658 SCIENCE PROGRESS 



could not be greater than 1 10. Yet its products, radium B and 

 radium C, are respectively chemically identical and non-separable 

 from lead and bismuth respectively, which is easy enough to 

 understand if the atomic weight is also 220 and impossible to 

 explain if it is no or less. The fact that, in the final chemical 

 court of appeal, the rare gases fit beautifully into the periodic 

 law, as a new family of zero group number, only if their 

 molecules are considered monatomic, and cannot be fitted in at all 

 if their molecules are considered polyatomic, has been strikingly 

 extended in the case of the radioactive emanations. The 

 sequence radium, (vacant), emanation, (vacant), polonium, bis- 

 muth, lead of the last two horizontal rows is completely 

 analogous to the other sequences — barium, caesium, xenon, 

 iodine, tellurium, antimony, tin; strontium, rubidium, krypton, 

 bromine, selenium, arsenic, germanium ; calcium, potassium, 

 argon, chlorine, sulphur, phosphorus, silicon ; magnesium, 

 sodium, neon, fluorine, oxygen, nitrogen, carbon. 



Now let us turn to the physical evidence. This is not so 

 logical in argument, perhaps, because the underlying causes are 

 not always clear. In the first place I would put the stopping 

 power of helium to the a-rays. Though helium is twice as dense 

 as hydrogen, it stops the a-ray to nearly the same extent as 

 hydrogen. The range of the a-particle of polonium in hydrogen 

 is 15*95 an d in helium 167 cms. at N.T.P. (T. S. Taylor, Phil. 

 Mag., 191 3 (vi.) 26, 402). This would be unexpected but for 

 Prof. Bragg's generalisation. He found the stopping power of 

 matter to be purely an additive or colligative property, and to 

 depend only on the numbers and kinds of atoms and not of the 

 molecules into which the atoms are combined. The stopping 

 power of any atom is approximately proportional to the square 

 root of its mass, not directly to its mass as might be supposed. 

 If helium be monatomic, then, since hydrogen is diatomic, at the 

 same pressure the number of atoms of hydrogen in the path of 

 the a-ray will be twice the number of helium atoms, but the 

 stopping power of each helium atom will be approximately ^4 

 times that of each hydrogen atom, so that the stopping power of 

 equal thicknesses of the two gases at the same temperature and 

 pressure will be approximately the same. Lest it should be 

 thought that helium is peculiar, because the a-particle is a 

 helium molecule, it may be said that argon, like helium, also 

 obeys the square-root law, only on the assumption that its 



