432 Dr. J. P. V. Madsen on 



That the incidence radiation from these substances is softer 

 than the emergence, is also supported by the experiments of 

 Kleeman and Eve already referred to, both these observers 

 having found the value of A, for the incidence rays from 

 these or similar substances to be of the order 4, measured by 

 Pb. In the case of the incidence radiation from Pb the 

 measurements were not sufficiently accurate to decide whether 

 the incidence radiation was softer than the emergence ; 

 it appeared to be of much the same quality, i. e., \ = 1'& 

 approximately. 



§iii. 



As the apparatus described in fig. 1 was unsuitable for 

 measuring the amount of secondary emergence radiation from 

 substances such as Al and C, and as we do not know that 

 the distribution of the emergence radiation is the same in 

 all directions for all substances, an experiment was set up as 

 shown in fig. 7. 



The lower portion of the diagram shows the arrangement 

 of the Pa, magnet, and iron case which is the same as pre- 

 viously described. Some distance above the Ra was supported 

 — upon an insulated stand — a flat cylindrical ionization- 

 chamber. The floor of this chamber was of sheet lead 1*25 

 mm. thick, the sides were of Zn, and for a lid was used a 

 thin Al plate. The conical chamber, shown by dotted lines, 

 was used in experiments to be described later. The radiators 

 were in the form of flat plates, the first of which in any 

 experiment was placed in the position p, p. Additional 

 thicknesses were added immediately below p, p and in contact 

 with the top plate, while for very great thicknesses plugs of 

 the material could be inserted in the conical hole in the Pb 

 block, through which the 7 radiation streamed. 



In the first set of experiments the Pb plates L, L were 

 used in the position shown, the large central hole in them 

 allowing the 7 rays to pass without obstruction to the ioni- 

 zation-chamber ; while a considerable amount of secondary 7 

 radiation from the radiating plates p, p was prevented from 

 reaching the chamber* A strong magnetic field was applied 

 during the experiments. The floor'of the ionization-chamber, 

 consisting of Pb 1*25 mm. thick, was sufficient, as has been 

 shown in previous papers, to give the full amount of emer- 

 gence ft radiation. Several readings were taken with each 

 thickness of radiator in position, and the readings were suffi- 

 ciently large with the chamber filled with air. From any 

 reading it is necessary to subtract an amount corresponding 



