126 Mr. L. Simons on the Beta-Ray Emission from 



thin that the graphite backing could easity be seen through 

 them. 



Films of the other substances were obtained in the following 

 manner. It was necessary to obtain an adherent conducting 

 surface in each case. The surface had to be conducting in 

 order that the electrostatic capacity of the ionization 

 chamber, of which the surface really formed the back, 

 should remain unaltered when the various screens were 

 substituted for one another. This conducting surface had 

 also to be made of some material of low atomic weight from 

 which there is no electronic emission. To obtain a degree 

 of uniformity, the graphited wax surface was placed 

 horizontally, face uppermost, and an inverted electric hot- 

 plate was brought down for a few moments to within a 

 distance of about 0'5 cm. from the surface. The graphite 

 surface cracks and the paraffin-wax comes up through the 

 fissures of retreat. If the process is watched carefully and 

 is stopped at the right moment, the whole surface is con- 

 ducting, as tested by an electroscope, and it will take a 

 powder in a way that polished graphite cannot. It also 

 remains quite plane. Pure As, Se_, Zr. Bi, and Sb were 

 very finely powdered in a mortar and lightly dusted over 

 the prepared plates, the larger grains being wiped off and 

 the plates being vigorously tapped in order to get rid of any 

 particles not properly adhering to the wax. Microscopic 

 examination showed that each plate was covered with a 

 uniform layer of minute grains. No estimate of the size of 

 the grains was made. Each plate was tested for electrical 

 conductivity. Barium peroxide was similarly dusted on a 

 plate for the Ba screen and red-lead for the Pb screen. An 

 eleventh screen was made containing a flat sheet of carbon 

 (cut from a dynamo brush) instead of the usual paraffin-wax. 

 Finally, each disk was provided with three screw-legs round 

 the edge outside the rim, and all the parts except the 

 prepared screen and the back of the disk were coated with a 

 thin layer of wax and rubbed with graphite. 



A small brass conical gauge-piece was made 1*45 cm. 

 high, which was placed on a sheet of glass and the screen 

 placed over it. The screw-legs were now adjusted until 

 each part of the surface of the screen just touched the apex 

 of the gauge-piece. The screens were then ready to be 

 placed in the ionization chamber, but before doing so each 

 was placed for a short while under the receiver of an 

 air-pump in which the pressure was maintained at about 

 2 mm. 



