Thus, as the result of Maxwell’s more exact researches on the motion of a system of spherical particles, we find that we again can obtain the equations-- T = ½ _mv_² _p_ = ⅓ N _mv_² = ⅔ NT = ⅔ _ρ_ T/_m_ From these results we obtain as before the laws of Boyle, Charles and Avrogadro. Again if _σ_ be the specific heat of the gas at constant volume, the quantity of heat required to raise a single molecule of mass _m_ one degree will be _σ_ _m_. Thus, when a molecule is heated, the kinetic energy must increase by this amount. But the increase of temperature, which in this case is 1°, is measured by the increase of kinetic energy of the single molecule. Hence the amount of heat required to raise the temperature of a single molecule of all gases 1° is the same. Thus the quantity _σ_ _m_ is the same for all gases; or, in other words, the specific heat of a gas is inversely proportional to the mass of its individual molecules. The density of a gas--since the number of molecules per unit volume at a given pressure and temperature is the same for all gases--is also proportional to the mass of each individual molecule. Thus the specific heats of all gases are inversely proportional to their densities. This is the law discovered experimentally by Dulong and Petit to be approximately true for a large number of substances. * * * * * In the next part of the paper Maxwell proceeded to determine the average number of collisions in a given time, and hence, knowing the velocities, to determine, in terms of the size of the particles and their numbers, the mean free path of a particle; the result so found differed somewhat from that already obtained by Clausius. Having done this he showed how, by means of experiments on the viscosity of gases, the length of the mean free path could be determined. An illustration due to Professor Balfour Stewart will perhaps make this clear. Let us suppose we have two trains running with uniform speed in opposite directions on parallel lines, and, further, that the engines continue to work at the same rate, developing just sufficient energy to overcome the resistance of the line, etc., and to maintain the speed constant. Now suppose passengers commence to jump across from one train to the other. Each man carries with him his own momentum, which is in the opposite direction to that of the train into which he jumps; the result is that the momentum of each train is reduced by the process; the velocities of the two decrease; it appears as though a frictional force were acting between the two. Maxwell suggests that a similar process will account for the apparent viscosity of gases.