56 
PEOFESSOR E. RUTHERFORD AND :ME. R. K. McCLUXG 
Ill previoii-s papers by Owens* and Rutherford,! the behaviour of thorium 
oxide as a radio-active substance has been carefully examined. It has been shown 
that thorium compounds give out a material emission of some kind, which possesses 
temporary radio-active projierties. This emanation is most apparent with thick layers 
of thorium oxide. In the present case the layer was not thick enough to give out 
much emanation, and the rate of discharge was due to the radiation alone. Owens 
has shown that the radiation from thoiium is approximately homogeneous. 
The value of X, the coefficient of absorption of thorium radiation in air, is ‘69 and 
d = 4-1 1 — — -96. 
Thus for a weight of ‘665 gramme the total energy radiated into the gas per unit 
iW 
area = same assumptions as for uranium, the energy radiated 
into the air per second = 1-2 X 10“^^ calorie, a somewhat greater value than for an 
equal weight of uranium oxide. 
Excited Radio-activity due to Thorium. 
Thorium compounds, in addition to the property of giving out a radio-active emana¬ 
tion, possess the power of exciting temporary radio-activity on all substances in their 
neighbourhood. The excited radiation is homogeneous in character, and is of a more 
penetrating type than the radiation from either uranium or thoiium. The intensity 
of the excited radio-activity can lie greatly increased by concentration on the negative 
electrode of small area by means of a strong electric field. On the assumption that 
the energy of the radiation excited on the electrode is dissipated in producing ions, 
an estimate can be formed of the energy stored up on the electrode. 
In a particular experiment a fine platinum wire, '018 centim. in diameter and 
1 centim. long, caused the separation of about 10 coulombs of electricity before the 
radio-active power was lost. This corresponds to an emission of 2 X 10“° calorie. 
This by no means inconsiderable quantity of energy is in some way derived from 
the surface of a platinum wire '056 sq. centim. in area, without the slightest appre¬ 
ciable change either in the weight or appeai'ance of the wire. 
Radium and Polonium^ 
The question of the equality of the velocity of the ions, produced by thorium 
radiation and the rays from the powerful radio-active substances radium and polonium, 
with the velocity of the ions produced by X rays, has not been specially investigated, 
but from the very close similarity of the types of these radiations, it seems very 
probable that the ions produced by all are the same. 
* ‘Phil. XIag.,’ October, 1899. 
t ‘Phil. Mag.,’ January and February, 1900. 
