1912] on the Origin of Radium. 403 



average life of radinin was not greater than a few thousand years. The 

 present estimate, due to Rutherford, is 2500 years. A few thousand 

 years hence the radium in existence to-day will for the most part 

 have disintegrated. Very little of the radium in existence at the 

 time the Pyramids were being built can still exist. Hence arose 

 one of the most interesting and crucial of the prol)lems of atomic 

 disintegration. Does the regeneration of radioactive constituents, 

 observed in the cases where the period is short compared to the span 

 of human life, apply also to radium — to an element, that is, with a 

 definite spectrum, atomic weight and chemical character, filling a 

 vacant place in the periodic system, and forming one of a family of 

 common elements ? After the separation of radium from a mineral 

 does the non -radium part of the mineral grow a fresh crop with lapse 

 of time, the quantity present before separation being the balance or 

 equilibrium quantity when the rate of production is equal to the rate 

 of supply ? A somewhat similar prediction made with reference to 

 the production of another well-defined element, helium, in the radio- 

 active process had only to be tested, as it was first in 1903 by 

 Sir AVilliam Ramsay and myself, to be proved correct. The question, 

 however, of the origin of radium is still, in spite of many discoveries, 

 not entirely solved. 



At first sight the experimental trial of the view appeared easy. 

 This problem is not analogous to the finding of a meteor after its 

 flight is spent. The quantities of radium which can be detected and 

 recognized unequivocally by radioactive methods are thousands of 

 times smaller than can be detected even by the spectroscope, sensitive 

 as the spectroscopic test of radium is. The first product of the dis- 

 integration of radium is a gas, the radium emanation, and the test 

 for radium consists in sealing up a solution of the substance for a 

 month, then boiling the solution in a current of air, and introducing 

 this air into the electroscope. For the instrument employed and 

 shown (Fig. 7), a millionth of a milligram of radium would be rather 

 an undesirably large quantity, whilst a few hundredths of this amount 

 is the best suited for accurate measurement. Tlie volume of radium 

 emanation, measured at N.T.P., obtainable from one gram of radium 

 is only • 6 cubic millimetre. If a thousandth part of this quantity 

 were distributed uniformly through the air of this room, estimated as 

 50,000 cubic feet, or about 1§ tons by weight, and the electroscope 

 were then filled with the air of the room, it would produce an effect 

 much greater than any dealt with in the work to be described. (The 

 effect of breaking a tube containing the emanation in equilibrium 

 with 8 mg. of radium outside in front of the fan supplying air to 

 the building was demonstrated by the electroscope.) 



Only two primary radio-elements, uranium and thorium, were 



known changing sulficiently slowly to account for the maintenance of 



radium in the earth to-day. Indeed, these two elements are the only 



ones known with atomic weights greater than that of radium, and are 



Vol. XX. (No. 106) 2 e 



