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the slow leakage of gas through a small aperture. Two cases of effusion are distinguished. In the first case, the diameter of the opening is small compared with the mean free path of the molecules (the pressure in the vessel is very low). In such a case, molecular effusion occurs in which collisions between molecules play no part, and the overall volume of gas escaping per unit time is , where S is the area of the aperture, μ is the molecular mass of the gas, R is the universal gas constant, T is the absolute temperature of the gas, and p1 and p2 are the gas pressures on the two sides of the aperture. The effusion method of measuring very low pressures (about 10–3–10–4 mm Hg) is based on this case.
In the second case, where the gas pressure is so high that the mean free path is smaller than the diameter of the aperture, the leakage of gas obeys the laws of hydrodynamics. The molecules escape from the aperture in the form of a jet, and the volume of gas discharged per unit time is inversely proportional to the square root of the density of the gas. This law underlies a method of determining the density of gases from the time of their discharge through small apertures (0.10–0.01 mm). However, if the pressure within the vessel is considerably greater than the external pressure, the amount of gas escaping is proportional to the pressure in the vessel.
the process of lava (magma) pouring out onto the earth’s surface. When the lava cools, effusive rocks are formed, bedded in the form of lava flows and lava sheets. Effusion, one of the manifestations of volcanic activity, is usually accompanied by an explosion, with discharges of small fragments (volcanic ash, sand, or tuff) or large chunks (volcanic bombs and slags). Viscous acidic lava sometimes will not flow, but rather is extended, forming volcanic domes.