Dissipation

“ Every known physical fact leads to the conclusion that diffusion, or a homogeneous distribution, of energy is the general aim of all happenings. No change whatever seems to have occurred, and probably none ever will occur, resulting in a concentration greater than the corresponding dissipation of energy. A partial concentration may be brought about in a system, but only at the expense of a greater dissipation, and the sum total is always an increase in dissipation. ”

“ But it is far more probable that the secular rate of dissipation has been in some direct proportion to the total amount of energy in store at any time after the commencement of the present order of things, and has been therefore very slowly diminishing from age to age. ”

“ This shows that the principle of the dissipation of energy has control over the actions of those agents only whose faculties are too gross to enable them to grapple individually with the minute portions of matter which are the seat of energy. ”

“ THE dissipation of energy is a continuous process, quite familiar to mankind in its main features and results, since the days of the ancient philosophers. It was recognized by them that all mechanical motions, being dissipated by friction, gradually diminish, and must finally cease unless maintained by external power. In the language of modern science, the motion which thus disappears is converted from molar into molecular motion. ”

“ On the one hand, we know by experiment, that oily and bituminous substances can be melted and partly changed into vapour by heat, and that they become harder and denser, in proportion as the more volatile parts have evaporated from them. On the other hand, coaly substances are destitute of fusibility and volatility, in proportion as they have been exposed to greater degrees of heat, and to other circumstances favourable to the dissipation of their more volatile and fluid parts. ”

“ Conditions for Saturation.—If a liquid is evaporating into a vacuum, the molecules on leaving find no opposition until they reach the limits of the vessel containing the vacuum. Evaporation under these conditions goes on with great rapidity and the space becomes saturated almost instantly. If, however, air be present at ordinary pressure, many of the ordinary water vapor molecules on leaving are struck and returned to the water by the air molecules directly above. Those escaping gradually work their way upward through the air. ”

“ The effects of this powerful agent begin to be distinctly felt at the pressure of about five atmospheres. Perhaps they might be noticed at a lower pressure, and I am inclined to attribute to this cause the unfavorable symptoms presented by workmen who have spent several months in compressed air; but this is a complex problem. In any case, if the necessities of industry subject men to pressures higher than six atmospheres, they will be in danger not only at the instant of decompression, but even from the effects of the compression. ”

“ If, however, coincidently with the application of pressure, opportunity be given for the excess of heat to be dissipated to a colder surrounding medium, the molecules, giving off their excess of energy, become relatively quiescent, and at a certain stage the gas becomes a liquid. ”

“ When we say that when two different gases mix by diffusion, as we have supposed, the energy of the whole remains constant, and the entropy receives a certain increase, we mean that the gases could be separated and brought to the same volume and temperature which they had at first by means of certain changes in external bodies, for example, by the passage of a certain amount of heat from a warmer to a colder body. ”

“ But the diffusing molecules carry with them in their free paths the momentum and the energy which they happen at a given instant to have. The diffusion of momentum tends to equalise the apparent motion of different parts of the medium, and constitutes the phenomenon called the internal friction or viscosity of gases. The diffusion of energy tends to equalise the temperature of different parts of the medium, and constitutes the phenomenon of the conduction of heat in gases. ”

“ Before entering on the details of ventilation, we must consider the physical causes at work which tend to purify the air, apart from all artificial contrivances. These are three in number—namely, diffusion, winds, and differences of temperature of masses of air.
(1) Diffusion causes the rapid mixture of gases placed together. Every gas diffuses at a certain rate—namely, inversely as the square root of its density. In any room which is not air-tight, diffusion is constantly occurring, air passing in and out at every possible point. ”

“ Also, if we regard the total quantities of the gases and , and also the pressure, as constant, the quantities of these gases which appear uncombined in a phase of dissipated energy, will increase with the temperature, if the formation of the compound under constant pressure is attended with evolution of heat. ”

“ The decomposition of bodies by heat is, moreover, assimilated to the phenomenon of volatilization: the rate of decomposition at a given temperature varying with the pressure, and with the nature of the atmosphere which surrounds the unstable body. ”

“ I have repeated this experiment often. The elements of the water dissociate by the heat, and instantaneously recombine in cooling, causing the detonation."
The degree of dissociation, as of evaporation, depends on the pressure: under four atmospheres (60 lbs.) , the heat required for dissociation is 1,870° . But the question arises, If the vapor dissociates at these temperatures, why do not the gases instantaneously combine and detonate? The reason is plain: in the boiler the atoms at once become mixed with a volume of steam sufficient to render them inexplosive. ”

“ Heat.—The most familiar of the active agencies that resist condensation is heat. Upon this the existing volume of the earth is immediately dependent, in some large part at least. As this heat is dissipated, the earth shrinks. This shrinkage increases the force of gravity, and hence the internal pressure increases, and, if further compression takes place as the result of this increased pressure, additional heat is developed, which checks further condensation until it is dissipated. ”

“ We may assume that in the lower regions of the solar atmosphere the free heat evolved by mechanical compression will be more than sufficient to dissociate the whole of the aqueous vapor, and thus the dissociated gases will be left at a higher temperature than was necessary to effect their dissociation. ”

“ To determine the effect of a change of temperature without change of pressure upon the general features of the surface of dissipated energy, we must know whether heat is absorbed or yielded by a mass in passing from the phase represented by the point in the primitive surface to the composite state consisting of the phases and which is represented by the same point. ”

“ Now, though we are accustomed to see water-atoms thus stirred into an activity which gives us the giant steam as servant, it has probably fallen to the lot of but few of us to encounter certain gaseous substances so utterly deprived of their self-assertiveness [Pg 214] as to collapse into a liquid mass, in which shape they are quite strangers to us. What gaseous body do we know better than the air we breathe? and what should we less expect to be reducible to the consistency of water? Yet science has lately brought prominently into notice that strange child of pressure and cold, Liquid Air ”

“ Hence instead of concentration we have dispersion.
The united action of the winds and the unity of the atmosphere result from this dispersion. The displacement of one molecule produces the displacement of another. The vast body of air becomes subject to one agitation. To these profound causes of coalition we must add the irregular surface of the earth, whose mountains furrow the atmosphere, contorting and diverting the winds from their course, and determining the directions of counter currents in infinite radiations. ”

“ If the vessel was so large that in different parts of it the temperature and pressure were different, the diffusion would, indeed, still be perfect, because at all ordinary temperatures and pressures hydrogen and carbonic acid gas remain gaseous. But if the vapour introduced is of such a nature that at moderate temperatures and pressures it condenses, wholly or in part, or liquefies, the diffusion will not take place with the same uniformity. We need not go further for illustration than to the case of our own atmosphere as it actually exists. ”

“ Pressure facilitates evaporation, and on more closely examining this reaction we arrive at the conclusion that vapour can never spontaneously condense itself when liquid drops already formed are not present, unless forces of another nature intervene to diminish the effect of the capillary forces. In the most frequent cases, these forces come from the dust which is always in suspension in the air, or which exists in any recipient. ”

“ If a gas through which some other substance is diffused in a gaseous state, expands in consequence of the removal of pressure, it will, when the rarefaction and consequent cooling reach a certain point, begin to let fall the suspended substance. Conversely, if, a gas, saturated even with some substance, is subject to increased pressure, and is allowed to retain the additional heat which that pressure generates; so far from letting fall what it contains, it will gain the power to take up more. See then, the inference respecting condensation in a nebulous spheroid. ”

“ The surface of dissipated energy has an important application to a certain class of problems which refer to the results which are theoretically possible with a given body or system of bodies in a given initial condition. ”

“ As the latter strikes outwards, the air in front of it will be driven outwards, condensed; and, on account of the elasticity of the air, the condensation will at once start to travel outwards in every direction,—a wave of denser air ”

“ What does science tell us of the drop of water? Two gases, the one exciting life and quickening combustion, the other a highly inflammable air, are, by the influence of a combination of powers, brought into a liquid globe. We can, from this crystal sphere, evoke heat, light, electricity, and actinism in enormous quantities ”

“ The foamy appearance of water, when the pressure of the air over it is diminished, is owing to the expansion and escape of the air previously dissolved by it, or existing in its pores. When a child first sucks it only presses or champs the teat, as observed by the great Harvey, but afterwards it learns to make an incipient vacuum in its mouth, and acts by removing the pressure of the atmosphere from the nipple, like a pump. ”

“ And as the force of expansion in dense air when heated, is in proportion to its density, this central air might afford another agent to move the surface, as well as be of use in keeping alive the central fires; though, as you observe, the sudden rarefaction of water, coming into contact, without those fires, may be an agent sufficiently strong for that purpose, when acting between the incumbent earth and the fluid on which it rests. ”

“ Air Saturation.—Gasoline, confined in a vessel, will vaporize up to a point where it completely saturates the air contained therein, and then ceases. If allowed to stand in the open air, it will, in time, entirely evaporate. ”

“ The physical properties indicated by the nature of the surface of dissipated energy have been only occasionally mentioned, as they are often far more distinctly indicated by the diagrams than they could be in words. ”

“ Because the diffusion of air which, surrounding him in every direction, and acting upon the internal as well as the external surfaces of his body, and probably surrounding every atom of his frame, establishes an equilibrium, in which every degree of pressure counteracts and sustains itself. ”

“ Whenever the elasticity of the vapour, corresponding to the temperature of the water, is greater than the atmospheric pressure, the evaporation will take place not only from its surface, but from every point in its interior; the liquid particles throughout the mass assuming the gaseous form, as rapidly as they are actuated by the caloric, which subverts the hydrostatic equilibrium among them, to constitute the phenomena of ebullition. ”

“ Again, if too much moisture enters the channels of a body, and thus introduces disproportion, the other elements, adulterated by the liquid, are impaired, and the virtues of the mixture dissolved. This defect, in turn, may arise from the cooling properties of moist winds and breezes blowing upon the body. In the same way, increase or diminution of the proportion of air or of the earthy which is natural to the body may enfeeble the other elements; the predominance of the earthy being due to overmuch food, that of air to a heavy atmosphere. ”

“ We thus arrive at this general law, which is applicable to all the substances of nature, solid, liquid, or gaseous: If the pressure supported by different bodies, taken at the same temperature, be augmented by a small quantity, quantities of heat will be disengaged from it, which will be proportional to their dilatability by heat. ”

“ A thousand detonations can produce no more effect upon the air, or upon the watery vapor in it, than a thousand rebounds of a small boy's rubber ball would produce upon a stonewall. So far as the compression of the air could produce even a momentary effect, it would be to prevent rather than to cause condensation of its vapor, because it is productive of heat, which produces evaporation, not condensation. ”

“ If we change the vibration of a thing, we practically change the manifested nature of that thing. The difference between solid ice, liquid water, semi-gaseous vapor, and gaseous steam is simply the difference caused by various rates of vibration caused by heat. ”

“ The drier the air, and the hotter the air, the greater is the amount of cloud which can be thus dissolved in it. When the cloud first forms, its quantity is far greater than the air is able to maintain in an invisible state. But, as the cloud mixes gradually with a larger mass of air, it is more and more dissolved, and finally passes altogether from the condition of a finely-divided liquid into that of transparent vapour or gas. ”