JuLy 1, 1915] 
NATURE 
495 

a large amount of energy, for in some cases they are 
expelled very close to the velocity of light, which is 
the limiting velocity possible for such particles. The 
expulsion of high-speed £ particles is usually accom- 
panied by the appearance of y rays, which correspond 
to X-rays, only of greater penetrating power than has 
so far been obtained from an X-ray tube even when a 
high voltage is employed. The emission of energy in 
the form of y rays is not negligible, for in some cases 
it is even greater than the energy emitted in the form 
of high-speed 8 particles, and may amount per atom 
to as much as 20 per cent. of the energy released in 
the form of a swift o particle. 
By the application of a high voltage to a vacuum 
tube it is quite possible to produce types of radiation 
analogous to those spontaneously arising from radium. 
For example, if helium were one of the residual gases 
in the tube, some of its atoms would become charged, 
and would be set into swift motion in the strong 
electric field. In order, however, to acquire a velocity 
equal to the velocity of expulsion of an a particle, say, 
from radium C, even in the most favourable case nearly 
four million volts would have to be applied to the 
tube. 
In a similar way, in order to set an electron in 
motion with a velocity of 98 per cent. the velocity of 
light, at least two million volts would be necessary. 
As we have seen, it has not so far been found possible 
to produce X-rays from a vacuum tube as penetrating 
as the y rays. The study of the radiations from radio- 
active substances is thus of especial interest, not only 
for the information obtained on the structure of the 
atoms themselves, but also in providing for investiga- 
tion special types of radiation of greater individual 
_intensity than can be obtained by ordinary experi- 
mental methods. The enormous energy of motion of 
swift a and £ particles must exist in the atom before 
its disintegration, either in a potential or a kinetic 
form, and may arise either from the passage of the 
charged particles through the intense electric fields 
within the atom, or from the very swift motion of 
these particles within the atom before their release. 
In any case, there can be no doubt that electric fields, 
and possibly magnetic fields, of enormous intensity 
exist within the very small volume occupied by the 
essential structure of the atom—fields many million 
times greater in intensity than we can hope to produce 
in laboratory experiments. 
In order to explain certain experimental results, I 
have suggested that the main mass of the atom is 
concentrated within a minute volume or nucleus, 
which has a positive charge, and is of dimensions 
exceedingly minute compared with the diameter of 
the atom. This charged nucleus is surrounded by a 
distribution of electrons which may extend to dis- 
tances comparable with the diameter of the atom, as 
ordinarily understood. The general evidence indicates 
that the a and primary f particles are expelled from 
the nucleus, and not from the outer structure of the 
atom. If this be the case, the a particle which carries 
a positive charge would have its velocity increased in 
passing through the strong repulsive field surround- 
ing the nucleus; on the other hand, the f particle 
which carries a negative charge must be retarded 
in its escape from the nucleus, and must possess 
great initial energy of motion to escape at all. There 
appears to be no doubt that the penetrating y rays 
have their origin in some sort of disturbance in the 
rings of electrons nearest to the nucleus, but do not 
represent, as some have supposed, the vibrations of 
the nucleus itself. 
a Rays. 
A brief account was given of the recent work of 
Rutherford and Robinson in determining with accu- 
NO. 2383, VOL. 95] 
| racy the velocity of expulsion of the a particles from 
certain radio-active substances. This was done by 
measuring the deflection of a pencil of a rays in 
strong magnetic and electric fields. With the aid 
of intense sources of radiation, it was found that the 
value of E/M—the ratio of the charge carried by the 
a particle carried to its mass—was 4820 units, a value 
to be expected if helium has an atomic weight 4 and 
carries two unit charges. This experiment also 
shows that the mass of the flying positive particle 
is not affected appreciably by its swift motion. From 
known data the initial velocity of the expulsion of 
the a particles from all other radio-active substances 
can be deduced with accuracy. 
If the expulsion of an a particle from an atom is 
the result of an internal explosion, we should antici- 
pate, from the analogy of a shot from a gun, that the 
residual atom would recoil in a direction opposite to 
the escaping B particle. The existence of these 
“recoil”? atoms can be shown in a variety of ways, 
for the velocity of recoil is sufficient to cause the 
atoms to leave the surface on which they are deposited 
and to pass through a considerable distance in air 
at a pressure of one millimetre before they are stopped. 
It is to be anticipated that the momentum of a recoil- 
ing atom should be equal and opposite to that of the 
escaping a particle. Since the deflection of a charged 
particle in motion in a magnetic field is inversely 
proportional to its momentum, the deflection of a 
stream of recoiling atoms should be the same as for 
the a particles if the atoms carry the same charge. 
Dr. Makower has examined the deflection of a pencil 
of recoil atoms in a magnetic field, and found it to be 
exactly half of that due to the a particle, proving 
definitely that the recoiling atom carries only one unit 
of positive charge in place of two for the a particle. 
We thus see that the simple application of momen- 
tum enables us to deduce the mass and energy of the 
recoiling atoms. Since the mass of the radio-active 
atoms is about fifty times that of the a particle, the 
velocity, and also the energy, of recoil is only about 
1/soth of that of the escaping particle. In a similar 
way, it can be shown that the ejection of a swift 
B particle should cause a vigorous recoil of the atom, 
though not so marked as in the case of the more 
massive a particle. 
B Rays. 
During the last few years notable advances have 
been made in our knowledge of the mode of emission 
of B particles from radio-active atoms. The work of 
Baeyer, Hahn, and Meitner, and of Danysz, has 
shown that the 6 rays from a radio-active substance 
like radium B or radium C contain a number of 
definite groups of rays which are expelled with definite 
velocities. This is best shown photographically by 
examining the deflection of a pencil of 8 rays in a 
magnetic field. In a uniform field, each of the groups 
of rays describes a circular path the radius of which 
is inversely proportional to the momentum of the 
B particle. By the application of special methods it 
has been found possible to obtain a veritable spectrum 
of the B rays. The spectrum of the 8 rays from 
radium B and radium C has been very carefully 
examined by the writer and Mr. Robinson, and found 
to give a large number of well-marked bands, each of 
which represents a group of f rays, all of which are 
expelled with identical speed. It was at first thought 
that most of the energy of the B rays was comprised 
in these groups, as some of the bands on the photo- 
graphic plate were very marked. Chadwick, however, 
has recently shown that the fraction of the rays which 
give a line spectrum is only a few per cent. of the 
| total radiation. The general evidence shows that the 
B radiation from these substances gives a continuous 

