246 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME Other things being equal, the substance having the smallest molecular weight would be selected for use as an antifreeze substance. Other important considerations are cost and the volatility of the antifreeze substance. Inasmuch as ethyl and methyl alcohols boil below the temperature of water, they are gradually lost when the automobile engine gets hot and would have to be renewed two or three times during a winter. The other substances would not be lost in this way because they are less volatile than water, and it might be more economical to use one filling of such a substance even at a higher price than several fillings of the cheaper, more volatile substances. Salt is mixed with ice in making ice cream in order to lower the melting-point of the ice. This is necessary because the sugar and other solutes dissolved in the water of the ice cream lower its freezing-point below the melting-point of ice. Salt is scattered on icy walks to remove the ice because it lowers the melting-point of the ice and thus causes it to melt. Ocean water does not freeze as readily as fresh water because of the salts dissolved in it. Brine is used in commercial ice plants to conduct to the cooling-coils the heat from the cans of water to be frozen. 4. The Osmotic Pressure Is Produced. If a membrane, such as parchment paper or an animal bladder, is placed between two solutions of different concentrations, it will be found that solvent particles will pass through the membrane in the direction of the more concentrated solution more rapidly than the solute particles will pass through the membrane in the opposite direction. The passage of solvent through a membrane into a solution is called osmosis; the passage of a solute through a membrane is called dialysis. Most membranes permit both dialysis and osmosis; but if a proper membrane could be secured, osmosis alone would take place. If osmosis were allowed to continue until the water ceased to rise in the tube, the hydrostatic pressure thus produced would be a measure of the osmotic pressure of the solution. It has been found that the osmotic pressure of dilute solutions of equimolecular concentration is the satne for different substances when there is no dissociation of the solute and that it is independent of the character of the solute. The osmotic pressure exerted by a substance in solution is equal to the gas pressure which that substance would exert if it were a gas occupying the same volume as that of the solvent, under the same conditions of temperature. The solution statie is very similar to the gaseous state in that osmotic pressure is affected in the same way by It is impor- temperature and volume changes as are gaseous pressures.
REGULARITIES OF SOLUTIONS 247 tant to note that the laws dealing with the behavior of substances in solution apply only to very dilute solutions. Osmosis is explained in a number of different ways. The sieve theory, for example, assumes that the solute particles are too large to pass through the openings in the membrane, while the solvent molecules are sufficiently small to do so. There is considerable evidence that each solute particle is joined to one or more solvent particles in solution. This fits into the sieve theory nicely and also explains why the gas laws do not apply to concentrated solutions. In such solutions so much of the solvent would thus be joined to the solute particles that there would be a much smaller volume of free solvent than there is assumed to be in making calculations involving the volume. Osmosis is also explained as the result of the solution of one of the phases in the membrane. If particles the size of solvated ^ solute particles could pass through a membrane whose openings were too small to admit the passage of the still larger colloidal particles, the solute and colloid would thus be separated. Of course, openings large enough to admit the passage of solute particles would also admit the solvent molecules, so osmosis always takes place during dialysis. Dialysis does not always take place, however, when there is osmosis. Osmosis and dialysis have many important applications in living organisms because every living cell is surrounded by a membrane through which it makes necessary interchanges with its environment. If oysters are placed in distilled water they swell because of osmosis, inasmuch as the solutions inside the oyster have a much higher osmotic pressure than distilled water — they have been in equilibrium with the sea water in which the oyster grew. Some unscrupulous markets have taken advantage of this idea to increase the size and weight of their oysters before selling them. If plant cells are placed in salt water, they wilt and lose their turgor. This is due to the fact that water passes from the plant cells to the salt water. Such a process is called plasmolysis. One type of remedy for intestinal stasis involves the use of saline laxatives. Such substances as sodium sulfate (Glauber's salt), magnesium sulfate (Epsom salts), magnesium citrate, and milk of magnesia flush out the intestinal tract by the water drawn from the blood as a result of the higher osmotic pressure their presence produces in the intestines. One objection to the use of saline laxatives is that they not only remove water from the blood but that they also bring about a loss of • When a solute combines with a portion of the solvent, the solute is solvated.