Most creative of living theoretical physicists; developed in 1905 the theory of Brownian movements and pointed the way thereby to the experimental verification of the molecular and kinetic hypothesis; first set up, also in 1905, the so-called Einstein photo-electric equation, which involves a new conception as to the nature of light, an equation subsequently completely verified experimentally; awarded the Nobel prize in 1921 for this last accomplishment; author of the special theory of relativity in 1905 and of the general theory of relativity in 1914, both of which have had great success in explaining otherwise unexplained phenomena and in predicting new ones

JAMES CLERK MAXWELL (1831-1879)

One of the greatest of mathematical physicists; born in Edinburgh, Scotland; professor of natural philosophy at Marischal College, Aberdeen, in 1856, of physics and astronomy in King's College, London, in 1860, and of experimental physics in Cambridge University from 1871 to 1879; one of the most prominent figures in the development of the kinetic theory of gases and the mechanical theory of heat; author of the electromagnetic theory of light - a theory which has become the basis of nearly all modern theoretical work in electricity and optics (see page 469)

Again, chemists tell us that if two globes, one containing hydrogen and the other carbon dioxide gas, be connected as in Fig. 51, and the stopcock between them be opened, after a few hours chemical analysis will show that each of the globes contains the two gases in exactly the same proportions, — a result which is at first sight very surprising, since carbon dioxide gas is about twenty-two times as heavy as hydrogen. This mixing of gases in apparent violation of the laws of weight is called diffusion.

We see, then, that such simple facts as the transference of odors and the diffusion of gases furnish very convincing evidence that the molecules of a gas are not at rest but are continually moving about.

63. Molecular motions and the indefinite expansibility of a gas. Perhaps the most striking property which we have found gases to possess is the property of indefinite, or unlimited, expansibility. The existence of this property was demonstrated by the fact that we were able to attain a high degree of exhaustion by means of an air pump. No matter how much air was removed from the bell jar, the remainder at once expanded and filled the entire vessel. The motions of the molecules furnish a thoroughly satisfactory explanation of the phenomenon.

The fact that, however rapidly the piston of the air pump is drawn up, gas always appears to follow it instantly, leads us to the conclusion that the natural velocity possessed by the molecules of gas must be very great.

64. Molecular motions and gas pressures. How are we to account for the fact that gases exert such pressures as they do against the walls of the vessels which contain them? We have found that in a room at sea level the air presses against the walls with a force of 15 pounds to the square inch. Within a pneumatic truck tire this pressure may amount to as much

as 100 pounds, and the steam pressure within the boiler of a locomotive is often as high as 240 pounds per square inch. Yet in all these cases we may be certain that the molecules of the gas are separated from each other by distances which are large in comparison with the diameters of the molecules; for when we reduce steam to water, it shrinks to re of its original volume, and when we reduce air to the liquid form, it shrinks to about of its ordinary volume.

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The explanation is at once apparent when we reflect upon the motions of the molecules. For just as a stream of water particles from a hose exerts a continuous force against a wall on which it strikes, so the blows which the innumerable molecules of a gas strike against the walls of the containing vessel must constitute a continuous force tending to push out these walls. In this way we account for the fact that vessels containing only gas do not collapse under the enormous external pressures to which we know them to be subjected.

65. Explanation of Boyle's law. It was discovered in the last chapter that when the density of a gas is doubled, the temperature remaining constant, the pressure is likewise found to double; that when the density is trebled, the pressure is trebled; etc. This, in fact, was the assertion of Boyle's law. Now this is exactly what would be expected if the pressure which a gas exerts against a given surface is caused by blows struck by an enormous number of swiftly moving molecules; for doubling the number of molecules in the given space (that is, doubling the density) would simply double the number of blows which are struck per second against that surface, and hence would double the pressure. The kinetic theory of gases which is here presented accounts in this simple manner for Boyle's law.

66. Brownian movements and molecular motions. It has recently been found possible to demonstrate the existence of molecular motions in gases in a very direct and striking way. It is found that very minute oil drops suspended in perfectly stagnant air, instead of being themselves at rest, are cease

lessly dancing about just as though they were endowed with life. In 1913 it was definitely proved that these motions, which are known as the Brownian movements, are the direct result of the bombardment which the droplets receive from the flying molecules of the gas with which they are surrounded; for at a given instant this bombardment is not the same on all sides, and hence the suspended particle, if it is minute enough, is pushed hither and thither according as the bombardment is more intense first in one direction, then in another. There can be no doubt that what the oil drops are here seen to be doing, the molecules themselves are also doing, only in a much more lively way.

67. Molecular velocities. From the known weight of a cubic centimeter of air under normal conditions, and the known force which it exerts per square centimeter (namely, 1033 g.), it is possible to calculate the velocity which its molecules must possess in order that they may produce this amount of force by their collisions against the walls. The result of the calculation gives to the air molecules under normal conditions a velocity of about 445 m. per second, and it assigns to the hydrogen molecules the enormous speed of 1700 m. (over a mile) per second. The speed of a projectile is seldom greater than 800 m. (2600 ft.) per second. It is easy to see, then, since the molecules of gases are endowed with such speeds, why air, for example, expands instantly into the space left behind by the rising piston of the air pump, and why any gas always fills completely the vessel which contains it (see mercury-diffusion air pump, opposite page 35).

68. Diffusion of gases through porous walls. Strong evidence of the correctness of the views given above is furnished by the following experiment:

Let a porous cup of unglazed earthenware be closed with a rubber stopper through which a glass tube passes, as in Fig. 52. Let the tube be dipped into a dish of colored water, and a jar containing hydrogen be placed over the porous cup; or let the jar simply be held in the position shown in the figure, and illuminating gas be passed into it by means of a rubber tube connected with a gas jet. The rapid passage of bubbles out through the water will