DIFFUSE MATTER IN SPACE 
373 
than aggregated stellar matter. Dynamical studies of stellar motions lead 
to the conclusion that the mass of invisible matter cannot exceed in any 
large ratio the mass of the luminous stars; it has in fact generally been 
concluded that it is not greater. 
For a density of 1 atom per cu. cm. and atomic radius 10 - 8 cm., the 
mean free path is 5-6.10 9 km. 
We have yet to decide whether this matter takes up a high or a low 
temperature, but provisionally we shall assume a temperature of 10 , 000 °. 
The mean speed for atomic weight 10 is then 4-6 km. per sec.; hence the 
duration of the free path is 40 years. But we shall find later that the inter 
stellar gas is ionised so that the attractions and repulsions of the electric 
charges will modify the free path. We assume single ionisation, and regard 
a deflection of 90° or .more as constituting an encounter; the results are 
then: 
Free path of atoms.—Length 7. 10 8 km. Duration 5 years. 
Free path of electrons.—Length 1-8.10 8 km. Duration 3| days. 
The collisions are sufficiently frequent to ensure that the material is 
a genuine gas with atomic velocities distributed according to Maxwell’s 
law. The term temperature can be applied with its ordinary significance 
as a measure of the energy of the random motions*. 
Energy is transferred from stellar radiation to this diffuse matter in 
four ways— 
(а) Line absorption by the atoms. 
(б) Ionisation of the atoms. 
(c) Scattering by free electrons. 
(. d ) Switches of electron orbits at encounter with atoms. 
Energy is radiated from the matter by the four converse processes. The 
processes (a) and ( 6 ) involve high-frequency radiation and therefore tend 
to raise the temperature to a high value of T x . The process (c) is independ 
ent of wave-length and tends to reduce the temperature to 3°-18. The 
fourth process requires closer examination. 
(a) Line absorption has practically no effect on the temperature of 
the material. An atom absorbs a quantum and is thereby raised to an 
excited state, but there is no mechanism for converting the energy so 
acquired into translatory energy; all the atom can do is to retain it for 
about 10~ 8 sec. and then by re-emission restore it to the field of radiation. 
Dense material can be heated by line absorption through the mechanism 
of superelastic collisions. An excited atom collides with an electron (or 
perhaps another atom) and the energy of excitation is released so as to 
cause an explosive rebound. Thus the energy passes from radiation via 
* We prefer not to use the term temperature of matter except in this sense.