CONTEMPORARY ADVANCES IN PHYSICS 



329 



llrey, Brickwedde and Murphy. At first they did not expect to 

 distinguish such an isotope in ordinary hydrogen; but they inferred 

 from thermodynamical theory that a greater than the normal pro- 

 portion (of H^H^ molecules among the ordinary H^H^ molecules) should 

 be obtained by Hquefying large quantities of gas and letting it re- 

 evaporate at a low pressure, taking for their investigation the last two 

 or three cubic centimetres of liquid out of several thousand. It 

 turned out later that they could detect H^, or "deuterium" as they 

 have named it, in ordinary hydrogen; but in these special samples the 

 evidence of it was far more patent. 



This evidence is the advent of "shifted lines" in the ordinary line- 

 spectrum of atomic hydrogen. The frequencies of atomic spectrum- 

 lines depend on the ratio of the masses of electron and nucleus, in a 

 manner which in times past has been of the utmost value in establishing 

 the present-day model of the atom ^*; the difference between the values 

 of this ratio for H^ and H^ is only that between 1/1850 and 1/3700, 

 and results in a frequency-difference of less than three parts in ten 

 thousand, and yet the corresponding wave-length-difference is easy to 

 detect with spectroscopes. The existence of H^ entails that each of 

 the familiar spectrum-lines of H^ should be attended by a faint com- 

 panion, displaced by this percentage toward lesser wave-lengths. 

 Urey, Brickwedde and Murphy observed the faint companions of the 

 four most prominent of the Balmer lines; others have since observed 

 them, and just before these pages started for the press, there appeared 

 the photographs ^^ taken by Ballard and White of four lines of the 

 Lyman series with their companions, which I reproduce as Fig. 9. 



H^H- 



[I'H- 



H'H= 



H'H- 



H'H2 



1st 



1st 



2nd 



3rd 



4th 



2nd order 1st order 



Fig. 9 — Lines of the Lyman series of ordinary hydrogen (H') accompanied by the 

 corresponding lines of H^. The first picture on the left shows the first line of the 

 series, photographed in second order; the others show the first four lines of the series 

 from left to right, photographed in first order. (S. S. Ballard & H. E. White; Physical 

 Review.) 



i» Cf. the ninth of this series of articles (October, 1925), or my Introduction to 

 Contemporary Physics, pp. 308-312. 



>9 1 am indebted to Messrs. Ballard and White for sending me the origmal of this 

 picture. 



