In addition to the recently discovered tare elements contained in the air, five of the better known ones exist ordinarily in the gaseous stateviz., hydrogen, oxygen, nitrogen, chlorine, and fluorine. Gases vary greatly in their general properties, being of varying colours, densities, solubilities in different fluids, and chemical affinities. They can all be reduced to the liquid or solid state by lowering the temperature and increasing the pressure sufficiently, and the highest temperature at which a gas can be liquefied by pressure is called its “critical temperature,” while the “critical pressure” is that under which it can be liquefied at its critical temperature. Many gases are readily absorbed by charcoal, and some of them are occluded by metals. Recently heated beechwood charcoal will absorb by what is called “surface action” go times its own volume of ammonia, while that from cocoanut shell will take up 171 volumes. The gases most easily liquefied are those which are most readily absorbed by charcoal, and in this condensed form they exhibit unusually active chemic al properties. For example, powdered charcoal saturated with hydrogen sulphide when brought into contact with oxygen, bursts into combustion owing to the rapid chemical action of the two gases. All gases tend to expand and the pressure or elastic force of a gas is the collective effect of the bombardment of its freely moving molecules against the containing vessel. A list of the critical temperatures and pressures and boilingpoints of the better known gases is appended: Equal volumes of gases at the same temperature and pressure contain an equal number of molecules, and the volume occupied by a given weight of any gas is inversely as the pressure. Thu densities of the gaseous elements are, for the most part, identical with their atomic weights. The densities of compound gases are onehalf of their molecular weights. One litre of hydrogen at o degrees C. and 760 mm. mercury weighs o08936 grm., and the weights of litres of other gaseous elements are ascertained by multiplying this factor by their atomic weights. Gases expand part of their volume at o degrees C. for every increase of i degrees C. in temperature at constant pressure. Gases exhibit a peculiar property of diffusion, so that if two vessels containing, say, oxygen and hydrogen respectively, be placed with their openings in contact, each gas will mingle with the other so thoroughly and automatically, that after a time, there is uniformity of composition of the gases contained in the several vessels. It has been ascertained that the relative velocities of diffusion of any two gases are inversely as the square roots of their densities. This diffusion is readily appreciated if it be borne in mind that gaseous matter is to be regarded as an aggregation of molecules in which the attractive force which unites them is reduced to a minimum because the spaces they occupy are relatively great, and that these molecules are therefore in a constant state of rapid motion and bombardmenta state of things that is of course greatly enhanced by the application of heat. It is this bombardment that is the foundation of the pressure or elastic force exercised by gas confined in a vessel at any given temperature and pressure.
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