84 B. B. Boltwood — Ultimate Disintegration Products 



ratios are not contradictory to the order of the ages attributed 

 by geologists to the formations in which the different minerals 

 occur. 



From the data which have been presented in the preceding 

 tables it is apparent that the requirements for a disintegration 

 product of uranium are fulfilled by lead within the limits of 

 probable experimental error. On the basis of this evidence 

 the assumption would appear to be justified that lead is the 

 final product of uranium. 



Helium. 



Few experimental determinations of the relative quantities 

 of helium in minerals of known composition are to be found 

 in the literature. A careful search has brought to light only 

 the following : Twelve determinations by Hillebrand* of the 

 "nitrogen" present in an equal number of samples of urani- 

 nites of known composition ; the determination by Ramsay 

 and Traversf of the per cent of helium in a sample of fergu- 

 sonite, the analysis but not the locality of which is given ; the 

 determinations by Strutt;}; of the amounts of helium in a num- 

 ber of minerals which had been analyzed for uranium only ; a 

 determination by Dunstan and Blake§ of the helium in an 

 analyzed sample of thorianite from the Sabaragamuwa province, 

 Ceylon ; and the determination of helium in another specimen 

 of the same mineral by Biichner.| 



Considering the great exactness of all the analytical work 

 carried out by Hillebrand, and the general method which he 

 followed in his determinations of " nitrogen," it is highly 

 probable that by dividing the values which he gives in his 

 paper by seven ( JST 2 : He = 28 : 4) a very reliable number for 

 the percentage of helium is obtained. 



It has been shown conclusively by a number of different 

 experimenters that the disintegration of radium is accompanied 

 by the production of helium, and it is further stated by De- 

 bierne^[ that the disintegration of actinium furnishes helium 

 also. If the assumption is made on the basis of analogy that 

 the entire change from uranium to lead is accompanied by the 

 production of helium,. then the quantities of matter involved 

 in this change can be represented, by the equation 



Uranium (238-5) = lead (206*9) + helium (31'6), 



in other words, that for every 207 parts of lead there will be 

 formed 32 parts of helium. 



From a knowledge of the amount of lead present in the min- 

 erals it is, therefore, possible to calculate the amount of helium 



*Tliis Journal, xl, 384, 1890 ; ibid., xlii, 390, 1891. 



fProc, Roy. Soc. Lond., lii, 316, 1898. Jlbid., Lond., Ixxvi (A), 88, 1905. 



%l. c. | Nature, lxxv, 165. 1906. IfC. E., cxli, 383, 1905. 



