690 
NATURE 
[FEBRUARY 19, 1914 
explanation in the hypothesis of a discrete struc- 
ture of radiant energy. 
Chief of these is the observed mode of transfer 
of energy from kathode-rays to X-rays, and vice 
versa. Kathode rays are electrons projected with 
enormous velocities. The stoppage of an electron 
by the target in the Réntgen tube generates an 
X-ray pulse. All electrons are stopped within a 
time, which is the shorter the greater their energy 
of motion. Hence the X-ray pulse generated is 
“thin” in proportion as its energy is great. The 
more rapid the kathode rays, the thinner, 
“harder,” and more penetrating are the X-rays. 
Now the beautiful recent work on the reflection 
and interference of X-rays, often referred to in 
Nature, has proved that these rays are covered 
by the wave-theory of light. The X-ray waves 
are some 10,000 times shorter than the shortest 
ultra-violet light waves known. They have, like 
ordinary light, a wave-length, or rather a range of 
wave-lengths, and the energy of every X-ray wave 
is proportional to its frequency, since the thinner 
and “harder” pulses have the smaller wave- ° 
lengths. 
But this is not all. When X-rays impinge on a 
target, electrons are projected from it; they in turn 
constitute kathode rays. The velocity of these 
electrons is independent of the intensity of the 
X-ray beam. It only depends upon its “hardness,” 
i.e., its frequency, or the reciprocal of its wave- 
length. To put it in the language of visible light, 
the velocity with which an electron is expelled 
from the target depends, not upon the “bright- 
ness” of the X-rays, but solely upon their 
“colour,” and is the greater the more that 
colour tends towards the “blue” end of the 
spectrum. 
Moreover, those electrons which are not expelled 
from the material exposed to the X-rays appear 
to be quite unaffected, and they form the vast 
majority of the electrons present, unless a parti- 
cular “characteristic frequency” is used for the 
existing rays, whereupon the electrons come out 
in enormous numbers. 
The handing on of a quantity of energy intact 
from X-ray to kathode-ray and back to X-ray was 
used to support an atomistic view of the X-rays 
themselves, until it was found that the same rules 
apply to the liberation of electrons by ultra-violet 
light. Here arose a dilemma: either ultra-violet 
light itself (and probably all radiation) is atomic, 
or there #s some mechanism by which radiant 
energy can be absorbed until a definite quantity 
(proportional to the frequency) is accumulated, 
whereupon an electron is expelled. The remark- 
able thing is that this energy of the electron is 
actually derived from the light, so that the latter 
does not simply liberate internal energy by some 
sort of “trigger” action. 
All this might not have ensured a hearing for 
an atomistic hypothesis of energy had not Prof. 
Max Planck (now rector of Berlin University) put 
forward a theory of radiation based upon quite 
other considerations, which also involved an 
atomic structure of energy, at least when radi- 
NO. 2312, VOL. 92] 
| counted for the fact that, as a body gets hotter, 
ated.2. He was endeavouring to explain the ex- 
perimental fact that the total heat of all wave- 
lengths radiated by a blackbody (not a blackened 
body, but the “ideal” black represented, say, by 
the mouth of a deep cave) is proportional to the ~ 
fourth power of its absolute temperature, and 
found that no formula completely representing the 
relation between the frequency and the amount 
of energy associated with it could be written down 
unless the energy was flung out by each molecular — 
radiator in definite amounts or ‘“‘quanta”’ propor- 
tional to the frequency, i.e., inversely proportional 
to the “wave-length.” This immediately ac- 
it passes from “red” heat to “white” heat (i.e., 
towards higher frequencies) until, when we reach 
the temperature of the sun, the maximum energy 
is well within the visible spectrum. 1 
The actual magnitude of the supposed quanta 
is excessively small. For a frequency of 1 vibra- 
tion per second, it would only amount to 
6x 10-27 erg, a quantity known as the “action 
constant.’’ For frequencies like that of green — 
light (600 billion per second) it would still only 
amount to some billionths of an erg, but such is” 
the marvellous sensitiveness of the eye, that it 
can detect light (say, from a star of the sixth 
magnitude) when the amount of energy passing 
through the pupil is only some 300 or 400 quanta 
per second. , 
What, then, is the mechanism of this radia- 
tion by quanta? Are we to suppose that it 
resembles the sound waves proceeding from the 
incessant but irregular rifle fire of a large army, 
in which each soldier gradually accumulates suffi- 
cient powder to fire his shot? Or is it atomistic, 
like the bullets? Or must we fall back upon Sir 
J. J. Thomson’s bold but rather appalling concep-— 
tion of a gigantic web of countless threads per- 
vading the universe, in which each thread con- 
nects a positive and a negative electric atom, and 
bears its trembling message along with the speed 
of light in a single direction? j 
Whichever view may be finally adopted, we may 
be sure that the investigation of this fascinating 
problem will teach us a great deal about the inter-— 
stellar ether which conveys the messages. The 
recent German attempt to explain away the ether, 
known as the electromagnetic “Principle of Rela- 
tivity,” has failed in its main object. Gehrcke, in 
his preface to Drude’s “Lehrbuch der Optik,” 
describes that principle and its temporary sway as 
“the most notable case of mob suggestion since 
the days of the N-rays.” The hypothesis of 
quanta is saved from a similar failure by keeping 
in close touch with experiment. In the hands of 
Nernst and Lindemann and Debye it has been 
used with brilliant success for investigating and 
explaining the fall in the specific heat of all bodies — 
as we approach the absolute zero of temperature. 
The specific heat probably begins by being pro- 
portional to the cube of the absolute temperature, 
so that the heat energy of the body is proportional — 
to the fourth power, thus recalling the Stefan- 
2 “ Vorlesungen iiber Warmestrahlung,” 2nd edition. (Leipzig, Barth.) 
