NA JURE 



289 



THURSDAY, JANUARY 25, 1906. 



HH.LIUM IN RELATION TO RADIO-ACTIVE 

 PROCESSES. 



Radio-activity. By Prof. E. Rutherford, F.R.S. 

 Second edition. Pp. xiv+580. (Cambridge: The 

 University Press, 1Q05.) Price 12s. 6d. net. 



pROF. RUTHERFORD'S book has no rival as 

 an authoritative exposition of what is known of 

 tin properties of radio-active bodies. A very large 

 share of that knowledge is due to the author himself. 

 His amazing activity in this field has exi ited universal 

 admiration. Scarcely a month has passed for several 

 years without some important contribution from him, 

 or from the pupils whom he has inspired, on this 

 branch of science ; and, what is more wonderful still, 

 there has been in all this vast mass of work scarcely 

 a single conclusion which has since been shown to be 

 ill-founded. The general scope of the present work 

 has been noticed in these columns in a review of the 

 firs! edition. Before passing to the discussion of 

 special points, it is only necessary to say that the 

 second edition fully maintains the reputation of its 

 predecessor for completeness and suggestiveness of 

 treatment. 



1 1 is natural to turn eagerly to the paragraph in 

 which Prof. Rutherford discusses what may be called 

 the burning question in radio-activity — Does the a 

 particle consist of an atom of helium ? Prof. Ruther- 

 ford is evidently still inclined to the view that this 

 is the case. He is influenced chiefly by the undoubted 

 fact that helium is a product of the changes occurring 

 in radium, by the approximate agreement in the 

 electrochemical equivalent of the a particles with the 

 value which is considered appropriate to helium 

 atoms, and by the slowness with which the final 

 products of radium are formed. This slowness seems 

 to exclude the possibility that helium, which appears 

 so soon, can be anything but a bye-product. 



It is difficult to regard the argument from the 

 electrochemical equivalent as having great weight, for 

 the assumptions which must be made before available 

 measurements can be brought to bear are many and 

 bold. 



In the first place, how do we know the electro- 

 chemical equivalent of a helium atom? In a case 

 like hydrogen or oxygen we are on safe ground, for 

 experiments on the electrolysis of its compounds enable 

 us to compare the quantity of electricity conveyed with 

 the amount of the element liberated. But no such ex- 

 periment can be made with helium, for it forms no 

 compounds. It is evident, therefore, that the desired 

 information can only be got by some indirect argu- 

 ment. Unfortunately, the most obvious kind of induc- 

 tion does not lead us to a very satisfactory conclusion. 

 It is found that if we call the charge which a mono- 

 valent atom can carry e, then a divalent atom will 

 carry a charge ie, a trivalent atom a charge T,e, and 

 so on. What, then, are we to expect of an atom like 

 NO. 1 89 1, VOL. 73] 



helium, which, so far as can be judged from its 

 chemical behaviour and from its position in the 

 periodic classification, has zero valency? Obviously 

 that it should not be able to carry a charge at all in 

 the same sense that the other atoms can do so. 



Helium, like other gases, can be ionised bv the 

 Rontgen rays, so there is reason to think, from this 

 point of view, that its atom can in some sense be 

 charged. 



It has been customary to assume that the appro- 

 priate charge to assign to it is that of a monovalent 

 atom. This may be the best view to take provision- 

 ally.. But' further lighl is much needed. 



Again, we have no very direct measurement of the 

 charge carried by an a particle. Prof. Rutherford 

 assumes for that case also the charge characteristic 

 of a monovalent atom, and he has been able to cal- 

 culate on this assumption the volume of radium 

 emanation and the heating effect of radium. The 

 results in both cases are in satisfactory agreement with 

 experimental data, and Prof. Rutherford is to be con- 

 gratulated on his remarkable success in showing that 

 such agreement can be obtained. But he would not, 

 in all probability, press the exactness of this agree- 

 ment as sufficient evidence that the charge of an a 

 particle is exactly that assumed, and not either half 

 or double, for he has had to use many data which 

 cannot but be considered subject to very serious un- 

 certainty. Indeed, the agreement obtained is so good 

 that even if all the premises are correct it must be 

 considered as to some extent fortuitous. 



Perhaps the most original chapters in Prof. Ruther- 

 ford's book are those in which he has so admirably 

 disentangled the complicated series of changes which 

 are involved in the disintegration of radium and its 

 emanation. The idea at one time entertained that 

 radio-activity was determined by high atomic weight 

 must now be abandoned. For it has been made quite 

 clear by these investigations that changes character- 

 ised by low radio-activity and slow atomic disintegra- 

 tion are followed by others of far greater rapiditv. 

 Radium, in losing atomic weight, turns into the 

 emanation, which, weight for weight, is far more 

 active than its parent. 



In the appendix some interesting properties of the 

 c. rays are described. This branch of the subject is, 

 however, at present in a very chaotic state, and we 

 shall not discuss it here. One remark may be 

 criticised — nearly the last in the book. Prof. Ruther- 

 ford thinks that ordinary matter may be emitting as 

 many or more a particles than uranium, if only the 

 velocity of these is less than that minimum velocity 

 which has been found necessary to produce the 

 characteristic phenomena. This is scarcely con- 

 sistent with the facts if the a. particle is a helium 

 atom. For why, in such a case, is helium only 

 found in appreciable quantity in radio-active minerals? 



In conclusion, we must once more congratulate 

 Prof. Rutherford on the admirable manner in which 

 he has brought his book up to date, and express a 

 hope that the present edition may have many suc- 

 cessors. R. J. Strutt. 



O 



