x] RADIO-ACTIVE PROCESSES 321 
the slightest experimental evidence has been adduced to show 
that the energy of radium is derived from external sources. 
J. J. Thomson (loc. cit.) has discussed the question in the 
following way :— 
“Tt has been suggested that the radium derives its energy from 
the air surrounding it, that the atoms of radium possess the faculty 
of abstracting the kinetic energy from the more rapidly moving air 
molecules while they are able to retain their own energy when in 
collision with the slowly moving molecules of air. I cannot see, 
however, that even the possession of this property would explain 
the behaviour of radium ; for imagine a portion of radium placed 
in a cavity in a block of ice; the ice around the radium gets 
melted; where does the energy for this come from? By the hypo- 
theses there is no change in the air-radium system in the cavity, 
for the energy gained by the radium is lost by the air, while heat 
cannot flow into the cavity from the outside, for the melted ice 
round the cavity is hotter than the ice surrounding it.” 
The writer has recently found that the activity of radium is 
not altered by surrounding it with a large mass of lead. A cylinder 
of lead was cast 10 cms. in diameter and 10 cms. high. A hole 
was bored in one end of the cylinder to the centre, and the radium, 
enclosed in a small glass tube, was placed in the cavity. The 
opening was then hermetically closed. The activity was measured 
by the rate of discharge of an electroscope by the y rays trans- 
mitted through the lead, but no appreciable change was observed 
during a period of one month. 
M. and Mme Curie early made the suggestion that the radiation 
of energy from the radio-active bodies might be accounted for by 
supposing that space is traversed by a type of Réntgen rays, and 
that the radio-elements possess the property of absorbing them. 
Recent experiments (section 215) have shown that there is present 
at the surface of the earth a very penetrating type of rays, similar 
to the y rays of radium. Even if it were supposed that the radio- 
elements possessed the power of absorbing this radiation, the 
energy of the rays is far too minute to account even for the energy 
radiated from an element of small activity like uranium. In 
addition, all the evidence so far obtained points to the conclusion 
that the radio-active bodies do not absorb the type of rays they 
ih 1soh\e 21 
