Gases are for the most part transparent and colourless, with a few exceptions, and of course, like the air of the atmosphere, invisible. They are little affected by the attraction of cohesion, but rather, on the contrary, the particles composing them have a constant tendency to separate from each other, so that their force of expansion is only limited by the pressure under which they may be kept, and the temperature they may be exposed to. They have a tendency to penetrate each other, as it were; for instance, if you take a jar of heavy gas, such as carbonic gas, set it with its mouth upwards, then invert over it another jar containing hydrogen, a gas nearly twenty-two times lighter; in a very short time the two gases will have become thoroughly mixed, the heavy carbonic acid having risen, and the light hydrogen fallen, until the gases are thoroughly mixed, each jar containing an equal quantity of each gas.
OXYGEN GAS.
This gas, so named from two Greek words signifying the maker of acid, was discovered by Dr. Priestly in 1774. He obtained it by heating the red oxide of mercury in a glass retort, when the gas escaped in considerable quantities. In the ensuing year Scheele obtained it by a variety of methods, and a few years afterwards Lavoisier discovered that it was contained in atmospheric air, where it exists in the proportion of about one-fifth, the remaining four-fifths being almost entirely nitrogen.
Oxygen gas may be obtained for the purpose of experiment, by heating to redness the black oxide of manganese in an iron bottle, to the mouth of which a flexible tube is attached to convey away the gas as fast as it is liberated from the manganese. The first portions should be allowed to escape, being mixed with the air in the tubes and bottle, and the remainder may be collected in a gasometer, or in glass jars inverted over water.
Another method to obtain the gas, and one to be used only in the absence of other ingredients, is to mix in a retort some of this same oxide of manganese with about half its weight of strong sulphuric acid, and apply heat to the retort, when the gas will come over in considerable quantities; the first portions must be allowed to escape as before.[6] If the gas is required very pure, a small quantity of the salt called chlorate of potassa may be heated in a retort, and oxygen gas will be evolved, and may be collected as before. If you have an iron bottle, the first mode is by far the cheapest, as the heat of a bright fire is sufficient for the operation, and a large quantity of gas is obtained in a short time from a very inexpensive material. The most rapid and convenient process of all is to heat a mixture of two parts chlorate of potash, and one of powdered black oxide of manganese, in a common clean oil flask, to which a cork and bent tube has been adapted. Care must be taken not to mistake sulphide of antimony for black oxide of manganese, as very serious accidents have arisen from this cause.
[6] Some boiling water should be added to the mass left in the retort directly the gas has ceased to come away, or it will adhere to the glass so firmly, that the retort will certainly be spoilt.
Oxygen is largely distributed over our globe, both in its uncombined state, and in union with other substances. Besides forming one-fifth of the atmosphere, it forms eight-ninths by weight of all the water in the ocean, rivers, and springs on the face of the whole earth. It also, in combination with various metals, forms the various earths and minerals of which the crust of the earth consists, so that it is the most abundant and widely distributed substance in nature, and in combination with other elements, forms nearly half the weight of the solid earth.
In its uncombined state it is a colourless gas, somewhat heavier than atmospheric air, without taste or smell. It is a powerful supporter of combustion, and is absolutely necessary for the support of animal life, which cannot exist for any time without a free supply of this gas, which is constantly consumed in the act of breathing, and is replaced by an equivalent portion of carbonic acid gas. The want of oxygen is partly the cause of the oppression felt in crowded rooms, where the air cannot be renewed so fast as is required for the number of persons who are constantly consuming the oxygen; and if an animal be confined under a glass jar inverted over water, it will presently die, just for the same reason that burning tapers are extinguished under similar circumstances.
If a jet of this gas be thrown upon a piece of charcoal, sulphur, or almost any combustible body in a state of ignition, it will make it burn with great vividness and rapidity. For a complete series of experiments with oxygen see “The Boy’s Play-book of Science.”