Permeability

“ But with the next Element added to our resources, in the next Round, Permeability will become so manifest a characteristic of matter, that the densest forms of this Round will seem to man's perceptions as obstructive to him as a thick fog, and no more. ”

“ The permeability of the membrane can very readily be changed, as will be seen later. There is reason for believing, for example, that the permeability of fish to salt increases after death—for stale fish strike through more quickly than fresh fish—and that permeability increases at temperatures near the freezing point of water. ”

“ Soil best suited for sewage farms.
Whilst practically no soil is entirely useless for sewage farming, with the exception perhaps of peat, owing to the quantity of moisture it contains, a soil that combines great permeability for air with high retaining and absorbing powers—such as a loamy sand with fairly large grains—is probably the best. ”

“ As the sulfate penetrates the concrete, the concrete fails and the external boundary conditions move into the concrete. Also note that the pressure (P) is zero from 0.75 to 1.00 meters. Since the concrete was cracked, the permeability of the cracked portion of the concrete overwhelmed the uncracked portion, resulting in virtually no pressure drop across the crack. This increases the pressure gradient across the remaining untracked concrete. ”

“ Now, a drop of water no less than a grain of sand is built up of a very great number of molecules, attached to one another by the force of cohesion—a force which is much stronger in the sand than in the water, but which nevertheless exists in both. ”

“ In the following discussion, we shall suppose both the solidity and the fluidity to be perfect, so far as any properties are concerned which can affect the conditions of equilibrium,—i.e., we shall suppose that the solid matter of the body is entirely free from plasticity and that there are no passive resistances to the motion of the fluid components except such as vanish with the velocity of the motion,—leaving it to be determined by experiment how far and in what cases these suppositions are realized. ”

“ We will now consider the stability of a fluid enclosed in a rigid envelop which is non-conducting to heat and impermeable to all the components of the fluid. The fluid is supposed initially homogeneous in the sense in which we have before used the word, i.e., uniform in every respect throughout its whole extent. Let be the ultimate components of the fluid; we may then consider every body which can be formed out of the fluid to be composed of , and that in only one way. Let denote the quantities of these substances in any such body, and let , denote its energy, entropy, and volume. ”

“ We have supposed the solid to be homogeneous. But it is evident that in any case the above conditions must hold for every separate point where the solid meets the fluid. Hence, the temperature and pressure and the potentials for all the actual components of the solid must have a constant value in the solid at the surface where it meets the fluid. Now, these quantities are determined by the nature and state of the solid, and exceed in number the independent variations of which its nature and state are capable. ”

“ When, furthermore, we proceed to examine the constitutive principles essential to any being in the concrete, we may be able to distinguish between principles which are determinable, passive and persistent throughout all essential change of that being, and others which are determining, specifying, differentiating principles. In water, for example, we may distinguish the passive underlying principle which persists throughout the decomposition of water into oxygen and hydrogen, from the active specifying principle which gives that substratum its specific nature as water. ”

“ But inasmuch as the effect of the attractive influence of the earth, or the weight of the liquid, increases with the quantity we use, while on the other hand the surface-tension, or its own moulding molecular force, remains precisely the same, we should, if we use a large quantity of liquid, expect the weight to be the particular force which determined its shape; and if we employ a small mass of liquid, then the surface-tension, growing proportionately greater, would become the more important. ”

“ The forces of cohesion and elasticity have been advanced in explanation, on the strength of the fact that the solubility of a salt in water is regulated by cohesion, and that of a gas by its elasticity. ”

“ If then a solid can be made to behave like a liquid, even with such pressures as are within our control, how are we to suppose that the solids would behave with such pressures as those to which they are subjected in the interior of the earth? The fact is 162that the terms solid and liquid, at least as we understand them, appear to have no physical meaning with regard to bodies subjected to these stupendous pressures, and this must be carefully borne in mind when we are discussing the nature of the interior of the earth. ”

“ This appearance, on the contrary, is perfectly consistent with the idea, that the fluidity of these bodies had been the effect of heat; for, whether we suppose the introduction of foreign matter into the porous mass of a stratum for its consolidation, or whether we shall suppose the materials of the mass acquiring a degree of softness, by means of which, together with an immense compression, the porous body might be rendered solid; the power of heat, as the cause of fluidity and vapour, is equally proper and perfectly competent. ”

“ But it may be urged, that even if we grant that this conception of a perfect fluid, as a body which has its parts perfectly moveable among each other, leads us necessarily to the principle of the equality of hydrostatic pressure in all directions, still this conception itself is obtained from experience, or suggested by observation. And to this we may reply, that the conception of a fluid, as contemplated in mechanical theory, cannot be said to be derived from experience, except in the same manner as the conception of a solid and rigid body may be said to be acquired by experience. ”

“ Even when a body exhibits no fluid properties at its actual temperature, if its surface has been formed at a higher temperature, at which the body was fluid, and the change from the fluid to the solid state has been by insensible gradations, we may suppose that the value of coincided with the superficial tension until the body was decidedly solid, and that they will only differ so far as they may be differently affected by subsequent variations of temperature and of the stresses applied to the solid. ”

“ There is great Difference between changing the Substance, and only covering the Surface of a Body. These Petrifactions, as they are called, being no other than Precipitations of Matter too heavy to be longer sustained in the Water; and which, being very fine, adapts itself to every Prominence and Cavity of the Body it settles upon, and exactly assumes its Shape. ”

“ It will now be necessary for me to give you some account of the mechanical properties of fluids—a science which, when applied to liquids, is divided into two parts, hydrostatics and hydraulics. Hydrostatics, treats of the weight and pressure of fluids; and hydraulics, of the motion of fluids, and the effects produced by this motion. A fluid is a substance which yields to the slightest pressure. If you dip your hand into a basin of water, you are scarcely sensible of meeting with any resistance.
Emily. The attraction of cohesion is then, I suppose, less powerful in fluids, than in solids? ”

“ These are, that any body heavier than a fluid will sink to the bottom of the vessel, wherein the fluid is contained, and in the fluid will weigh as much as its own weight exceeds the weight of an equal quantity of the fluid; any body uncompressible of the same density with the fluid, will rest any where in the fluid without suffering the least change either in its place or figure from the pressure of such a fluid, but will remain as undisturbed as the parts of the fluid themselves ”

“ 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. ”

“ But, in truth, the pressure of any ambient fluid, how great soever, can be no intelligible cause of the cohesion of the solid parts of matter. For, though such a pressure may hinder the avulsion of two polished superficies, one from another, in a line perpendicular to them, as in the experiment of two polished marbles ; yet it can never in the least hinder the separation by a motion, in a line parallel to those surfaces. ”

“ This law holds good while the body remains in a gaseous state in which the maintenance of the volume is essentially dependent on heat. When a body becomes liquid or solid, its volume is dependent in part on forms of resistance other than heat, and the force of the law is abated, though the principle still holds good. ”

“ From these and other analogous results we learn that all bodies have a greater or less degree of porosity. The distance at which the particles of fluid bodies are maintained is strikingly proved by the fact, that hydrated salts dissolved in water occupy no more space than that which is equal to the water contained in the crystalline body ”

“ For this it will be necessary to consider the variations of the energy of the fluids only so far as they are immediately connected with the changes in the solid. We may suppose the solid with so much of the fluid as is in close proximity to it to be enclosed in a fixed envelop, which is impermeable to matter and to heat, and to which the solid is firmly attached wherever they meet. ”

“ As the cause of the solidity or firmness of bodies is the pressure of the air or atmosphere, so likewise it must depend on the make and size of the pores of such bodies; for bodies whose pores are smallest, must be acted upon with a greater power than those whose pores are largest, or whose pores are so large as not only to admit light, but also common air into them. ”

“ And indeed experience shews, that no other resistance, than what arises from the power of inactivity in the parts of the fluid, is of moment, when the body moves with any considerable swiftness.
11. There is besides these yet another species of resistance, found only in such fluids, as, like our air, are elastic. Elasticity belongs to no fluid known to us beside the air. ”

“ It seems probable, however, that if the fluid in contact with the solid is not renewed, the system will generally find a state of equilibrium in which the outermost portion of the solid will be in a state of isotropic stress. ”

“ The others, when highly concentrated, that is, brought to the utmost degree of strength of which they are capable, always become an invisible vapour, permanently elastic, until it comes in contact with water, or some other substance with which they are capable of uniting. ”

“ But to take another instance; let the nature in question be Consistency. A Hostile Instance is Air. For metal can be fluid and can also be consistent; and so can glass; water also can be consistent, when it is frozen; but it is impossible that air should ever be consistent, or put off its fluidity. ”

“ At exceedingly high temperatures much greater pressure, and therefore much greater density, can be attained without liquefaction or solidification. And, in considering the effect of pressure on the materials of a solid globe, we must not fall into the mistake of supposing that the strength of such solid materials can protect the material from compression and its effects. We must extend our conceptions beyond what is familiar to us. ”

“ This regular arrangement of the floating marly substance in the body of salt, which is that of the structure of a coated pebble, or that of concentric spheres, is altogether inexplicable upon any other supposition, than the perfect fluidity or fusion of the salt, and the attractions and repulsions of the contained substances. It is in vain to look, in the operations of solution and evaporation, for that which nothing but perfect fluidity or fusion can explain. ”

“ Being also fluid, and in perpetual agitation, its parts are constantly mixed together; so that, instead of its heat being all accumulated in its surface, as in the case of a solid opaque body, it is diffused through its whole mass. ”

“ If a solid in the form of a right parallelepiped is subject to different pressures on its three pairs of opposite sides by fluids in which it is soluble, it is in general necessary for equilibrium that the composition of the fluids shall be different. The fundamental equations which have been described above are limited, in their application to solids, to the case in which the stresses in the solid are isotropic. ”

“ But the remarkable transformations and combinations continually going on in every living body, which are, in fact, the essential conditions of its life, are themselves dependent on certain physical conditions which must be always present. Professor Allen remarks: 'The sensitiveness of nitrogen, its proneness to change its state of combination and energy, appear to depend on certain conditions of temperature, pressure, etc., which exist at the surface of this earth. ”

“ We may also suppose that in the narrow space or spaces between the solid and the envelop, which are filled with fluid, there is no motion of matter or transmission of heat across any surfaces which can be generated by moving normals to the surface of the solid, since the terms in the condition of equilibrium relating to such processes may be cancelled on account of the internal equilibrium of the fluids. ”

“ That these veins are of a formation subsequent to the hardening and consolidation of the strata which they traverse, is too obvious to require any proof; and it is no less clear, from the crystallized and sparry structure of the substances contained in them, that these substances must have concreted from a fluid state. Now, that this fluidity was simple, like that of fusion by heat, and not compound, like that of solution in a menstruum, is inferred from many phenomena. ”