Presence of Neutrons in a Discharge Tube. 599 



which a thin sheet of aluminium foil was waxed on. The 

 effective area of the aluminium window was 2\ sq. cm. for A, 

 and "6 sq. cm. for A', the thickness of: the foil being '001 cm. 

 in each case. The tubes were exhausted by a Gaede rotary 

 mercury pump. In preliminary experiments a willemite 

 screen placed at A and A' showed that at suitable pressures 

 a good beam of positive rays was obtained. As a rule, 

 hydrogen was used in the tubes, and in many cases a steady 

 flow of hydrogen was maintained by the usual capillary tube 

 method. Experiments with air were a'so made. The tube 

 was run by a 10-inch Cox coil with hammer break. 



(b) The detection. — Three experimental arrangements 

 were used. In the first an ionization chamber filled with 

 mercury vapour was used. The vessel itself was a cylin- 

 drical iron pot, one of the flat faces being provided with a 

 thin mica window for the admission of the ionizing rays. 

 The outside of the vessel was wound with German-silver 

 heating coils embedded in alundum cement. The floor of 

 the vessel was covered with mercury, and by raising the 

 temperature to the boiling-point of mercury a large partial 

 pressure of mercury vapour was obtained. The iron electrode 

 was insulated by a quartz tube at the end of a long cylin- 

 drical neck. The quartz insulation w;is waxed into the neck 

 and kept cool by a water-jacket. The electrode was con- 

 nected to a Wilson electroscope which served to measure the 

 ionization current through the vessel. Using: a verv strong 

 source of y rays, a saturation current was obtained about six 

 times greater than that obtainable when the vessel was filled 

 with air at the same temperature. So far, however, it has not 

 been found possible to obtain great sensitiveness with the 

 arrangement. The rapid ebullition of the mercury produces 

 a considerable development of electricity within the chamber, 

 probably of frietional origin. In view of the interest which 

 the mercury vapour ionization chamber would possess in X-ray 

 work, further experiments with it are being undertaken. 



For the present purpose, 1 used also a second arrangement 

 which allowed of greater sensibility, at any rate as far as 

 7 rays are concerned. This was an ordinary a-ray electro- 

 scope with thick lead lining provided with a thin window 

 either of lead or of platinum. The window of the electro- 

 scope was placed close up against the window of the positive 

 ray tube. 



Finally, a zinc-sulphide screen, such as is used for the 

 detection of a particles, was also used ; it was placed close 

 against the window of the tube and examined through a 

 microscope of wide aperture. 



4. Results. — The tube was run in all stages of hardness 

 from an alternative spark-gap of less than a millimetre 



