PNEUMATICS 247
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ol .a.um. Ddens- in air- pups. Tsrmal pper- ti, ketic tbry, eieri- pressures lower than the fortieth of a millimetre. If, however, there is placed somewhere in the duct leading from the mercury pump to the receiver a non-volatile sub stance which absorbs mercury vapour, the mercury vapour will be arrested. With similar precautions for the absorp tion of water vapour or other vapour which may be present, it is possible to obtain very low pressures indeed. Thus Crookes 1 in his radiometer experiments obtained pressures as low as 00015 mm. or the 2 millionth of an atmosphere. The form of pump used was of Geissler s type. The water vapour was absorbed by phosphoric anhydride. Sulphur was used to absorb the mercury vapour ; and on the further side metallic copper was placed to absorb any sulphur vapour which might tend to pass. The best test of a good vacuum is the electrical test. Disruptive discharge through a long tube filled with gas is possible only when the gas is very rare, but there is a point in the rarefaction of a gas at which the discharge passes most easily. In other words, the dielectric strength of a gas reaches a minimum as its pressure is diminished, and it is possible to obtain such a high vacuum that electric discharge will not take place through any considerable length of the exhausted space (see ELECTRICITY). In this con nexion we may notice a simple but instructive experiment of Dewar s. He carefully exhausted a vacuum tube, in which he had previously inserted a piece of carbon. In- the last stages of the exhaustion the carbon was strongly heated so as to drive off most of the gas which is always condensed on its surface. After the completion of the exhaustion, the tube was sealed up and the carbon allowed to cool. As it cooled, it condensed over its surface the greater portion of the small quantity of gas left in the vacuum tube. The vacuum was thus vastly improved, so that it was im possible to pass an electric spark between two platinum electrodes which had been previously fused into the glass. A gentle heat applied to the carbon, however, was sufficient to drive off from its surface enough of the occluded gas to raise the pressure to tha point necessary for the passage of the electricity. As the carbon cooled again, the high vacuum was restored and the discharge ceased. It is hardly necessary to do more than mention the other class of air-pumps. Let the essential valves in an exhaust ing pump have their actions reversed, and the apparatus will become a condensing pump. The condensing syringe is the usual form of such a pump ; but, compared to the exhausting instrument, it has very limited applications. For the mere obtaining of high pressures hydraulic means are preferable, being at once more manageable and less dangerous. Besides, even moderately slow compression of a gas is accompanied by considerable rise of tempera ture, just as rarefaction of a gas is accompanied by appreciable lowering of temperature. In the former case work is done in compressing the gas against its own pres sure, and this energy appears as heat which raises the temperature of the gas. In the latter case the gas in expanding draws upon its own energy and so cools. The thermal properties of gases are treated as a branch of mathematical physics under HEAT and THERMO DYNAMICS, where also will be found discussed such experi mental details as bear intimately upon the theory. The article ATOM contains a concise statement of the modern kinetic theory of gases ; and in CONSTITUTION OF BODIES and DIFFUSION certain more special aspects of the same theory are regarded. For the mathematical theory see MOLECULE. The principles embodied in these articles have bei/n assumed hroughout the present article. The cooling of a gas by its own expansion may be observed in one of its effects during exhaustion in an ordinary receiver. Frequently a cloud of minute drops of water a veritable fog forms in the exhausted air. The reason simply is that the air has become cooled below its dew-point, or the temperature corresponding to the pres sure of water vapour present. If the receiver is first filled with dry air and then exhausted, no cloud forms. Also if the air is carefully filtered through cotton wool, no cloud 1 See his Bakerian Lecture, Phil. Trans., 1878; clxix. 300. forms, even though the air be thoroughly saturated with water vapour. This latter fact was established by Mr Aitken in his beautiful series of experiments on dust, fogs, and clouds. 2 It thus appears that the formation of fog and cloud depends not only upon the humidity of the air but also upon the amount of dust in the air. The little particles of dust are necessary as nuclei upon which the vapour can begin to condense. The more numerous the dust particles the finer are the drops which form on them. As these coalesce into larger drops and fall, they bring down the dust nuclei with them ; and hence the tendency of rain is to clear the atmosphere, and make conditions less favourable for the formation of more cloud and rain. Thus rain, fog, and dew all require for their formation a free solid surface, colder than the temperature of saturation, on which to condense. In a dustless atmosphere no cloud can ever form. It has been already pointed out that when a fluid is in motion it Fluid can no longer be regarded as even approximately possessing the pro- friction, perties of the ideal perfect fluid. The postulate that the stress be tween contiguous portions is perpendicular to the common interface cannot be for a moment admitted. A few examples will make this clear. Thus, if a vessel filled with a liquid be set in rotation, the liquid will soon be found to be rotating with the vessel ; and if the motion of the vessel be stopped, the motion of the liquid inside will gradually subside. These phenomena show the existence of a stress tangential to the fluid elements, whereby the relative motion of the different parts of the system is gradually destroyed, until the vessel and its contents behave as one solid body. Again, a fluid stream flowing along a tube or canal moves fastest in the middle, slowest at the bounding walls, and with all possible intermediate speeds at intermediate places. This retardation, due in the first instance to the action of the walls upon the fluid in direct contact therewith, and then to the friction between the successive contiguous sheets of fluid, plainly implies the existence of tangential stress. The action of the wind in causing waves on the smooth surface of a sheet of water is a further illustration. In the discussion of fluid motion, however, it is customary to consider first the properties of the ideal fluid in this respect. Under HYDROMECHANICS, and especially under ATOM, will be found the mathematical theory treated in full so far as the motion of a non-viscous fluid is concerned. The possibility of setting up vortex-motion in a fluid depends General upon its imperfection, upon the property of viscosity or fluid effects of friction which is possessed by all known fluids. Some of the more viscosity, obvious effects of this property have been already noticed. Its effects indeed are conspicuous wherever there is relative motion of the contiguous parts of a fluid. A current of air moving through a mass of air at rest soon loses its momentum ; a solid body moving through still air likewise has its motion retarded. The loss of energy which a meteoric stone so suffers as it speeds through the atmosphere appears in the form of heat, which is suffi cient to make the stone glow to incandescence or to the temperature of rapid combustion. The waves of rarefaction and condensation, which constitute sound physically, gradually decay in virtue of viscosity. It appears that the rate of this decay is quickest for the shortest waves ; so that a sound after travelling through a long distance may lose its shriller constituents and so be modified and mellowed. It is viscosity also which supports the minute dust particles and cloud-forming drops of water in our atmo sphere. These are bulk for bulk heavier than the atmosphere, and in tranquil air are slowly sinking. The slowness of their down ward motion is directly due to the effect of fluid-friction. The term fluid-friction is applied because of the similarity of Fluid its effects to the effects of friction between solid masses. Other- friction wise there need be no further resemblance. The true nature of depends friction between solid surfaces is not known ; possibly, as suggested on cliffu- by Sir W. Thomson, it may be in great measure electrical. In the sion. case of gases, however, the origin of friction is more apparent. Its laws can be deduced from the kinetic theory, and depend directly upon the principle of diffusion. According to the kinetic theory of gases, the molecules of a gas are in constant motion amongst themselves. Compared to their own dimensions, they are by no means closely packed, so that any individual molecule travels a comparatively large space between its encounters with other molecules. Any two contiguous regions are continually inter changing molecules. This diffusion of a gas into itself we have no means of measuring experimentally, as we cannot deal with the individual molecules. Suppose, however, that we have two con tiguous layers of a gas flowing in parallel directions with different speeds. The general drift of molecules in the one layer is faster
2 Trans. Roy. Soc. Edin., 1880-81.