23. Gases, solids, liquids 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 the straight line except when it meets another molecule or collides with the walls of its container. 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 Bernouilli, a Petersburg academi­cian, as early as 1738. Even today the "Bernouilli 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 energy 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 energy 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 a 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.

The fourth state of matter is considered below.

One of the oldest fields of science is the one of electric­al discharges in gases. Considering special properties of mat­ter in discharge tubes William Crookes put forward the idea that such gases should be considered a fourth state of matter namely the plasma state.

The plasma is first of all an ionised gas in which the charges of both signs are approximately equal. The plasma is caused by heating the matter to very high temperatures. Under such conditions the so-called ionised gas is produced with a great mass of free electrons forced away from the atoms.

The production of the plasma can take place in a number of ways. Among them we find: spark discharge, arc discharge, chemical reactions of high energy and nuclear reactions. One should also mention here the bombardment of electromagnetic fields or particles of any other origin provided sufficient energy is available to provide the required work of the neu­tral atoms and ions.

An electrical conductivity which can te compared to that of some metals and strong electrolytes exists in the plasma.