Osmotic pressure

“ Thus the recognition of the fact that a substance in solution is really a gas, has at a single stroke put us in possession of the laws of osmotic pressure—laws slowly and laboriously discovered by the long series of investigations on the pressure of gases.
Osmotic pressure plays a most important rôle in the arena of life. It is found at work in all the phenomena of life. When osmotic pressure fails, life itself ceases. ”

“ It is probable that osmotic pressure is not really of the same nature as gas-pressure, but depends on equilibrium of vapour-pressure. The vapour-molecules of the solvent are free to pass through the semi-permeable membrane, and will continue to condense in the solution until the hydrostatic pressure is so raised as to produce equality of vapour-pressure. ”

“ Again Bancroft has shown that the pressure for finite solutions in osmosis varies with the heat of dilution, which again varies with the specific nature of the solvent and the solute. [43] All this brings us back to Gibbs's fundamental position that osmotic pressure "is a function of the temperature and the n potentials. [44] From this point of view, Graham's original doctrine, that osmosis is the conversion of chemical affinity into mechanical power, [45] is at once true to the mathematical theory and the laboratory facts. ”

“ Osmosis takes place when two fluids of different osmotic pressure are separated by animal membrane. Difference in osmotic pressure is due to differences in molecular concentration, the greater the number of molecules the greater is the pressure, and the greater rapidity of flow is from the fluid of less pressure to the fluid of greater pressure. The molecular concentration of tissue and blood fluid is constantly being equalized by the process of osmosis. ”

“ All the thermodynamic relations we have deduced hold on any theory of solution and favour no one theory rather than another. Whether osmotic pressure be due to physical impact or to chemical affinity it must necessarily have the gas value in a dilute solution, and be related to vapour pressure and freezing point in the way we have traced. ”

“ Similarly, any other increase in concentration of synthetic products is compensated for by entrance of water because of increased osmotic pressure, unless the products are insoluble and, therefore, incapable of effecting the osmotic pressure. ”

“ An osmotic growth also shows indications of movements which are connected with its nutrition, and these movements are rhythmic, like those of respiration or circulation in a living organism. The growth of an osmotic production shows itself not as a continuous process but periodically. The water traverses the membrane, raises the pressure, and distends the cell; at first the cell wall resists by reason of its elasticity, it then suddenly relaxes, yielding to the osmotic pressure and bulging out at a thinner spot on the surface ”

“ The process is called osmosis, and the pressure that forces the liquid up the tube is called osmotic pressure. [pg 273] It is by this sort of diffusion that chicks which are being incubated get air, and that growing plants get food. It is in this way that the cells of our body secure food and oxygen and get rid of their wastes. There are no little holes in our blood vessels to let the air get into them from our lungs. The air simply diffuses through the thin walls of the blood vessels. ”

“ It is clear, therefore, that osmotic pressure plays an important part in the physical mechanism of cell activities and in the regulation of the proportion of water contained in the protoplasm, with its consequent effects upon the chemical reactions which may go on in the cell. ”

“ The importance of the osmotic pressure of sea-water in biology will be easily understood from the fact that a frog placed in sea-water loses water by exosmosis and soon becomes 20% lighter than its original weight, while a true salt-water fish suddenly transferred to fresh water gains water by endosmosis, swells up and quickly succumbs. The elevation of the boiling point is of little practical importance, but the reduction of vapour pressure means that sea-water evaporates more slowly than fresh water, and the more slowly the higher the salinity. ”

“ Gardeners are well aware of the noxious influence of a low osmotic pressure. They water the soil around the roots of a plant, so that the water may take up {51}some of the salts from the soil before being absorbed by the plant. Pure water poured over the heart of a delicate plant may burst its cells owing to its low osmotic pressure. ”

“ Solution: Kirchhoff's, Gibb's, Duhem's and Van t'Hoff's researches. § 2. Osmosis: History of phenomenon—Traube and biologists establish existence of semi-permeable walls—Villard's experiments with gases—Pfeffer shows osmotic pressure proportional to concentration— Disagreement as to cause of phenomenon. ”

“ In very dilute solutions of simple substances, where only one kind of dissociation is possible and the dissociation of the ions is complete, the number of pressure-producing particles necessary to produce the observed osmotic effects should be equal to the number of ions given by a molecule of the salt as shown by its electrical properties. ”

“ It was found, however, that, by the process known to chemists as osmosis, this light gas in time partially escaped, and the remainder became mixed with air, the consequence being that the graduations were no longer correct. They are more durable as at present constructed. The liquid rises and falls in the tube with the variations of atmospheric pressure and temperature acting together. If the pressure were constant, the confined air would expand and contract for temperature only, and the instrument would act as a thermometer. ”

“ Even far below the critical temperature the molecules have an enormous degree of activity, and tend to fly asunder, maintaining what appears to be a gaseous, but what technically is called a vaporous, condition—the distinction being that pressure alone suffices to reduce the vapor to the liquid state. Thus water may change from the gaseous to the liquid state at four hundred degrees above zero, but under conditions of ordinary atmospheric pressure it does not do so until the temperature is lowered three hundred degrees further. ”

“ There will, however, be somewhere an elevation at which the heat evolved by the joint compression of the elementary and combined gases will be just sufficient to dissociate the latter, and here will be the meeting surface of the combined and the uncombined constituents of water. There will be a sphere containing combined oxygen and hydrogen surrounded by an atmospheric envelope containing large quantities of aqueous vapor, and the temperature at this limiting surface will be equal to that of the oxyhydrogen flame under a corresponding pressure. ”

“ Hence, hydrogen, starting from zero to the highest pressures, exhibits a gradually, although only slightly, varying coefficient of expansion, whilst for air and other gases at the atmospheric and higher pressures, the coefficient of expansion increases with the increase of pressure, so long as their compressibility is greater than should follow from Mariotte's law. But when at considerable pressures, this kind of discrepancy passes into the normal (see Note 25) , then the coefficient of expansion of all gases decreases with an increase of pressure, as is seen from the researches of Amagat. ”

“ All moving bodies of visible matter produce heat as according to their velocity. The flow of gases only induces thermal reduction from molecular friction. By this term it must not be understood that the molecules actually come in contact, and rub against each other. There is no pressure, however great, that can cause molecular contact. The area of the volume of the molecule can be reduced by enormous pressure, and the tension thus brought to bear on their rotating envelopes induces heat. ”

“ When the tendency of each to pass into the other is exactly counterbalanced, we have what is called a state of equilibrium between the two phases. This tendency of the molecules to pass off into the atmosphere, even at lower temperatures, gives rise to a certain amount of pressure, called "vapor tension." The atmosphere, or any artificial pressure which may be applied, tends to overcome this. At every temperature only a certain amount of water vapor can exist under a given external pressure, viz., the vapor tension of water at that temperature. ”

“ Cautious application of heat often increases, not only the sensibility, but also the power of self-recovery. High temperature, however, produces erratic behaviour by causing violent molecular disturbance.
Influence of pressure.—Pressure also has pronounced effect on molecular response. Moderate increase of pressure increases the sensibility, but too great an increase may cause a loss of sensibility. In substances which are nearly neutral, pressure variation may even cause reversal of response. ”

“ In fact, the quantity 4πρ2K, which we may call with van der Waals the molecular pressure, is so great for most liquids (5000 atmospheres for water) , that in the parts near the surface, where the molecular pressure varies rapidly, we may expect considerable variation of density, even when we take into account the smallness of the compressibility of liquids.
The pressure at any point of the liquid arises from two causes, the external pressure P to which the liquid is subjected, and the pressure arising from the mutual attraction of its molecules. ”

“ If a gaseous solution be brought into an atmosphere of another gas which is limited—for instance, as in a closed vessel—then a portion of the gas held in solution will be expelled, and thus pass over into the atmosphere surrounding the solution, and will produce its partial pressure. Let us imagine that water saturated with carbonic anhydride at 0° and under the ordinary pressure is brought into an atmosphere of a gas which is not absorbed by water ”

“ At 9°·6 the tension of dissociation is equal to the atmospheric pressure. At a higher pressure the crystallo-hydrate may form at temperatures above 9° up to 28°·7, when the vapour tension of the hydrate equals the tension of the chlorine. It is evident that the equilibrium which is established is on the one hand a case of a complex heterogeneous system, and on the other hand a case of the solution of solid and gaseous substances in water. ”

“ No doubt evaporation is generally speaking taking place from the upper surface of a liquid, and the vapour thus produced acts as a gas, so that in this respect some slight qualification of the above statements might be necessary. But nothing can be more obvious than the fact that the upper pressure of the gas indicates some profound difference between its molecular structure and that of a solid or a liquid, We are so accustomed to this manifestation of gaseous pressure that its extraordinary character is apt to be overlooked on accounts of its familiarity. ”

“ 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. ”

“ A fourth factor involved in the deposition of mineral matter from solution is pressure. Pressure increases the solvent power of water with respect to minerals directly; it produces the same effect indirectly by its effect on the solution of gases. As water charged with gas comes to the surface, the pressure is relieved and some of the gas escapes. Such mineral matter as was held in solution by the help of the gas which escapes is then precipitated. (5) Precipitation is also sometimes effected by the mingling of waters containing different mineral substances in solution. ”

“ It is the lightest of all gases. The small density of hydrogen determines many remarkable properties which it shows; thus, hydrogen passes exceedingly rapidly through fine orifices, its molecules (Chapter I.) being endued with the greatest velocity. [24] At pressures somewhat higher than the atmospheric pressure, all other gases exhibit a greater compressibility and co-efficient of expansion than they should according to the laws of Mariotte and Gay-Lussac ”

“ Cooling hydrogen in a capillary glass tube to the temperature of liquid oxygen, he expanded it quickly from 100 atmospheres to one, and obtained the appearance of an instantaneous ebullition. Olszewski confirmed this result by expanding from a pressure of 190 atmospheres the gas cooled by liquid oxygen and nitrogen boiling under reduced pressure, and even announced that he saw it running down the walls of the tube as a colourless liquid. ”

“ Every known substance may exist in either of three states, the solid, liquid or gaseous. For every substance there is a critical temperature above which it can exist only as a gas, and cannot be liquefied by any pressure, but below which a suitable pressure will cause liquefaction. Below its critical temperature a gas is called a vapor, above it a permanent gas. Now in the free atmosphere some of the gases are never below their critical temperatures and, therefore, cannot be liquefied by any pressure, without special cooling ”