_ assemblage of minute particles—electrons. 

May 26, 1923] 
NATURE 
711 

that anything can be too small or too trifling to deserve 
his attention.”” The modern development of the valve, 
through the researches of those who have brought it 
to its present excellence, has rested on a still smaller 
entity, the electron, a body with a mass of 0900 x 10727 
grams, about 1/1800 of the atom of hydrogen, carrying 
a negative charge of 1591x10~” electro-magnetic 
units ® of electricity, first glimpsed by Crookes, then 
proved to exist by J. J. Thomson. 
The appearance of a Crookes tube or vacuum tube 
when carrying an electric discharge is well known, 
When the pressure is sufficiently reduced, the tube is 
non-luminous except for a beam of light which proceeds 
normally from the cathode—the negative electrode— 
and penetrates into the tube a distance depending on 
the pressure ; this beam constitutes the cathode rays : 
if the rays strike the glass at the end of the tube, a vivid 
fluorescence is produced. 
Crookes showed that the beam constituted a current 
of negative electricity ; it could be deflected by a 
magnet. Experiments by Perrin and J. J. Thomson 
proved conclusively the existence of the negative charge. 
Thomson showed also that the stream consisted of an 
He meas- 
ured the velocity of the particles and the ratio e/m of 
the charge on each to its mass. Further experiments, 
of which perhaps those of Millikan are the most im- 
portant, have led to a determination of the charge on 
the electron, and from this and a knowledge of the 
ratio e/m the values of e and m are found. These 
values are the same whatever be the nature of the 
cathode from which the rays take their origin—the 
mass and charge of an electron are the same whatever 
be its source. Thus now it is scarcely too much to 
say that nearly all electrical phenomena are conditioned 
by the presence and motion of electrons. The current 
in a cable is a stream of electrons ; a conductor is a 
body through which they move freely ; an insulator 
checks their activity. The power that drives our 
motors comes from them; the light of the electric 
lamp, the heat that comes from an electric radiator, 
have their origin in these tiny particles; the plate 
current of the valve rectifier referred to above is a 
stream of electrons; when the grid is negatively 
electrified, it adds negative electrons to the stream ; 
when it is positive, some of the electrons from the 
filament are stopped in their passage through its inter- 
Stices to neutralise the positive electricity it possesses. 
Electrical engineering in its many branches is closely 
bound up with the properties of an electron discovered 
by men whose sole object it was to advance natural 
knowledge. Nor is this all: for from the electron 
* One electro-magnetic unit is the charge transferred by 1 ampere cir- 
culating for ro seconds. 
‘came X-rays, though this, perhaps, is scarcely the 
correct way of putting it, as J. J. Thomson’s discovery 
really followed that of Réntgen. About 1894, physicists 
in many countries were experimenting with Crookes’s 
cathode rays. A chance observation made by a skilled 
worker revealed the fact that the cathode rays produced 
an effect outside the tube in which they were generated, 
R6ntgen in the autumn of 1895 was conducting an 
investigation with a vacuum tube wrapped in light- 
proof paper, and noted that a fluorescent screen of 
barium platino-cyanide lying near shone out when the 
tube was excited ; if he placed opaque objects between 
the screen and the tube, shadows were cast on the 
screen, showing that rays, the X-rays, proceeded from 
the tube in straight lines; and it was quickly found 
that the rays penetrated substances opaque to light, 
the penetration depending on the density of the sub- 
stance. There is no need to dwell on the results that 
have followed from this and their significance to 
engineers. X-rays can penetrate 4 to 5 mm. of lead, 
12 mm. of tin, 75 mm. of carbon steel, 100 to 150 mm. 
of aluminium, and 300 to goo mm. of wood. By their 
‘aid hidden cracks or faulty welds can be shown upon 
metal structures, while they have been employed for 
many industrial purposes, besides their use in surgery 
and medicine. 
For some time the nature of X-rays was a mystery. 
Their rectilinear propagation and the absence of re- 
fraction when they fell obliquely on the surface of a 
-medium other than air were difficult of explanation. 
Now it is known that they are produced by a very 
rapid change of motion of electrons. When the velocity 
of an electron is altered, an electro-magnetic wave is 
produced, and, starting from the electron, travels 
outward with the velocity of light. The frequency in 
this wave—in the number of vibrations per second 
produced—depends on the suddenness ot the change of 
velocity of the electron. If this is very great, the 
frequency in the resulting wave is also very great. 
When a beam of cathode rays falls on the glass 
walls or on the anti-cathode of an X-ray bulb, 
the electrons are stopped almost instantaneously. 
Electro-magnetic rays of very high frequency— 
X-rays—are produced. Their wave-lengths are now 
known to lie between 12x 1078 cm. and o*17 x 1078 
em. The wave-length of visible light is between 
7700x108 cm. and 3600x10-8 cm., that of ultra- 
violet light lies between 3600 x 10-8 cm. and 200 x 10-8 
cm., and it is to this minuteness of wave-length 
that the absence of refraction is due. In the 
hands of Sir William and Prof. W. L. Bragg, it has 
been the means of revealing the inner structure of 
materials in a manner which is of the utmost im- 
portance to engineers. 
Terrestrial Magnetism and the Orientation Faculty of Birds. 
ee possible existence of a “ magnetic sense” in | mysterious power of geographical orientation which is 
animals has for long been a subject of specula- 
tion, and Lord Kelvin is numbered among those to 
displayed by many animals and by primitive man. 
The idea has often been invoked in the case of the 
whom the idea has proved attractive. No direct | especially remarkable powers of orientation which are 
evidence in its favour has ever been obtained ; but, on 
the other hand, there is no actual proof that some form 
of physiological sensibility to the phenomena of terres- 
trial magnetism may not exist and be a factor in that 
NO. 2795, VOL. II11] 
possessed by migratory birds and by homing pigeons, 
and it is to be feared that much loose talk has at times 
been indulged in on this particular point. A recent 
author (F. Cathelin, ‘‘ Les Migrations des Oiseaux,” 
