9. Form statements which are not true to the fact. Ask your fellow-students to correct them.

LESSON TWENTY-TWO

SOLIDS, LIQUIDS, GASES, AND PLASMA

According to the molecular theory a gas consists of a large number of molecules which are far apart in comparison to their diameter. Each molecule moves freely and rapidly in a straight line except when it meets another molecule or col­lides with the walls of its container (Fig. 17). As a result of numerous collisions, the speed of any one molecule constantly changes in direction and, generally, in amount. A gas being compressed, these collisions become more frequent. In addi­tion, the speed a given molecule travels with is greatly in­creased when a gas is heated.

The law explaining gas pressure on the walls of a container due to the collision of separate molecules was first established

and stated by Daniel Bernuilli, a Petersburg acade­mician, as early as 1738. Even today the "Bernuilli effect" is considered to be one of the basic laws of thermodynamics.

The principal physical property distinguishing a gas from a liquid and a so­lid, consists in its ability to expand and occupy all the space available to it. Gases, therefore, have neither de­finite volume, nor definite shape, on the contrary, they take both the volume and shape of the container into which they are placed. They readily diffuse. The ease with which the molecules of a gas diffuse shows not only that they are in rapid motion, but also that the distance between them is large compared with the space occupied by the molecules themselves, in other words, compared with their own size.

Solid bodies and their behaviour are the next subject we shall deal with. It is not difficult to distinguish the property of a solid from that of a gas. It may be stated as follows: in the solid phase or condition the molecules attract each other strongly and, hence, they are unable to move about freely and rapidly as in the case of a gas. Of course, they are also in motion but they are limited both in the manner and range of move­ment. In fact, they are limited to vibrational and sometimes rotational motions. This explains the definite size and shape of solid bodies which neither flow as do liquids, nor occupy all available space as gases do. We may consider them as vibrating about their mean positions with a motion like that of a weight vibrating at the end of a spring. It is these vibra­tions which produce the sensation of temperature. In a hot body the power of the vibrations is large and the speed of the molecules in passing through their mean positions is found to be rather high. On the other hand in a cold body the power of the vibrations is less, the speed of molecular motion being greatly reduced.

What feature distinguishes a liquid from the two other states of matter? We find that a liquid is not so compressible as a gas. The reason for a certain degree of compressibility of liquids is easily understood. The molecules of a liquid are much closer together than those of a gas. They are free to move but owing to the closeness of different molecules to each other they are unable to travel far without colliding with other molecules. Under such conditions, on the one hand the mole­cules of a liquid are more free to move than the molecules of a solid, but on the other hand, they are less free to travel than those of a gas. In short, the molecule of a liquid is in a state of constant motion without any definite direction. It is quite free to travel from place to place within the liquid itself; but it cannot easily leave its surface, i.e., evaporate, because of the rather strong force of attraction observed between the molecules of any liquid. This force serves to fix the volume of a given liquid although its shape is changeable. In other words, liquids have a definite volume at a given temperature but they do not occupy all the available space and always take the shape of the container. They are found, in general, to diffuse much more slowly than gases do.

Besides gases, solids, and liquids there is a fourth state of matter, namely, a plasma state. It is caused by heating the matter to as high a temperature as 4000-5000°C and even higher. Under such conditions, the so-called ionized gas is produced with a great mass of free electrons forced away from the atoms. Generally speaking, "plasma" is a rather new term in science, the physical properties of the plasma being a prob­lem that has not been fully solved yet.