Page:Elementary Text-book of Physics (Anthony, 1897).djvu/251

§ 223] one degree is, by Maxwell's law, the same for all gases, so that $$\Delta \tfrac{1}{2}mV^2$$ is a constant for all gases; and hence $$C_{0}m$$ is a constant for all gases. $$C_{0}$$ cannot be directly observed, but we may set $$\frac{C_{v}}{C_{0}} = \beta,$$ and observe $$C_{v}.$$ If $$\beta$$ is the same for all gases, $$C_{v}m$$ should also be constant. This is Dulong and Petit's law for gases. It holds quite closely for all gases of the same type of molecular structure, and the departures from it are readily explained by the probability that $$\beta$$ is not the same for all gases.

The phenomena exhibited by the radiometer afford a strong experimental confirmation of the kinetic theory of gases. These phenomena were discovered by Crookes, In the form first given to it by him, the instrument consists of a delicate torsion balance suspended in a vessel from which the air is very completely exhausted. On one end of the arm of the torsion balance is fixed a light vane, one face of which is blackened. When a beam of light falls on the vane it moves as if a pressure were applied to its blackened surface. The explanation of this movement is, that the molecules of air remaining in the vessel are more heated when they come in contact with the blackened face of the vane than when they come in contact with the other face, and are hence thrown off with a greater velocity, and react more strongly upon the blackened face of the vane. At ordinary pressures the free paths of the molecules are very small, their collisions very frequent, and any inequality in the pressures is so speedily reduced that no effect upon the vane is apparent. At the high exhaustions at which the movement of the vane becomes evident, the collisions are less frequent, and hence an immediate equalization of pressure does not occur. The vane therefore, moves in consequence of the greater reaction upon its blackened surface.

'''223. Influence of the Size and Attractions of the Molecules. Critical Temperature.'''—On the elementary theory which has just been developed all gases should conform precisely to the so-called gaseous laws, whereas in fact they only conform to those laws approximately. It was shown by van der Waals that the