Page:Radio-activity.djvu/481

 the primary active substance has been isolated and its activity measured.

262. Origin of radium. The changes in radium are thus fairly rapid, and a mass of radium if left to itself should in the course of a few thousand years have lost a large proportion of its radio-activity. Taking the above estimate of the life of radium, the value of λ is 5·4 × 10^{-4}, with a year as the unit of time. A mass of radium left to itself should be half transformed in 1300 years and only one-millionth part would remain after 26,000 years. Thus supposing, for illustration, that the earth was originally composed of pure radium, its activity per gram 26,000 years later would not be greater than the activity observed to-day in a good specimen of pitchblende. Even supposing this estimate of the life of radium is too small, the time required for the radium practically to disappear is short compared with the probable age of the earth. We are thus forced to the conclusion that radium is being continuously produced in the earth, unless the very improbable assumption is made, that radium was in some way suddenly formed at a date recent in comparison with the age of the earth. It was early suggested by Rutherford and Soddy that radium might be a disintegration product of one of the radio-elements found in pitchblende. Both uranium and thorium fulfil the conditions required in a possible source of production of radium. Both are present in pitchblende, have atomic weights greater than that of radium, and have rates of change which are slow compared with that of radium. In some respects, uranium fulfils the conditions required better than thorium; for it has not been observed that minerals rich in thorium contain much radium, while on the other hand, the pitchblendes containing the most radium contain a large proportion of uranium.

If radium is not produced from uranium, it is certainly a remarkable coincidence that the greatest activity of pitchblende yet observed is about five or six times that of uranium. Since radium has a life short compared with that of uranium, the amount of radium produced should reach a maximum value after a few thousand years, when the rate of production of fresh radium*