58 
PROFESSOR E. RUTHERFORD AXD MR. R. K. McCLUXG 
In some experiments Curie mentions using a specimen of radium 100,000 tunes 
more active than uranium. It this applies to measurement for equal weights and 
areas of radium and uranium, then the total energy radiated out into the gas by 
1 gramme weight of radium is not less than 10—^ calorie per second, or 3200 calories 
per year. 
It is evident that, unless energy is supplied from external sources, the substance 
cannot continue emitting energy at such a rate for many years, even supposing a 
considerable amount of energy may possibly be derived from rearrangements of the 
components of the molecule. 
In the light of the results on the amount of energy given out by radio-active 
substances, it is of interest to consider some speculations as to the origin of the rays, 
and of the su|)ply of energy required for a continuous emission of the radiation. 
We will first briefly review the state of our present knowledge of the radio-active 
substances. Uranium, the first of the radio-active sulxstances discovered, has been 
closely investigated. Becquerel has shown that it gives out radiation constant!}' 
from year to year, even when placed in the dark. The radio-activity is preserved in 
solution, and persists if the substance is recrystallised in the dark. The radiation 
given out is independent of the gas around it, and of the pressure of the gas, and is 
not much affected by considerable changes of temperature. The same radiation is 
given out by all the uranium compounds. The radio-activity appears to depend on the 
uranium molecule alone, and not what it is comliined with. Pitcliblende and other 
uranium minerals are active, and, as far as exiieiiments have gone, continue radiating 
indefinitely. 
In considering the question of the emission of energy per unit weight of uranium, 
an important point arises which it is difiicidt to decide satisfactorily by experiment, 
viz., whether the radio-activity is confined to the surface or possessed by the whole 
mass of the substance. At first sight the radio-activity appears to be superficial, 
since the intensity of the radiation does not increase with increase of thickness of 
uranium. Such an action, however, is to be expected, even though there is volume 
radio-activity, since the radiation can C)nly penetrate to the surface from a very short 
de])th below the surface. The inci-ease of the intensity of the radiation with increase 
of thickness for thin layers and the action of solutions support, as far as they go, the 
supposition that the activity is throughout the volume. The energy given out in the 
interior of tlie substance would most probably be dissipated as heat in the material. 
If the radio-active power is possessed by the whole volume, it follows from the above 
supposition that the mineral pitchblende must have been radiating energy since its 
formation as a mineral. If we supjiose the radiation lias heen going on constantly at 
its present rate in the course of 10,000,000 years, each gramme of uranium has 
radiated at least 300,000 calories. 
It is difficult to suppose that such a quantity of energy can be derived from 
regroiqiing of the atoms or molecular recombinations on the ordinary chemical theory. 
