[Illustration: FIG. 8.--Osmosis Shown by carrot placed in water.] =20. Osmosis.=--If two liquids are separated by a membrane or porous partition, they tend to pass through and mix. This action is called osmose, or _osmosis_. Such a movement of liquid molecules in osmosis may be illustrated by filling a beet or carrot that has had its interior cut out to form a circular opening (see Fig. 8) with a thick syrup. The opening is then closed at the top with a rubber stopper through which passes a long glass tube. If the carrot is immersed in water, as in Fig. 8, a movement of water through the porous wall to the interior begins at once. Here, as in the experiment of the hydrogen and air passing through the porous cup, the lighter fluid moves faster. The water collecting in the carrot rises in the tube. This action of liquids passing through porous partitions and mingling is called _osmosis_. Gases and liquids are alike in that each will _flow_. Each is therefore called a _fluid_. Sometimes there is much resistance to the flow of a liquid as in molasses. This resistance is called _viscosity_. Alcohol and gasoline have little viscosity. They are _limpid_ or _mobile_. Air also has some viscosity. For instance, a stream of air always drags some of the surrounding air along with it. Important Topics 1. Liquids behave as if they were composed of small particles in motion. 2. This is shown by (1) Diffusion, (2) Solution, (3) Evaporation, (4) Expansion, (5) Osmosis. Exercises 1. Give an example or illustration of each of the five evidences of molecular motion in liquids. 2. When is air saturated? What is the explanation? 3. Why does warming a liquid increase its rate of evaporation? 4. Air molecules are in rapid motion in all directions. Do they enter a liquid with a surface exposed to the air? Give reason. 5. What are some of the inconveniences of living in a saturated atmosphere? 6. Fish require oxygen. How is it obtained? (3) MOLECULAR FORCES IN LIQUIDS