576 The Evoliitio7i of Matter 



curve. Tlius the activity of a rod exposed to radium emanation for 

 1 minute decays in accordance with the curve of Fig, 2, which 

 represents the activity as measured by the a-rays. If the electro- 

 scope be screened from the a-rays, it is found that the activity of the 

 rod in ^- and y-rays increases for some 35 minutes and then diminishes. 

 (Fig. 3.) 



These complicated relations have been explained satisfactorily 

 and completely by Rutherford on the hypothesis of successive changes 

 of the radio-active matter into one new body after another \ The 

 experimental curve represents the resultant activity of all the matter 

 present at a given moment, and the process of disentangling the 

 component effects consists in finding a number of curves, which 

 express the rise and fall of activity of each kind of matter as it is 

 produced and decays, and, fitted together, give the curve of the 

 experiments. 



Otlier methods of investigation also are open. They have enabled 

 Rutherford to complete the life-history of radium and its products, 

 and to clear up doubtful points left by the analysis of the curves. 

 By the removal of the emanation, the activity of radium itself has 

 been shown to consist solely of a-rays. This removal can be 

 effected by passing air through the solution of a radium salt. The 

 emanation comes away, and the activity of the deposit which it 

 leaves behind decays rapidly to a small fraction of its initial 

 value. Again, some of the active deposits of the emanation are 

 more volatile than others, and can be separated from them by the 

 agency of heat. 



From such evidence Rutherford has traced a long series of dis- 

 integi-ation products of radium, all but the first of which exist in 

 much too minute quantities to be detected otherwise than by their 

 radio-activities. Moreover, two of these products are not them- 

 selves appreciably radio-active, though they are born from radio- 

 active parents, and give rise to a series of radio-active descendants. 

 Their presence is inferred from such evidence as the rise of /3 and <y 

 radio-activity in the solid newly deposited by the emanation ; this 

 rise measuring the growth of the first radio-active offspring of one of 

 the non-active bodies. Some of the radium products give out a-rays 

 only, one /3- and 7-rays, while one yields all three types of radiation. 

 The pedigiee of the radium family may be expressed in the following 

 table, the time noted in the second column being the time re- 

 quired for a given quantity to be half transformed into its next 

 derivative. 



> Rutherford, Eadio-activity (2nd edit.), Cambridge, 1905, p. 379. 



