What is AIR?

The air is a mixture of gases. Many years ago it was found that animals which were confined in a limited amount of air died after a short time. Later, it was ascertained that, in breathing, animals do not consume all of the air which is inspired, but only a part of it, and that part is now known as oxygen the other chief part, known as nitrogen, is of no direct use for sustaining life. Air is substantially a mixture of oxygen gas and nitrogen gas. It really contains small quantities of other gases, also varying proportions of moisture in consequence of the evaporation of water from the land and water surfaces of the earth wh ch is always going on. For any given temperature, there is a maximum amount of water vapour which a given volume of air is capable of taking up or dissolving, and under these conditions it is saturate d with moisture at that particular temperature. Instruments which have been devised for determining the amount of moisture contained in a given volume of air are termed hygrometers. A cubic metre of air at 20 degrees C. can take up 17157 grms. of water, but at o degrees C. it can only hold 487 grms., so that when cooled, the excess of water held in gaseous solution is deposited as mist, dew, rain, hail, or snow. As w ill be seen under the description of oxygen, that gas can be prepared in the laboratory by a number of methods, but it is a natural constituent of the air, the composition of which is materially the same throughout the world. Animal life could not exist if this oxygen gas were not present in the air, and the most interesting point is that the air contains such a much larger quantity of the other gas nitrogen. I11 breathingthe act of inspirationthe oxygen of the air is sucked up or absorbed in the lungs, and that reacting upon the blood forms carbon dioxide, which gas is given out in the breaththe act of respiration. The inert nitrogen which is present in the air makes the oxygen weak or dilute enough to enable this to be done, as oxygen is fatal to human life when continuously breathed in its pure state. It is the oxygen of the air that also sustains the combustion of wood, coal, and coke in firegrates. In burning, the tar bon of which coal and coke are largely composed combines or enters into chemical combination with the oxygen of the air, forming carbon dioxide, and gives out heat at the same time. Charcoal is a form of carbon, and if a piece of it be heated to redness and then placed in a globe or glass jar containing oxygen gas, it becomes much brighter in redness, much hotter, and gradually disappears, being burnt up and thereby converted into carbon dioxide by combining with the oxygen. This act of burning, or combustion as it is termed, is essentially dependent upon the presence of oxygen. If an ordinary nightlight be lighted and placed in a widemouthed stoppered buttle, and the bottle then closed, the nightlight will go on burning until the oxygen contained in the air present in the bottle has been used up, when it will go out. On the other hand, as is well known, it will go on burning in the open air until it is all consumed. The air always contains a certain quantity of carbon dioxide, and this constituent is of more importance than might appear without careful consideration, bearing in mind how essential it is to the growth and development of vegetable I fe, and how the quantity of oxygen contained in the air is maintained by way of compensation. It has been estimated that the amount of carbon dioxide given off in each twenty four hours through the lungs of an adult human being is that which results from the oxidation of between 7 and 11 ounces of carbon, and this necessitates the consumption of about i| pounds of oxygen gas mhaled; and if now we multiply this quantity by the number of the world s inhabitants, the amount of oxygen gas thus removed from the air is represented by an astounding figure. The quantity, indeed, is so large that, unless there existed some compensating process, life would in course of time become impossible on the earth. It is now known that plant life gives back to the atmosphere the oxygen which animal life removes from it. This is the more important because, whereas 100,000 parts of air ordinarily contain about 33 parts by volume of carbon dioxide, larger proportions would foul it to such an extent as to produce headache and other sickness, while the presence of from 200 to 300 parts is seriously prejudicial to health. Vegetable life absorbs carbon dioxide from the air, as also from decomposing organic matter in the soil, and, assimilating the carbon in ts tissues by a variety of chemical processes, gives out again the oxygen which is contained in it, and thus maintains the composition of the air practically constant and universal. The immensity of this process can be imagined when we reflect upon the vast extent of forestry and plant life which coers the surface of the earth, and makes us realize at the same time the great importance of carbon dioxide as 1 constituent of the air side by side with that of oxygen and nitrogen, its other chief constituents. The air of towns generally contains small proportions of ammonia, sulphur dioxide, sulphuretted hydrogen, and organic matters, whilst nitric acid is produced by lightning flashes. At times, ozone in strail proportion also forms an aerial constituent, particularly in the upper layers, whilst recent investigations have revealed the presence, as normal constituents, of a number of rare gases in minute quantities including argon, helium, neon, krypton, and xenon. The relative proportions in which these arc present are shown below: Argon degrees

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Definition of  AIR