18 SCIENCE PROGRESS 



PHYSICS. By James Rice, M.A., University of Liverpool. 

 In the January number of the Phil. Mag. Dr. R. T. Beatty 

 gives the first part of an investigation which he is carrying 

 out on the energy emitted by luminescent hydrogen in the 

 visible region, and the partition of it among the various qualities 

 corresponding to the lines H„, H^, H y , and H s . A Geissler 

 tube was used as the source, and the light from it was analysed 

 by a specially designed monochromator. The intensity was 

 measured by a photoelectric cell, previously calibrated by 

 means of a carbon filament glow lamp. Special devices were 

 used to measure the potential difference between the electrodes 

 of the hydrogen tube and the current passed through it. The 

 tendency for the tube to become heated by the passage of the 

 discharge, and so alter the pressure of the gas, was obviated 

 to some extent by employing a clockwork arrangement in 

 the primary circuit of the induction coil which alternated 

 periods of quiescence with periods of emission on the part of 

 the tube. By so doing, the tube was kept in a constant 

 condition, and readings of current and potential could be 

 repeated time after time in a satisfactory manner. The 

 photoelectric cell was placed with its window close to the 

 emergence slit of the monochromator and the current through 

 it, and therefore the intensity of the light falling on it was 

 measured by an ingenious null method. The tube was used 

 with the hydrogen dry, and also in the presence of water 

 vapour, and the currents corresponding to the incidence on 

 the cell of the light corresponding to the four lines mentioned 

 were measured. Results were obtained over a wide range 

 of pressures, and the ratios of the light intensities in the various 

 lines tabulated and graphed. According to Bohr's theory of 

 the Rutherford atom of hydrogen, its single electron may 

 rotate around its positive nucleus in any one of a series of 

 circular orbits whose radii bear to one another the same ratios 

 as the squares of the natural numbers, and radiation or absorp- 

 tion of light occurs when the electron leaves one of these 

 orbits and assumes another. Thus the lines of the Balmer 

 series are due to the passage of the electron to the second 

 circular path from any of the paths outside this, H a corre- 

 sponding, for instance, to a passage from path 3 to path 2, 

 Hp to passage from path 4 to path 2, and so on. In the normal 

 condition the electron of a hydrogen atom is rotating in the 



