34 POPULAR SCIENCE MONTHLY. 



factors have, however, not yet been determined with sufficient accuracy 

 to make at present more than a rough estimate of the age of any par- 

 ticular mineral. An estimate of the rate of production of helium by 

 radium has been made by Eamsay and Soddy by an indirect method. 

 It can be deduced from their result that 1 gram of radium produces per 

 year a volume of helium of about 25 cubic mms. at standard pressure 

 and temperature. They, however, consider this to be an under esti- 

 mate. On the other hand, if the particle is a helium atom, it can 

 be calculated that 1 gram of radium produces per year about 200 cubic 

 mms. of helium. 



Let us consider, for example, the mineral fergusonite. Eamsay 

 and Travers have shown that it yields about 1.8 c.c. of helium and 

 contains about 7 per cent, of uranium. It can 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 

 % c.c. of helium per year, the age of the mineral is readily seen to be 

 about 40 million years. If the above rate of production of helium is 

 an overestimate, the time will be correspondingly longer. I think 

 there is little doubt that, when the data required are known with 

 accuracy, this method can be applied with considerable confidence to 

 determine the age of the radioactive minerals. 



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 our extraordinarily delicate methods for 

 the 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 British Association 

 meeting of the work done on this subject in the Cavendish Laboratory, 

 and has brought forward experimental evidence that strongly sup- 

 ports 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 conclusion can be considered to have been definitely 

 established. 



