190 G. F. BECKER ISOSTASY AND RADIOACTIVITY 



of helium Avliieh would escape from a system in equilibrium in a given 

 time. Then, after it had been shown by Mr. Strutt that some minerals, 

 notably zircon, retain at least most of the helium developed in them, 

 the time within whicli this helium could be evolved could be calculated 

 and Avas supposed to give an inferior limit for the age of the crystal.®^ 

 Similarly, after Mr. Boltwood had shown it probable that lead is a stable 

 end product of the decay of radioactive substances, he suggested that the 

 Ph/lJ ratio would serve as a means of determining the age of plumlufe- 

 rous uraninites. 



These interesting and perfectly legitimate efforts, however, led to diffi- 

 culties of which more hereafter. 



Largely through researches in radiology, several investigators, chief 

 among them Sir Ernest Eutherford, have developed new ideas of the 

 structure of atoms, and indeed of the nature of the chemical elements. 



It is Avell known that the Periodic Table of Mendeleef, arranged in the 

 order of the atomic weights of the elements, has been of great service to 

 chemistry and led in the hands of its inventor to the prediction of new 

 elements, which were duly discovered; yet the table was empirical and 

 exhibited puzzling irregularities. In recent years it has been given a 

 new and more satisfactory inteip relation, originating with Mr. A. van 

 den Broek,^^ who arranges the elements according to the number of 

 positive electric charges on the nucleus of the atom as conceived by 

 Rutherford. These charges advance by units from 1, the so-called 

 "atomic number" of hydrogen, to 92, the atomic number of uraniuin. 

 To eacli atomic number is supposed to belong an element (or possibly 

 a grou]) of elements), and only three or four gaps in the series now. 

 remain to be filled by discoveries. 



Now comes the astounding feature of the subject. It has been defi- 

 nitely discovered bv ^fr. Soddv, Sir Ernest Eutherford, and others that 

 a single atomic number may be borne by each of several substances which 

 may have different atomic weights and, in the case of radioactive sub- 

 stances, different stabilities, but which are inseparable by ordinary chem- 

 ical or physical properties. They display the same chemical reactions, 

 the same electrochemical behavior, the same spectrum, the same volatility. 

 It would appear, according to Eutherford, that the charge on the nucleus 

 is the fundamental constant which determines the physical and chemical 

 properties of the atom.^^ Soddy calls the members of a group of elements 



^1 Joly and Rutherford have devised a means of estimating the age of rocks from the 

 pleochroic halos in mica foils, these halos being due to radloactivitv. Phil. Mag., vol. 2.5, 

 ini3, p. 644. 



M Nature, vol. 92, lOl."?. pp. ."^72 and 476. 



°3 Nature, vol. 92, 191.S, p. 423, and Phil. Mag., vol. 27, 1914, p. 488. 



