128 MR. . W. C. KAYE ON THE 



A calcium-chloride solution key, K. which was operated from a distance, made or 

 broke the earth connection to the gold-leaf system. 



TO EARTH 



/TO EARTH 



Fig. 2. 



The gold leaf of the electroscope was viewed through a low-power microscope, having 

 a scale in the eye-piece. Its time of travel over a definite range was taken with a 

 stop-watch, and was usually from 20 to 60 seconds. When things were working well, 

 an accuracy of 1 per cent, could he obtained. In a comparison of the radiations from 

 a pair of metals, they were used alternately until the readings were concordant. 



Before the merciiry-break was employed, a standard bulb with a platinum anticathode 

 was joined in series with the experimental one. It was provided with an ionisation 

 chamber and electroscope, and thus served as a check on the constancy of the current 

 passing through the two bulbs. But if the mercury-break was working well, the use 

 of this standard bulb was not found to be necessary. 



Results. 



The results for the aluminium, copper, and platinum screens will be discussed fully, 

 and may be regarded as typical of the rest. 



Of the anticathodes, aluminium, iron, nickel, copper, silver, platinum, and lead 

 received special attention. 



Rays direct from Aluminium Window. With a potential difference of 28,000 volts 

 on the tube the relative ionisation values for the rays emerging from the aluminium 

 window unobstructed by any screen are given in the table below for some of the 

 radiators. The value for platinum is made equal to 100. 



These radiation values, although they do not follow the order of the atomic weights, 

 divide themselves into four well marked groups Bi to Ta, Sn to Pd, Zu to Fe, and Ca 

 to C. The metals of the iron-zinc group are characterised by abnormally high 

 radiation values. BARKLA and SADLER,* working with secondary Rontgen rays, have 

 arrived at a grouping almost identical, and characterised by similar features, 



* 3ARKLA and SADLER, 'Nature,' p. 344, Feb. 13, 1908. 



