190 Prof. E. Rutherford on Radioactivity 



Discussion of the Results. 



Before entering on the question of the cause and nature of 

 induced radioactivity, a brief review may be given of the 

 results obtained : — 



(1) All thorium compounds examined produce radioactivity 

 in substances in their neighbourhood, if the bodies are all un- 

 charged. With charged conductors the radioactivity is 

 produced on the — charged body. In strong electric fields, 

 tbe radioactivity can be concentrate 1 on the surface of thin 

 wires. Thorium oxide is the most active of the thorium 

 compounds in causing radioactivity, but loses its power if it 

 is heated for several hours at a high temperature. 



(2) The power of producing radioactivity is closely con- 

 nected with the presence of the ''emanation " from thorium 

 compounds; and is in some way dependent upon it. 



(3) The radiation excited in bodies is homogeneous, and of 

 a more penetrating character than the radiations from thorium 

 or uranium. The radiation is confined to the surface of the 

 substance, and is independent of whether the substance is a 

 conductor or non-conductor and of the nature of its surface. 



(4) The intensity of the radiation emitted falls off in a 

 geometrical progression with the time, decreasing to half its 

 value in about 11 hours. The decay of intensity is inde- 

 pendent of the state of concentration of the radioactivity or 

 the nature of the substance. 



(5) The amount of induced radioactivity increases at first 

 nearlv proportional to the time of exposure, but soon tends 

 to a value when the intensity of the radiation varies very 

 little with increase of the time of exposure. 



(6) The amount of induced radioactivity produced in a 

 olven time on a conductor depends on the potential-difference 

 between the electrodes, and tends to a constant value for large 

 E.M.F/s. 



(7) The amount of radioactivity is independent of the 

 pressure of the gas, except at low pressures when the amount 

 on the — charged conductor decreases with the pressure. The 

 amount is not much affected whether the gas is hydrogen, air, 

 or carbonic acid. 



(8) Xo increase of weight has been observed by making a 

 bodv radio-active. The radiation from a platinum wire is 

 not much altered by placing the wire in a flame, hot or cold 

 water, or nitric acid. Hydrochloric and sulphuric acids 

 rapidlv remove the radioactivity from its surface. The 

 solution, when evaporated, leaves the active portion behind. 



Three possible explanations of the phenomena of induced 



