Properties of the <x Rays from Radium. 165 



successive changes, which has been advanced from data of 

 quite another character. 



By passing the rays through an absorbing screen of thick- 

 ness d, the range in air was found to be reduced by the 

 distance pd, when p was the density of the screen compared 

 with air. This is an expression of the fact that the absorption 

 of the rays is proportional to the density of matter traversed. 

 In a thick layer of radioactive material of one kind, the rays 

 emitted into the gas come from different depths, and con- 

 sequently pass through a distance in air varying from zero 

 to the maximum range, corresponding to the rays from a thin 

 film of that material. It is thus obvious that the a particles 

 escaping into the gas will have different velocities, that is, 

 the rays will be complex. In radium in radioactive equili- 

 brium, which contains four distinct products emitting a 

 particles, the rays are still more complex in character, as 

 each set of rays has all ranges in air between zero and its 

 maximum. 



In order to obtain results of a definite character, it is thus 

 advisable not to use radium itself as a source of rays, but a 

 thin film of radioactive matter of one kind, so that the rays 

 all escape into the gas with the same velocity. 



This condition is fulfilled by using as a source of rays a 

 wire made active by exposure to the radium emanation. 

 The active deposit on the wire after removal contains the 

 three products radium A, B, and C. Since radium A is half 

 transformed in three minutes, it has practically disappeared 

 in the course of fifteen minutes. The rays then are only 

 emitted from the one product radium C, since radium B does 

 not emit rays at all. The deposit on the wire is so thin 

 that no absorption takes place in the active matter itself, 

 so that all the rays escape without change in their velocity. 

 The a particles projected into the wire are absorbed completely 

 by the wire itself. The activity of radium C dies down with 

 the time, falling to half value in the first 28 minutes after 

 removal. 



In order to produce a well-marked photographic action, 

 using a narrow pencil of a. rays at the distance of about 

 7 cms. required in the experiments, it was necessary to 

 employ an intensely active wire as a source of radiation. 

 For this purpose, a thin wire, about 1 cm. long, was charged 

 negatively to —800 volts in the presence of the accumulated 

 emanation from about 20 milligrams of radium bromide in 

 solution. The wire to be made active was the only nega- 

 tively charged body exposed in the presence of the emanation, 

 and consequently the active deposit was concentrated upon 



