thus : Two atoms are said to be in collision during all the time their volumes overlap after coming into contact. They necessarily in virtue of inertia separate again, unless some third body intervenes with action which causes them to remain overlapping; that is to say, causes combination to result from collision. Two clusters of atoms are said to be in collision when, after being separate, some atom or atoms of one cluster come to overlap some atom or atoms of the other. In virtue of inertia the collision must be followed either by the two clusters separating, as described in the last sentence of § 19, or by some atom or atoms of one or both systems being sent flying away. This last supposition is a matter-of-fact statement belonging to the magnificent theory of dissociation, discovered and worked out by Sainte-Clair Deville without any guidance from the kinetic theory of gases. In gases approximately fulfilling the gaseous laws (Boyle's and Charles'), two clusters must in general fly asunder after collision. Two clusters could not possibly remain permanently in combination without at least one atom-being sent flying away after collision between two clusters with no third body intervening *.
§ 23. Now for the application of the Boltzmann-Maxwell doctrine to the kinetic theory of gases: consider first a homogeneous single gas, that is, a vast assemblage of similar clusters of atoms moving and colliding as described in the last sentence of § 19; the assemblage being so sparse that the time during which each cluster is in collision is very short in comparison with the time during which it is unacted on by other clusters, and its centre of inertia, therefore,, moves uniformly in a straight line. If thore are i atoms in each cluster, it has 3i freedoms to move, that is to say, freedoms in three rectangular directions for each atom. The Boltzmann-Maxwell doctrine asserts that the mean kinetic energies of these Si motions are all equal, whatever be the mutual forces between the atoms. From this, when the durations of the collisions are not included in the time-averages, it is easy to prove algebraically (with exceptions noted below) that the time-average of the kinetic energy of the component translational velocity of the inertial centre t, in any direction, is equal to any one of the 3i mean kinetic energies asserted to be equal to one another in the preceding statement. There are exceptions to the algebraic proof
* See Kelvin’s Math, and Phys. Papers, vol. iii. Art. xcvn. § 33. In this reference, for “scarcely” substitute “not.”
t This expression I use for brevity to signify the kinetic energy of the whole mass ideally collected at the centre of inertia.
corresponding to the particular exception referred to in the last footnote to § 18 above; but, nevertheless, the general Boltzmann-Maxwell doctrine includes the proposition, even in those cases in which it is not deducible algebraically from the equality of the 3i energies. Thus, without exception, the average kinetic energy of any component of the motion of the inertial centre is, according to the Boltzmann-Maxwell
doctrine, equal to ^ of the whole average kinetic energy of
the system. This makes the total average energy, potential and kinetic, of the whole motion of the system, translational and relative, to be 3i(l + P) times the mean kinetic energy of one component of the motion of the inertial centre, where P denotes the ratio of the mean potential energy of the relative displacements of the parts to the mean kinetic energy of the whole system. Now, according to Clausius’ splendid and easily proved theorem regarding the partition of energy in the kinetic theory of gases, the ratio of the difference between the two thermal capacities to the constant-volume thermal capacity is equal to the ratio of twice a single component of the translational energy to the total energy. Hence, if according to our usual notation we denote the ratio of the thermal capacity, pressure constant, to the thermal capacity, volume constant, by &, we have,
t-i- 2-~
3»(1 + P)‘
§ 24. Example 1.—For first and simplest example, consider a monatomic gas. We have i=1, and according to our supposition (the supposition generally, perhaps universally, made) regarding atoms, we have P = 0. Hence, k —1 = §.
This is merely a fundamental theorem in the kinetic theory of gases for the case of no rotational or vibrational energy of the molecule; in which there is no scope either for Clausius’ theorem or for the Boltzmann-Maxwell doctrine. It is beautifully illustrated by mercury vapour, a monatomic gas according to chemists, for which many years ago Kundt, in an admirably designed experiment, found A —1 to be very approximately f; and by the newly discovered gases argon, helium, and krypton, for which also Jc—1 has been found to have approximately the same value, by Rayleigh and Ramsay. But each of these four gases has a large number of spectrum lines, and therefore a large number of vibrational freedoms, and therefore, if the Boltzmann-Maxwell doctrine were true, k — 1 would have some exceedingly small value, such as that
