808 Mr. H. Smith: Comparison of Positive Rays 



pressure by means of a mercury pump, and also by means of 

 charcoal cooled in liquid air. This tube could then be closed 

 by means of the tap T. Another charcoal tube, CT, attached 

 to the camera, was then immersed in liquid air in order to 

 keep the pressure in the camera as low as possible during 

 the whole exposure. By this means sharp positive-ray photo- 

 graphs were obtained. The discharge was made to pass 

 through a capillary tube (about 1 mm. in diameter) C, and a 

 photograph of the spectrum was obtained by attaching 

 a small direct-vision spectroscope to a camera (S in fig. 1). 



Fig.l. 



TO HYDROGEN 

 GENERATOR 



A. Anode. 



K. Cathode, Water-cooled. 



P. Wires from electrostatic plates to storage-cells. 

 Horseshoe magnet for magnetic field not shown. 



The helium was prepared from thorianite by heating with 

 sulphuric acid. The gas evolved was passed through a 

 charcoal tube cooled in liquid air, and its spectrum observed 

 in a small Geissler tube. As soon as the spectrum o£ helium 

 became strong and the spectrum of hydrogen difficult to 

 detect, the helium was considered pure, and the generating 

 apparatus together with the charcoal tube were shut off by 

 means o£ a tap. The helium was now contained in a tube 

 connected to the apparatus, and by turning a tap could be 

 let in at will. The method of filling the positive-ray bulb 

 was as follows : — The whole apparatus was washed out several 



