188 Prof. E. Rutherford on Radioacticity 



try and throw some light on the question as to what the 

 induced radioactivity is really due to. 



If the radioactivity is caused by some radio-active dust 

 deposited on the substance, we should expect to find evidence 

 of it by examining the surface with a microscope, or by noting 

 whether there is any increase in weight. A tine piece of platinum 

 wire, which had been carefully weighed, was made strongly 

 radio-active by five days' exposure to thorium oxide covered 

 over with paper. Within the limits of accuracy of the balance 

 no certain variation of the weight could be detected. The 

 increase of weight, if any, was certainly less than ^ of a 

 milligramme. On examination by a microscope no collection 

 of dust particles on the surface could be observed. We may 

 conclude from this experiment, that if the radioactivity is due 

 to the deposition of radio-active particles on the surface, these 

 particles must be extraordinarily radio-active compared with 

 their weight. A rough estimate shows that the radioactivity 

 of the surface-layer must be at least a million times greater 

 than that of uranium or thorium. 



The amount of radiation from an active surface is always 

 lessened by mechanical actions, such as rubbing the surface 

 with a cloth or fine sand-paper. In order to completely 

 remove the radioactivity, it is necessary to remove the 

 surface-layer by long scouring with sand- or emery-paper. 



A blast of air directed against a radio-active plate has no 

 appreciable effect on the amount of radiation given out. 



A radio-active platinum wire or plate can be heated white- 

 hot without much altering the amount of radiation given out 

 from it. A strongly active fine wire is more affected than a 

 plate; but that is probably chiefly due to action of the flame- 

 gases upon it. 



Chemical Actions. 



The radioactivity of a platinum plate is not much affected 

 by dipping it in water, caustic soda, or nitric acid, whether 

 hot or cold. Sulphuric or hydrochloric acid has the power of 

 rapidly destroying the intensity of the radiation in a few 

 minutes. A copper-sulphate solution, if only slightly acid, 

 does not act on the wire rapidly. The following example of 

 a test shows the effect o£ several of the solutions on a radio- 

 active platinum plate. After each immersion the plate was 

 washed in water and dried over a Bunsen-flame. After 

 exposure ot 4 minutes to gradually heated water and 2 minutes 

 to boiling water, the rate of discharge fell from 100 divisions 



