206 Hackyurr— The Secondary Radiation excited by y Rays. 
rest and from each other in their power of emitting secondary radiation. But 
there is no segregation of the rest of the elements into groups, such as exist 
for the secondary radiations excited by other means. ‘The numbers for these 
elements do not greatly vary. It would be impossible to measure accurately 
the difference between them by comparimg them directly with lead. They were 
therefore compared with an element belonging to the same chemical period. 
The four standards used were: lead, silver, nickel, and aluminium, whose relative 
radiating powers were very carefully determined in the manner described. An 
allusion may be made here to some modifications of this method used to reduce 
the labour of obtaining the remaining numbers. In many cases it was the 
differences of the radiating powers of the two elements which were measured and 
reduced to the standard in the usual way. Or three plates were used, and the 
known differences of two were used to standardize the unknown difference of a 
second pair. In this method the position of the radium is indifferent, since a 
change in the order of arrangement is the only one made. ‘The differences may 
then be increased by bringing the radium close to the plates. 
The absence of any periodic grouping made it unnecessary to include in the 
list a great number of elements, unless those of high atomic weight. or these 
the list was made as complete as possible. But no effort was made to ascertain 
the exact order of arrangement in the case of those elements which have almost 
equal intensities of secondary radiation. 
TABLE I. 
| Secondary Atomic | 
EISEINES. ueeeaauee Weight. | 
| 
Uranium, ae 117 | 239°5 
Uranium Oxide, ; 120 | — 
Bismuth,. .  . 100 | 2085 | 
Lead, : : 100 206°9 
Mercury, . . . 94 200°3 
Platinum, ; j 95 194°8 
18-ct. Gold, : : 91 =: 
Tungsten, : : 96 184:0 
Tin, C 6 c 71 118°5 
Cadmium, : : 68 112°4 
Silver, . j : 69 107°9 
Selenium, 67 UPA 
Zine, ‘ 70 65°4 
Copper, 67 63°6 
Cobalt, 69 59°0 
Nickel, 69 58°7 
Iron, 69 55°9 
Aluminium, . ; 75 | 27:1 
