CH. Iv] NATURE OF THE RADIATIONS 91 
Examined in these ways, 1t has been found that there are three 
different types of radiation emitted from radio-active bodies, which 
for brevity and convenience have been termed the a, 8, and ¥ rays. 
(a) The @ rays are very readily absorbed by thin metal foil 
and by a few centimetres of air. They have been shown to consist 
of positively charged bodies projected with a velocity of about 
1/10 the velocity of light. They are deflected by mtense mag- 
netic and electric fields, but the amount of deviation is minute 
in comparison with the deviation, under the same conditions, of 
the cathode rays produced in a vacuum tube. 
(i) The 8 rays are far more penetrating in character than the 
a rays, and consist of negatively charged bodies projected with 
velocities of the same order as the velocity of light. They are far 
more readily deflected than the « rays and are in fact identical 
with the cathode rays produced in a vacuum tube. 
(i) The y rays are extremely penetrating, and non-deviable 
by a magnetic field. Their true nature is not yet known, but they 
are analogous 1n some respects to very penetrating Réntgen rays. 
The three best known radio-active substances, uranium, thorium, 
and radium, all give out these three types of rays, each in an amount 
approximately proportional to its relative activity. Polonium 
stands alone in giving only the a@ or easily absorbed rays?. 
66. Deflection of the rays. The rays emitted from the 
active bodies thus present a very close analogy with the rays which 
are produced in a highly exhausted vacuum tube when an electric 
1 In an examination of uranium the writer (Phil. Mag. Jan. 1899) found that 
the rays from uranium consist of two kinds, differing greatly in penetrating power, 
which were called the a and B rays. Later, it was found that similar types of rays 
were emitted by thorium andradium. On the discovery of very penetrating rays from 
uranium and thorium as well as in radium, the term y was applied to them by the 
writer. The word ‘‘ray ” has been retained in this work, although it is now settled 
that the a and 8 rays consist of material particles projected with great velocity. The 
term is thus used in the same sense as by Newton, who applied it in the Principia 
to the stream of corpuscles which he believed to be responsible for the phenomenon 
of light. In some recent papers the a and 8 rays have been called the a and 6 
‘‘emanations.” This nomenclature cannot fail to lead to confusion, since the 
term ‘‘radio-active emanation” has already been generally adopted in radio- 
activity as applying to the material substance which gradually diffuses from thorium 
and radium compounds, and itself emits rays. 
