ATMOSPHERE 83 also made apparent by removing the air from the interior of any tube, the lo^r end of which is immersed in water or any o'ther fluid. This fluid will be pressed up the tube to a height corresponding to the pressure upon its surface. If this be at the level of the sea, water will rise 33 feet and mercury 29 inches. The common suction pump is but such a tube, furnished merely with a piston for lifting out the air, and then the water follows it. The power required is of course equal to the weight of the column of water to be lifted. The pressure of the air is also well illustrated by the common leather toy "sucker" a disk of soft leather, with a string knotted at one end passed through its centre. When moistened and applied to any smooth surface, care being taken to expel the intervening air, it is attracted to it by the external pressure. By the same principle the patella or limpet, and some other shell fish, hold fast upon the smooth rock. So great is this pressure, that the force exerted upon the body of a moderate-sized man must be about 15 tons sufficient to crush him, as it inevitably would, if applied to only a portion of the body, but quite harmless when pressing with perfect elasticity everywhere alike, from the external parts inwardly, and from those within outward. Let the pressure be taken off from any portion, as by the cupping instrument, and one is im- mediately sensible of the power that is exerted npon the parts around, painfully pressing them into the vacant space of the instrument ; or if taken from the whole body, as is the case with an aeronaut in a balloon at great height, the result may by the expansion of internal organs prove fatal. Inversely, a great increase of atmospheric pressure may be equally inju- rious and even fatal, as experienced by divers at great depth under water, or by the work- men engaged in labor in the caissons now em- ployed in forming a foundation for subaqueous structures. 2. Physical properties. The most important physical property of the atmosphere is its expansion by heat and contraction by cold. The amount of this expansion or contraction is f^g of its bulk at 32 F. for every degree of temperature above or below that point. At very low degrees of temperature, however, this law does not hold, and cannot do so, as is evident from the fact that if it were absolute the air when cooling to 492 below 32, that is, at 460 F., would be condensed to nothing. The latter temperature has for this reason been accepted by C16ment and Desormes as that of absolute cold, while according to Pouillet the temperature of the outermost limits of our at- mosphere is equal to that of the interplan- etary space beyond, being about 230 below zero. The expansion of air by heat is easily exemplified by heating air confined in a blad- der. Its expansion soon swells the bladder and ranges it to burst. As its bulk increases, its density diminishes. The colder and heavier air around it lifts it up. On this principle were constructed the first balloons. It is this prin- ciple also that gives rise to the currents of air or wind, the colder air flowing along the surface to fill the spaces left by the ascending warm air. Thus the trade winds blow from the temperate regions toward the torrid equato- rial belt. The whirling tornado, and all the phenomena of the winds, owe their origin to local heating and rarefaction of the atmosphere. The rays of the sun pass through the upper strata of the atmosphere, imparting to them little heat. This the air receives chiefly near the surface. As we ascend, the temperature diminishes one degree for every 300 or 400 ft. Near the equator perpetual snow covers the mountains at the height of 15,207 ft. ; in lat. 60 it is found at 3,818 ft., and in 75 at 1,016 ft. The main cause of this is not that the solar rays possess less heat in the higher regions, as the contrary has been proved, but that the portions of the earth's crust projecting far up into the atmosphere, as is the case with high mountains, possess less of the interior heat of the earth, being more subject to cooling by radiation, which has caused their temperature to descend to such a very low degree, that even a inidday tropical sun cannot raise it to 32 F. Another physical property of the atmosphere is its refraction and reflection of light. If the sun's rays did not illuminate the mass of the atmosphere, it would be of a black color ; but a partial refraction of the most refrangible rays takes place, and this gives the blue color to the sky, while that of the clouds comes from the reflection of the light upon the particles of vapor floating in the atmosphere. This blue color is too faint to be perceived in any small quantity of air ; it is only the great depth of the atmosphere that makes it visible, as the color of the ocean is only apparent when the waters are seen in mass. 3. Chemical properties. The atmosphere consists chiefly of a mixture of three gases, oxygen, nitrogen, and carbonic acid, with a very variable quantity of watery vapor. The normal quantities are by weight 23'2 per cent, oxygen, 76'7 nitrogen, and about O'l carbonic acid, while the watery vapor varies from almost utter absence to saturation or more than 80 per cent., according to locality, climate, season, and other circumstances. To this must he added the fact that the atmos- pheric oxygen is found in two different condi- tions according to circumstances, one being the neutral state or ordinary oxygen, the other its active condition, when it is called ozone. This differs from ordinary oxygen, first, by being more condensed so as to be one half heavier, 100 cubic inches of ordinary oxygen weighing 32 grains, while the same bulk of ozone has a weight of 48 grains ; secondly, by i causing many chemical reactions which ordi- nary oxygen is incapable of producing. It is also a most powerful disinfectant, one part of ozone purifying 3,000,000 parts of putrid air, by burning up as it were the miasmatic exhala- tions. In the arts it has already been applied as a bleaching and purifying agent. Its great
