296 Dr. T. S. Taylor on the Ionization Curve for the 



for successive distances within errors of experiments ; and 

 it will be seen that the values of a obtained are all consi- 

 derably higher than those o£ Bishop. Even allowing the 

 possibility of an error as large as 10 per cent, in the values 

 of r, the values of a could only be reduced to 5'5, 8*0, and 

 6*9 respectively, and there seems little doubt that Bishop's 

 results are all far too low owing to an incorrect assumption 

 of the value of n Q . 



Yours truly, 

 Merton College, Oxford. E. W.B. GlLL. 



XXIII. A Determination of the Ionization Curve for the 

 Alpha Rays from Polonium in Mercury Vapour, By 

 T. S. Taylor, PLD.* 



Introduction and Description of Apparatus. 



THE investigation, herein described, was undertaken for 

 the purpose of determining the Bragg ionization curve 

 for alpha rays in mercury vapour. (Since the vapour- 

 pressure of mercury is very low at ordinary temperatures, it 

 was necessary to design and use a type of apparatus adapted 

 to temperatures as high as 300° to 350° centigrade. 



After various attempts to construct a piece of apparatus 

 suitable for the work, the apparatus shown in fig. 1 was 

 finally found to work satisfactorily. In the main, it is 

 merely a modification of the general type of apparatus that 

 has always been used in determining the ionization curve for 

 alpha rays in air and other gases. 



The ionizati on-chamber, shown in the top of the central 

 portion of the diagram, consisted of an insulated iron plate 

 A about 3 centimetres in diameter, and two iron gauzes 

 below it. The iron plate was about 2*5 millimetres above 

 the upper gauze and was joined to the electrometer by means 

 of the connexion that supported it in the tube. The upper 

 iron gauze was joined to a constant potential of 80 volts and 

 the lower one was earthed. The two gauzes were separated 

 and insulated about 2*5 millimetres from each other by 

 means of mica disks. The mica disks were held tightly 

 together by having thin iron disks above and below them 

 which were held together by means of screws extending 

 through the mica disks. These mica disks were of such size 

 as to just slip easily into a glass tube 5*5 centimetres in 

 diameter. The connexions to the two gauzes and the iron 

 disk A were sealed through the glass tube as shown. An 

 earthed iron guard-ring represented by the shaded portion 

 * Communicated by the Author. 



