How is the pressure produced? The molecules are not packed together solidly in a gas, for when steam changes to water it shrinks to about 1/1600 of its former volume. Air diminishes to about 1/800 of its volume on changing to liquid air. The pressure of a gas is not due then to the gas filling all of the space in which it acts, but is due rather to the _motion_ of the molecules. The blow of a single molecule is imperceptible, but when multitudes of molecules strike against a surface their combined effect is considerable. In fact, this action is known to produce the pressure that a gas exerts against the walls of a containing vessel. Naturally if we compress twice as much gas into a given space there will be twice as many molecules striking in a given time, which will give twice as much pressure. If gas is heated, it is found that the heat will cause a swifter motion of the molecules. This will also make the molecules strike harder and hence cause the gas to expand or exert more pressure. =17a. Brownian Movements.=--Direct photographic evidence of the motion of molecules in gases has been obtained by studying the behavior of minute drops of oil suspended in stagnant air. Such drops instead of being at rest are constantly dancing about as if they were continually receiving blows from many directions. These motions have been called _Brownian Movements_ (see Fig. 7). It has been proved that these movements are due to the blows that these small drops receive from the swiftly moving molecules of the gas about them. If the drops are made smaller or the gas more dense, the movements increase in intensity. These effects are especially marked at a pressure of 0.01 of an atmosphere. [Illustration: FIG. 7.--Photograph of Brownian movement. This record is prepared by the aid of Siedentopf's ultra-microscope and a plate moving uniformly across the field from left to right.] Important Topics It is assumed that air and all gases are made up of molecules in rapid motion; that this motion is dependent upon temperature and pressure. Evidence of this is shown by (a) diffusion, (b) expansion, (c) pressure. Brownian Movements. Questions 1. What is the molecular (kinetic) theory of gases? 2. What three kinds of evidence help to confirm the theory? 3. What have you seen that seems to show that a gas consists of molecules in motion? 4. How many meters long is a 10-ft. pole? 5. A 50-kg. boy weighs how many pounds? 6. What are three advantages of the metric system? 7. What will 12 qts. of milk cost at 8 cents a liter? 8. A cube 1 meter each way will contain how many cubic centimeters? How many liters? What will a cubic meter of water weigh? (2) MOLECULAR MOTION IN LIQUIDS