540 SCIENCE PROGRESS 



for measuring the very diverse intensities of the direct and 

 scattered beams. Chadwick uses the scintillation method of 

 measurement, but he employs an arrangement whereby the 

 number of scattered particles reaching the fluorescent screen 

 is increased ; and, further, he employs a device suggested by 

 Sir E, Rutherford, which permits the measurement of the 

 intensities of the original and scattered beams on the same 

 screen. The respective atomic numbers of platinum, silver, 

 and copper are 78, 47, and 29, and it is found that the charges 

 on the nuclei of these elements are respectively 77-4 e, 46-3 e, 

 and 29-3 e, within an error of about i to i| per cent., the 

 accuracy of the experiments being limited by the probability 

 fluctuations. It is further shown that in the neighbourhood 

 of 10 ~" cms. from the platinum nucleus the law of force varies 

 as ^jr^, where p lies between 1*97 and 2*03. 



Prof. T. R. Merton {Proc. Royal Society, No. A, 691) describes 

 some interesting experiments designed to investigate the effect 

 of concentration on the spectra of luminous gases. It was 

 suggested by Lord Rayleigh that the broadening of the spectral 

 lines of an element in a Bunsen flame is due to temporary 

 association of the atoms or to the influence of neighbouring 

 atoms on the radiating particles. If a large quantity of sodium 

 is added to a flame containing a small quantity of lithium, or a 

 large quantity of lithium to a flame containing a small amount 

 of sodium, it might be expected, from the chemical similarity of 

 lithium and sodium, that temporary associations of lithium 

 and sodium atoms take place, with resulting broadening of the 

 lithium or sodium lines respectively ; but Prof. Merton finds 

 no trace of this effect in experiments conducted on flames or 

 arcs in vacuo. In the case of helium and hydrogen lines 

 produced by vacuum tubes, the broadening of the lines is 

 produced by electrical resolution due to the electric fields of 

 charged atoms in the immediate neighbourhood of the radiating 

 particles. A vacuum tube filled with pure helium under high 

 pressure (over 50 mm. of mercury), and excited by a condensed 

 discharge, produces broadened spectral lines ; if small quantities 

 of hydrogen are now passed into the tube the helium lines 

 appear broadened as before, and the hydrogen lines appear 

 quite sharp but faint ; but, on the addition of more hydrogen, 

 the latter become diffuse, and eventually definitely broadened 

 when the quantity of added hydrogen is large. It therefore 

 appears that charged atoms of the same kind affect the radiating 

 atoms and cause resolution to a marked extent, but the effect 

 of a charged particle of another kind in close proximity is small. 

 When a high-pressure helium vacuum tube containing a little 

 hydrogen is excited by an uncondensed discharge the hydrogen 

 lines as seen in a direct vision spectroscope appear of uniform 



