740 Mr. & Mrs. Soddy and Mr. A. S. Russell on the 



subtracted in fig. 4, but there it is too small to be o£ any 

 significance. With the 0*47 mg. radium this correction 

 amounted to 1*78, and with the 6*7 mg. radium to 30*6. 

 The corrected leak with the whole hemisphere in was 3*85 

 for the 0'47 mg., and 51*8 for the 6*7 mg. The natural 

 leak was 2' 8 in most of the experiments. This shows that 

 the correction is very small and is nearly proportional to the 

 quantity of radium. The curve (fig. 5) shows that the 

 observational points lie on the theoretical curve with very 

 great accuracy. 



The gratifying result follows that according to these 

 experiments the 7-rays of radium are exponentially absorbed 

 by lead as a single homogeneous radiation, with the value of 

 the absorption coefficient 0*5, from the thickness at which 

 the /5-radiation is completely absorbed up to 7*7 cm. This 

 is so completely at variance with the result obtained with 

 ordinary dispositions, and the conclusions of many inves- 

 tigators who claim to have evidence of the existence of two 

 and even three types of homogeneous 7-rays from radium, 

 that it may be regarded as a very remarkable result. It also 

 follows that the 7-rays are not scattered at all in passage 

 through matter. The sharpness and delicacy of 7-ray photo- 

 graphs, when precautions are taken to remove /3-rays by a 

 magnet, also bear this out (Mme. Curie's Thesis, " Radio- 

 active Substances," p. 65) . To indicate the difference between 

 the case of the 7-rays and that of the /3-rays, which are well 

 known to be scattered by thin films of absorbing materials, 

 it is of interest to contrast the preceding result with some 

 obtained with the /3-rays of uranium X, also in a hemi- 

 spherical ionization chamber. A copper hemisphere, 16 cm. 

 in diameter, was mounted above an electroscope as shown in 

 fig. 6, with the absorbing plates clamped on to the top. The 

 uranium X preparation, usually on a very small thin piece of 

 micro-cover glass, was laid on the centre of the top plate, 

 and could be covered at will with thick disks of zinc, lead, 

 or aluminium, when it was desired to examine also the 

 " reflected " radiation. Sometimes the uranium X was 

 placed directly on the centre of the uppermost plate. 



To compare with these results others were taken in a brass 

 electroscope of ordinary cylindrical pattern and size with the 

 absorbing sheets clamped up to the base. The preparation 

 was supported 9 cm. below the electroscope on a square of 

 micro-cover glass 11 cm. sq. and 0*125 mm. thick by means 

 of a light frame attached to the table on which the electro- 

 scope rested. The design was intended to minimize the 

 reflected radiation as much as possible. The preparations 



