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Related post: has found the widest acceptance among physicists. From the observa- tions of osmotic pressure by means .of Pfeffer's ferrocyanid of copper cell, Cheap Loratadine Van't Hoff (1885 and 1887) arrived at the conclusion that the phenomena of osmotic pressure are governed by the same laws as the pressure of gases, quantitatively as well as qualitatively. This would imply, that the pressure in both cases is produced by the same cause, namely, the kinetic motion of the molecules bombarding the walls of the container. Dis- solved molecules would behave precisely like gaseous molecules. The membrane would play a purely passive part; being important only by making the kinetic motion manifest to the senses. According to the Buy Loratadine Online Van't Hoff theory, the molecules of both solvent and solute are in constant motion, moving with the same energy as if they were in the form of a gas. Every molecule, by striking against the walls of the container, or against other mole- cules, exerts a certain pressure, which is precisely the same whether the substance Buy Loratadine is in the form of a gas or of a solution. If the same number of molecules per cubic space exist in two solutions separated by a semipermeable Purchase Loratadine membrane, the pressure will be 40 626 MANUAL OF PHARMACOLOGY equal on each side of the membrane. The nature of the molecules is immaterial in this, as long as they all penetrate through the membrane with equal readiness. If all the molecules can pass through the membrane, an exchange will take place; but since as many pass to one side as to the other, there will be no difference in the pressure in the two compartments. If, on the other hand, the molecules on the one side can pass through readily, while some of the molecules on the other side are not capable of diffusing through the membrane, then the pressure in the two compartments will become unequal. In an osmotic cell, the non-diffusible molecules will all Loratadine Online rebound from the membrane, and so remain confined to their own side. The diffusible molecules will, like gases, tend to pass through, irrespective of the other molecules, until they exist in equal concentration on each side, i.e., until the partial pressure of each kind of molecule is the same on both sides. If a solution consisting of x molecules of salt and (y x) molecules of water per cubic centimeter were inclosed in a closed vessel and separated from water containing y molecules per cubic centimeter by a semipermeable membrane, then water would pass into the solution until it also contained y molecules of water per cubic centimeter. But it would then also contain x molecules of salt. Its total molecular concentration per cubic centimeter would therefore be y -f- x; higher by x than that of the liquid outside of the vessel. The pressure in the vessel would therefore be increased by * molecules. The process might be represented as follows: Partial Molecular concentration: y HzO x salt y H-f) x salt - xH-f) y HiO tends to become- Total Molecular concentration: y \y y y -+ x This excess of total molecular concentration constitutes the osmotic pressure, An experiment analogous to osmosis can be performed on gases directly. Palladium is permeable for H but not for N. Ramsay, 1894, found that when a palladium vessel is filled with N, closed, and immersed in Order Loratadine an atmosphere of H, the pressure rises within the vessel until it exceeds the outside pressure by the partial pressure of the H. How- ever, this experiment could also be explained by the Buy Cheap Loratadine other theories. Application of Gas Laws to Osmotic Pressure. Van't Hoff claimed as chief support of his theory, that the osmotic pressure obeys quantitatively the laws of gas pressure. As applied to osmotic pressure, these would read: Boyle-M ariotte' s Laiv: With constant temperature, the osmotic pressure of a solution is proportional to its concentration. Under standard conditions, a gram-molecule distributed over a liter of space, exerts 22.34 atmospheres of pressure. Gay-Lussac's Law: With equal concentration, the osmotic pressure grows by 73 for each degree C. Avogadro'sLaw: With equal osmotic pressure and equal temperature, equal volumes of solutions contain the same number of dissolved molecules, and the same number as would be contained in an equal volume of gas of the same temperature and presence. Experimental Evidence. The experimental confirmation Purchase Loratadine Online of these laws is unfor- tunately very incomplete (Kahlenberg), because of the difficulty of constructing strictly semipermeable membranes which will resist the high pressures. The experi- ments which form the original basis of the Van't Hoff theory were very inaccurate. Since then, only a few rigorous determinations of absolute osmotic pressure have been attempted (especially by Morse, Holland and co-workers, 1911 and 1912). These showed a good agreement with the theory; but they were confined to a single type of membrane (ferrocyanid of copper) and to a single substance (cane sugar), within a limited range of concentration and temperature. Outside of these limits, there were considerable discrepancies. The numerous determinations of relative osmotic pressure, which Order Loratadine Online can be easily made, are inconclusive; for while they agree with this theory, they also harmonize with the competing theories. On the other hand, Kahlenberg, using different, but typically semipermeable membranes, finds that the gas laws do not hold at all; and that the phenomena can be explained quite simply on an entirely different basis. The evidence for the Van't Hoff theory is therefore incomplete. Advantages of the Van't Hoff Theory. This is very attractive in that it reduces all the physical phenomena of gases and solutions to a common basis, and thereby permits convenient and far-reaching theoretical and practical deductions. For instance, by PHYSICAL FACTORS OF SALT ACTIONS 627 postulating that osmotic pressure is directly proportional to the molecular concentration, it justifies the substitution of an easy calculation for the very difficult experimental Generic Loratadine determination of the pressure. Since it assumes that the membrane is purely passive, it permits the calculation of osmotic pressure in situations which are inaccessible to direct experimentation. In a word, if it is correct, it makes the difficult method of direct experimentation more or less superfluous, substituting for it an easily applied formula. This would be highly convenient and desirable but before relying on such deductions, the correctness of the formula must be more securely established. Kohlenberg's Theory. This has the advantage that it furnishes the simplest explanation of the nature of osmotic pressure; and that it is in the best accord with these phenomena as exhibited in living structures. The theory refers osmosis to solution-affinity, and may be best understood from an experiment made by L'Hermite in 1854. L'Hermite's Experiment. A layer of chloroform is poured into a vessel, then a layer of water, and finally a layer of ether. Even if any mechanical mixing of the strata is carefully avoided, it will be found that the ether layer gradually diminishes, until finally there are only two layers, the upper containing mainly the water, the lower the chloro-
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