vi Supplement to “ Nature,” May 12, 1923 

particles of the matter. In a mass of helium at 
ordinary temperatures the average speed of the atoms is 
rather less than 1 mile per second ; at 4 million degrees 
it is roo miles per second. This is a high speed, but 
not a speed to feel uncomfortable over. Sir Ernest 
Rutherford describes atoms of helium moving at the 
rate of 100,000 miles a second. I cannot vie with him. 
I usually find that my physical colleagues are rather 
disappointed with our jog-trot atoms in the stars. 
MATERIAL AND ALTHERIAL HEAT. 
We must imagine then a typical giant star as a 
mass of material with average density about that of 
air swollen to at least a thousand times the bulk of 
the sun. The atoms of which it consists are rushing 
in all directions with speeds up to too miles a second, 
continually colliding and changing their courses. Each 
atom is being continually pulled inwards by the 
gravitation of the whole mass, and as continually 
boosted out again by collision with atoms below. 
The energy of this atomic motion constitutes a great 
store of heat contained in the star; but this is only 
part of the store. The star contains a store of another 
kind of heat—zetherial heat, or ether-waves like those 
which bring to us the sun’s heat across go million 
miles of vacant space. These waves also are hastening 
in all directions inside the star. They are encaged 
by the material, which prevents them leaking into 
outer space except at a slow rate. An zther-wave 
making for freedom is caught and absorbed by an 
atom, flung out in a new direction, and passed from 
atom to atom; it may thread the maze for hundreds 
of years until by accident it finds itself at the star’s 
surface, free now to travel through space indefinitely, 
or until it ultimately reaches some distant world, and 
perchance entering the eye of an astronomer, makes 
known to him that a star is shining. 
The possession of this double store of heat is a 
condition which we do not encounter in any of the 
hot bodies more familiar to us. It is a new phase of 
matter beyond the reach of laboratory experiment, 
although happily the theory is so simple that there 
cannot be much uncertainty as to behaviour. It is 
true that a red-hot mass of iron contains a little of 
this therial heat in addition to the heat comprised 
in the motion of its molecules, but it is less than a 
billionth part of the whole. Only in the giant stars 
does the etherial portion rise to importance. A 
red-hot metal emits etherial heat, but it keeps no 
appreciable store ; it converts the material heat into 
this form as it is required for use. The star rejects 
this hand-to-mouth method ; and although it is con- 
tinually changing elements of heat from one form to 
the other, it keeps a thousand years’ supply always in 
readiness and emits its radiation by leaking etherial 
heat from the store. In older theories this feature 
was not realised; it was supposed that convection 
currents must exist continually bringing up hot matter 
from the interior to replace the surface-matter which 
had radiated. and cooled. Now it is seen that the 
difficulty is rather in the other direction—how does 
the star dam back the store of ether-waves so that 
they do not escape from it faster than we observe? 
This change of view has necessitated modifications of 
the older theories of Lane and others, and has on the 
whole considerably simplified the problem. 
In the hot bodies of the laboratory the heat is almost 
entirely in the material form, the etherial portion 
being insignificant. In the giant stars the heat is 
divided between the two forms in roughly equal 
amounts. Can we not imagine a third condition in 
which this time the heat is almost wholly etherial, 
the material portion being insignificant? We can 
imagine it, no doubt ; but the interesting, and I believe 
significant, thing is that we do not find it in Nature. 
Licut PRESSURE. 
You have heard of the pressure of light—that light 
actually has mass and weight and momentum and 
exerts a minute pressure on any object which obstructs 
it. A beam of light or zther-waves is like a wind, a 
very minute wind as a rule; but the intense ztherial 
energy inside the star makes a strong wind. This 
wind distends the star. It bears to some extent the 
weight of the layers overhead, leaving less for the 
elasticity of the gas to bear. That, of course, has to 
be taken into account in our calculation of the internal 
temperatures—making them lower than the older 
theory supposed. Just as ether and matter share 
the heat-energy between them, so the etherial wind 
and the material elasticity share the burden of support- 
ing the weight of the layers above. We are able to 
calculate the proportions in which they share it. To 
a first approximation the same proportion holds 
throughout nearly the whole interior, and the proportion 
depends only on the total mass of the star—not on 
the density or even on the chemical composition of 
the material. Moreover, in order to make this calcula- 
tion we do not need any astronomical knowledge ; 
all the constants in the formula have been determined 
by the physicist in his laboratory. We need to know 
the average molecular weight of the material, but I 
shall tell you later how we are able to fix that approxi- 
mately in spite of not knowing what elements to expect 
in the star’s interior ; that happens to be one of the 
benefits of dealing with very high temperatures. 
Let us imagine a physicist on a cloud-bound planet, 
who has never heard tell of the stars, setting to work 

