ATMOSPHERE (Greek, *vaporous sphere"), in ordinary usage, the gaseous en velope that surrounds the earth. The atmos phere consists chiefly of the gases, oxygen and nitrogen, not chemically combined, but mechani cally mixed in the proportion of about 21 vol umes of oxygen to 79 of nitrogen. It also contains small quantities of carbon dioxide, or ganic matter, water vapor, argon and other substances. (For a more precise statement of its composition, see Ant). At the surface of the earth it has a density of about 1/800th of that of water, though this varies somewhat with the height above the sea-level at which the determi nation is made, and with the temperature and barometric pressure prevailing at the time. The presence of free nitrogen in the atmosphere may be attributed, probably, to the comparative inertness of that gas, so far as any tendency to form chemical compounds is concerned. The presence of free oxygen cannot be explained in this manner, however, because oxygen is one of the most active chemical substances known. It appears more probable that oxygen is present in the free state simply on account of the im mense quantity of that element that the earth contains. In past geological times, it com bined with practically all of the oxidizable min erals that were near enough to the surface of the earth to be accessible to it, and the present supply of free oxygen in the atmosphere must be regarded as merely the excess of that element that remained unused, after all the possible oxidations had been effected. According to this view, the earth (at least in its more super ficial parts) is a gigantic, burned-out cinder ; and this accords with the estimates that chem ists and geologists make, that nearly one-half of the weight of the earth's crust consists of oxygen. It is likely that in past ages, and particularly in the carboniferous period when the vegetation that gave rise to our modern coal fields was flourishing, the quantity of carbon dioxide present in the atmosphere was consider ably greater than at the present time. Part of this gas was absorbed by plants, its carbon being stored in the coal beds and its oxygen returned to the air ; but it is likely that by far the greater portion combined with lime and other similar earths to produce the present vast deposits of limestone and other carbonated min erals and rocks. At the present day, carbon dioxide is being absorbed from the atmosphere by plants, and returned to it again by animals, and by factories in which coal is burned. We have no means of knowing whether the balance is being preserved, so far as this constituent of the atmosphere is concerned, or not, because the mass of the entire atmosphere is too vast for the composition to be sensibly changed by these causes, since the time when exact analyses became possible.
Galileo observed that water cannot be drawn up by a suction pump, or other equivalent device, to a greater distance than about 34 feet. He
did not succeed in explaining the existence of this limiting height, but his friend and amanu ensis, Torricelli, who succeeded him as profes sor at Florence, afterward made the shrewd guess that water rises in such a pump for the reason that the atmosphere exerts a certain pres sure upon all terrestrial objects, and that when a portion of this pressure is removed from the water in the suction tube of the pump, it is the pressure of the atmosphere upon the water external to the pump that causes the water in the pump-tube to rise; and he saw that if that were the case, it would follow that a pump could only draw' water up to the particular height at which the pressure due to the water column so edrawn upp would precisely balance that of the atmosphere. The limit of 34 feet corresponds (as is easily shown by a simple calculation) to a pressure of about 15 pounds to the square inch; and hence Torricelli inferred that the atmosphere exerts a pressure of that amount upon all objects. Mediating upon this hypothesis, it occurred to him that if his explan ation were indeed correct, the atmosphere would be able to raise mercury (which is about 14 times as heavy as water) to only about one fourteenth of the height to which it can raise water. He accordingly (in 1643) procured a glass tube some 35 inches long, and closed at one end. Placing it with the open end up ward, he filled it with mercury. He then cov ered the open end to prevent the escape of the mercury and inverted the tube se that its mouth dipped into a basin also filled with mercury. Upon uncovering the open end of the tube, he was gratified to see that the mercury in the tube at once sank until its upper surface stood at about 30 inches above that in the basin. This experiment proved that the atmosphere exerts a pressure equal to that due to a column of mer cury 30 inches high; or, in other words, equal to about 14.7 pounds per square inch. Additional proofs were soon given, also. Thus Pascal sug gested that if the explanation were true, the pressure ought to be less at the top of a moun tain, than in a lower place; because the moun tain projects up into the atmosphere so far that there is a sensibly smaller height of air above it than there is above a point in a valley. The experiment was actually carried out by M. P.errier, who carried an apparatus like Tor ricelli's (now known as a to the summit of a mountain in Auvergne called the Puv de Dome, and found at the top of this mountain (which is 4,800 feet high) the atmos phere could sustain only about 27 inches of mercury, although after returning to the plains below, the full height of 30 inches was again observed. Shortly afterward (in 1650) the air-pump was invented by Guericke, and the pressure of the atmosphere was demonstrated beyond any doubt whatever, by numerous direct experiments.