CHAPTER 5
THE ATMOSPHERE OF THE EARTH
50.The Origin of the Atmosphere. -As the earth cooled down from the intensely hot condition which it is supposed to have had at first, the substances which had not chemically combined and formed liquids and solids, or which required a low temperature for their con-BLUE HILL OBSERVATORY, MILTON, MASS.
One of the first places in America where conditions of the upper atmosphere were studied.
solidation were left still in the gaseous state around the solid core. This gaseous envelope composed of these substances surrounding the earth we call the atmosphere. Some of these gases are inert and do not readily combine with other substances. Others have formed extensive combinations, but they exist in such large quantities that they were not thereby exhausted.
51.The Composition of the Air.
Experiment 57. -(To be performed only by the teacher.) Having rounded out a cavity in a small flat cork, cover the cavity and surface around it with a thin layer of plaster of Paris. After the plaster hasset and become thoroughly dry float the cork on adish of water with the cavity side up. Place a piece of phosphorus as large as a pea in the cavity and carefully light it. (Great care must be taken in handling phosphorus as it ignites at a low temperature and burns with great fierceness. It must always be cut andhandled under water.)
Fig. 47.
As soon as the phosphorus is lighted, cover it with a wide-mouthed bottle. Be sure that the mouth of the bottle is kept slightly under water. The water will be found to rise in the bottle. The phosphorus soon ceases to burn. White fumes are formed, but these soon clear up. A clear gas is left in the bottle, but this cannot be air, for if it were, the phosphorus would have continued to burn in it, since it burns in air. If it were not for this property of not permitting phosphorus to burn, the gas left in the bottle could not be distinguished by ordinary means from air.
The gas fills more than three fourths of the bottle, so that more than three fourths of the air is composed of a gas which does not support combustion. This gas is called nitrogen. The other constituents of the air must also be transparent colorless gases, since the air is transparent and colorless. The most important of these is called oxygen. The phosphorus united with this and formed the white fumes. These fumes dissolved in the water, leaving the nitrogen.
Be careful to put the cork on which the phosphorus was burned in a place where it cannot cause a fire.
Although the air appears like a simple gas and was so considered until the end of the eighteenth century it has been shown to be composed of several different colorless gases. One of these, oxygen, supports combustion; another, nitrogen, neither burns nor supports combustion. Chemists have found that these two gases are mixed in the air in the proportion of about one part of oxygen to four parts of nitrogen.
Another heavy colorless gas called carbon dioxide is found in the air in the proportion of about 3 parts to 10,000. There are in addition very small quantities of several other gases, but these are not of sufficient importance to be studied here. Besides the gases, the air contains other matter, such as water vapor, dust particles and microbes.
Experiment 58. -Obtain four bottles of oxygen from the chemical laboratory. If not obtainable, place a piece of sodium peroxide (oxone) about as large as the end of a finger in a side necked test tube provided with a medicine dropper filled with water, as shown in Fig. 48. Putthe end of the delivery tube under the mouth of an invertedbottle filled with water arranged on the shelf of a pneumatic trough. Drop water slowly on to the sodium peroxide andFig. 48.
Fig. 49.
collect the gas generated. Fill several bottles. Oxygen can also be prepared by heating a mixture of about one part manganese dioxide and two parts potassium chlorate in a test tube and collecting the gas over water (Fig. 49). Does the appearance of this gas differ in any way from air? Smell of it. Has it any odor? Into one of the bottles of oxygen insert a splinter of wood having a spark at the end. It bursts into flame. Does the same thing take place when the stick with the spark upon it is held ina bottle of air?
Hold a lighted match at the mouth of another of the bottles containing oxygen. Does the gas itself burn as illuminating gas does when a match is applied to it? If the oxygen in the air were increased or decreased, it would have a great effect upon combustion. Attach apiece of sulphur to a short piece of picture wire. IgniteFig. 50
it and place the wire in a bottle of oxygen (Fig. 50). Does the sulphur burn strongly? How about the wire? Does it burn too?
The oxygen is the most important part of the air to animals, for without it they could not live. They breathe in oxygen and breathe out carbon dioxide. All their heat and energy is due to the power they have of combining oxygen with carbon and forming carbon dioxide. Plants also need it.
Plants need carbon dioxide as much as animals need oxygen. By far the greater part of plants is made from the carbon which they get from this gas. The growth of a plant is due to the power it has of tearing apart the carbon dioxide by the help of the sun and building the carbon into its structure. It returns the oxygen to the air to be used again by the animals and plants.