RECENT ADVANCES IN SCIENCE 287 



the particles are beginning to penetrate the surface-layer of 

 the metal. This is a property which distinguishes a-rays from 

 canal-rays, but the author states that, even at the highest 

 potential reached (about 54,000 volts), there was no indication 

 that the positive particles were beginning to acquire the great 

 ionising power of the a-rays. 



Radioactivity and X-Rays. — The Phil. Mag. for May 

 contains the second part of a paper by Messrs. A. Holmes and 

 R. W. Lawson on the end product of thorium in certain radio- 

 active minerals (the first part was published in the Phil. Mag. 

 December 19 14). The authors come to the conclusion that 

 thorium lead (thorium E) is an unstable product with a half- 

 period approximately io 6 years. It is highly probable that 

 it disintegrates with the loss of a /3-particle and so produces an 

 isotope of bismuth. It seems unlikely that this thorium 

 bismuth (thorium F) is stable, and further work will be needed 

 to determine its half-period and whether the product of its 

 disintegration is a thallium isotope or a polonium isotope. 



R. W. Varder (Phil. Mag, May) gives the results of experi- 

 ments on the absorption of homogeneous -rays by aluminium 

 and compares them with a formula deduced from theoretical 

 consideration by Dr. N. Bohr. 



H. Richardson (Proc. Roy. Soc. A. vol. xci., A. 630 June) 

 discusses the results of some work carried out by him on the 

 absorption in lead of the y-rays emitted by radium B and 

 radium C. In addition to the penetrating type of radiation 

 whose absorption coefficient /j, is 0*5 (cm. -1 ) in lead, radium C 

 emits also the soft types for which /i =46, /m = &o, and/* = i'5, 

 and which are practically absorbed in 1*5 cm. of lead. Radium 

 B emits not only radiations closely similar to three soft types 

 referred to (a fact already noted by Rutherford and Andrade), 

 but also a radiation for which yu, = 40 in aluminium. The 

 absorption of the radiations in different elements has been 

 examined, to find evidence, if possible, of the anomalous 

 absorption investigated by Barkla for elements of lower 

 atomic weight, in virtue of which the absorption coefficient of 

 the radiation characteristic of an element is abnormally high 

 for an absorber whose atomic number is slightly less than that 

 of emitting element. Working with uranium, lead, mercury, 

 gold, and barium as absorbers, the author finds no evidence 

 of an anomalous rise in the absorption coefficients of the 



