186 PHYSICS OF THE ELECTRON 



Travershave shown that it yields about 1.8 cc. of helium per gram and 

 contains about 7 per cent of uranium. It can readily be deduced 

 from known data that each gram of the mineral contains about one 

 four-millionth of a gram of radium. Supposing that one gram of 

 radium produces ^ cc. of helium per year, the age of the mineral is 

 readily seen to be about 40 million years. If the above rate of pro- 

 duction of helium by radium is an overestimate, the time will be 

 correspondingly longer. I think there is little doubt that when the 

 data required are accurately known this method can be applied, 

 with considerable confidence, to determine the age of the radioactive 

 minerals. 



XI. Radioactivity of Ordinary Matter 



The property of radioactivity is exhibited to the most marked 

 extent by the radioactive substances found in pitchblende, but it is 

 natural to ask the question whether ordinary matter possesses this 

 property to an appreciable degree. The experiments that have so 

 far been made show conclusively that ordinary matter, if it possesses 

 this property at all, does so to a minute extent compared with 

 uranium. It has been found that all the matter that has so far been 

 examined shows undoubted traces of radioactivity, but it is very 

 difficult to show that the radioactivity observed is not due to a 

 minute trace of known radioactive matter. Even with the extra- 

 ordinarily delicate methods of detection of radioactivity, the effects 

 observed are so minute that a definite settlement of the question 

 is experimentally very difficult. J. J. Thomson has recently given an 

 account at the Meeting of the British Association of the work done on 

 this subject in the Cavendish Laboratory, and has brought forward 

 experimental evidence that strongly supports the view that ordinary 

 matter does show specific radioactivity. Different substances were 

 found to give out radiations that differed in quality as well as in 

 quantity. A promising beginning has already been made, but a great 

 deal of work still remains to be done before such an important con- 

 clusion can be considered to be definitely established. 



