpipe. The hydrogen is introduced through the tube a and ignited at d; oxygen is then introduced through c. When the jet is directed against a piece of freshly-burned lime, we have the oxyhydrogen lamp, the incandescent lime of which gives an intense light.

Synthesis. The synthesis of water is accomplished by the use of Ure's eudiometer tube, Fig. 49. The tube, having been filled with water or mercury, is connected with an oxygen supply and a definite amount of this gas run into the closed limb; in the same manner twice as much hydrogen is introduced, the measurements being made with the columns of liquid in the two limbs at exactly the same height. A small

air-space is left in the open end of the tube to act as a cushion. The aperture is then

FIG. 48.
b

Oxyhydrogen blowpipe.

FIG. 49.

T

Ure's eudiometer.

tightly closed with the thumb, and an electric spark passed through the mixed gases, by means of the platinum wires, which are fused into the closed end. A slight flash appears on the passage of the spark, and immediately suction is felt by the thumb. On removing the latter carefully, and filling the open limb with the additional amount of water necessary, the gases in the closed limb will be found to have disappeared. It is, perhaps, more striking to explode equal volumes of the two gases, when it will be found that a quantity equal to one-half of the oxygen introduced remains, which may easily be proved to be that gas by applying a glowing taper.