Familiar Talks on Science: World-Building and Life; Earth, Air and Water.. Gray Elisha

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but it is now determined that they are the lowest forms of vegetable germs. Bacteria is the general or generic name for a large class of germs, many of them disease germs. By analysis of the air in different locations and in different parts of the country it has been determined that on the ocean and on the mountain tops these germs average only one to each cubic yard of air. In the streets of the average city there are 3000 of them to the cubic yard, while in other places where there is sickness, as in a hospital ward, there may be as many as 80,000 to the cubic yard. These facts go to prove what has long been well known, that the air of a city furnishes many more fruitful sources for disease than that of the country. Some forms of bacterial germs are not considered harmful, and they probably perform even a useful service in the economy of nature. Within certain limits, other things being equal, the higher one's dwelling is located above the common level the purer will be the air. This rule, however, has its limits, as the oxygen of the air is heavier than the nitrogen, so that the air at very great altitudes has not the same proportion of oxygen to nitrogen that it has at a lower level. An analysis that was made some years ago of the air on the west shore of Lake Michigan, especially that section where the bluffs are high, shows that it compares favorably with that of any other portion of the United States.

      In view of the foregoing, it is of the highest importance to the sanitary condition of any city, town, or village that it be not too compactly built. If more than a certain number of people occupy a given area, it is absolutely impossible to preserve perfect sanitary conditions. And there ought to be a State law, especially for all suburban towns, which are the homes and sleeping places for large numbers of business men who spend their days in the foul air of the city, stipulating that the houses shall be not less than a certain distance apart. Oxygen is the great purifier of the blood, and if one does not get enough of it he suffers even though he breathes no impurities. The power to resist the effects of bad air is much greater when one is awake and active than when asleep, and this is why it is more important to sleep in pure air than to be in it during our waking hours. It is best, however, to be in good air all of the time. By pure air I do not mean pure oxygen, but the right mixture of the two gases that make air. Too much of a good thing is often worse than not enough. Pure food to eat, pure water to drink, and pure air to breathe would soon be the financial ruin of a large class of doctors.

      CHAPTER VII

      AIR TEMPERATURE

      The most recent definition of heat is that it is a mode of motion; not movement of a mass of substance, but movement of its ultimate particles. It has been determined by experiment that the ability of any substance to absorb heat depends upon the number of atoms it contains, rather than its bulk or its weight.

      It has also been stated that the atmosphere at sea-level weighs about fifteen pounds to the square inch, which means that a column of air one inch square extending from sea-level upward to the extreme limit of the atmosphere weighs fifteen pounds. The density of the air decreases as we ascend. Each successive layer, as we ascend, is more and more expanded, and consequently has a less and less number of air molecules in a given space. Therefore the capacity of the air for holding heat decreases as we go higher.

      We deduce from these facts that the higher we go the colder it becomes; and this we find to be the case. Whoever has ascended a high mountain has had no difficulty in determining two things. One is that the air is very much colder than at sea-level, and the other that it is very much lighter in weight. We find it difficult, when we first reach the summit, to take enough of oxygen into our lungs to carry on the natural operations of the bodily functions. To overcome this difficulty, if we remain at this altitude for a considerable time, we shall find that our lungs have expanded, so as to make up in quantity what is lacking in quality.

      If a man lives for a long time at an altitude of 10,000 feet he will find that his lungs are so expanded that he experiences some difficulty when he comes down to sea-level. And the reverse is true with one whose lungs are adapted to the conditions we find at sea-level, when he ascends to a higher altitude. There is a constant endeavor on the part of nature to adapt both animal and vegetable life to the surroundings. While no exact formula has been established as to the rate of decrement of temperature as we ascend, we may say that it decreases about one degree in every 300 or 400 feet of ascent. There is no exact way of arriving at this, as in ascending a mountain the temperature will be more or less affected by local conditions. If we go up in a balloon we have to depend upon the barometer as a means of measuring altitude, which, owing to the varying atmospheric conditions, is not a reliable mode of measurement. It is easily understood that a cubic foot of air at sea-level will contain a great many more atoms than a cubic foot of air will at the top of a high mountain; or, to state it in another way, a cubic foot of air at sea-level will occupy much more than a cubic foot of space 10,000 feet higher up. Suppose, then, that the amount of heat held in a cubic foot of air at sea-level remained the same, as related to the number of atoms. In its ascent we shall find that at a high altitude the same number of atoms that were held at sea-level in a cubic foot have been distributed over a so much larger space that the sensible heat is greatly diminished or diluted, so to speak. It was an old notion that heat would hide itself away in fluids under a name called by scientists latent heat. This theory has been exploded, however, by modern investigation.

      If we place some substance that will inflame at a low temperature in the bottom of what is called a fire syringe (which is nothing but a cylinder bored out smoothly, with a piston head nicely fitted to it, so that it will be air-tight) and then suddenly condense the air in the syringe by shoving the plunger to the bottom, we can inflame the substance which has been placed in the bottom of the cylinder. In this operation the heat that was distributed through the whole body of air, that was contained in the cylinder before it was compressed, is now condensed into a small space. If we withdraw the plunger immediately, before the heat has been taken up by the walls of the syringe, we shall find the air of the same temperature as before the plunger was thrust down. This, however, does not take into account any heat that was generated by friction.

      Let us further illustrate the phenomenon by another experiment. If we suddenly compress a cubic foot of air at ordinary pressure into a cubic inch of space, that cubic inch will be very hot because it contains all the heat that was distributed through the entire cubic foot before the compression took place. Now let it remain compressed until the heat has radiated from it, as it soon will, and the air becomes of the same temperature as the surrounding air. What ought to happen if then we should suddenly allow this cubic inch of air to expand to its normal pressure, when it will occupy a cubic foot of space?

      Inasmuch as we allowed the heat to escape from it when in the condensed form, when it expands it will be very cold, because the heat of the cubic inch, now reduced to the normal temperature of the surrounding air, is distributed over a cubic foot of space.

      This is precisely what takes place when heated air at the surface of the earth (which is condensed to a certain extent) rises to the higher regions of the atmosphere. There is a gradual expansion as it ascends, and consequently a gradual cooling, because a given amount of heat is being constantly distributed over a greater amount of space. At an altitude of forty-five miles it will have expanded about 25,000 times, which will bring the temperature down to between 200 and 300 degrees below zero.

      When we get beyond the limits of the atmosphere we get into the region of absolute cold, because heat is atomic motion, and there can be no atomic motion where there are no atoms.

      We have now traced the atmosphere up to the point where it shades off into the ether that is supposed to fill all interplanetary space. As Dryden says:

      There fields of light and liquid ether flow,

      Purg'd from the pond'rous dregs of earth below.

      By interplanetary space we mean all space between the planets not occupied by sensible material. It is the same as interatomic space, or the space between atoms, except in degree, as the same substance that fills interplanetary space also fills interatomic space, so that all the atoms of matter float in it and are held together from flying off

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