March io, 192 i] 



NATURE 



41 



Betty and Bobtail at Pine-Tree Farm. By Lilian 

 Gask. Pp. 224. (London : G. G. Harrap and 

 Co., Ltd., 1920.) 6s. net. 

 We suppose that a book by this well-known 

 author requires no commendation, but perhaps an 

 appreciation in these pages may have a peculiar 

 value. The story of a little girl's visit to a farm 

 and what she saw of dog and sheep, weasel and 

 vole, bat and eagle, and other creatures — it is 

 not a work of science, of course, but a work of 

 art ; and how it is done who shall say ? We could 

 tell the same story, but no child would turn an 

 ear. One must have the secret of the Pied Piper. 

 It seems clear, however, that part of the success 

 of the book must be due to its truthfulness — for 

 the natural history seems all right, except a tale 

 about golden eagles hunting the deer in Scotland. 

 Another part of the success of the book must be 

 due to restraint in giving information, for many 

 books for young folks fail utterly in their Sand- 

 ford-and-Mertonism. The boy explaining why 

 bats are not birds would have been a bore if he 

 had said another word, but he stops just in time. 

 Goethe said something about this sort of thing ! 

 The rest of the attractiveness of the book is due 

 to the art of the writer. We should add, however, 

 that the coloured illustrations by Miss Helen 

 Jacobs are charming, and the book is beautifully 

 printed. We commend it heartily for young 

 children. 



Letters to the Editor. 



{The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications .] 



The Disintegration 1)f Elements by a-Particles. 



In earlier papers one of us has stated that long- 

 range particles which can be detected by their scin- 

 tillations on a zinc sulphide screen are observed when 

 a-particles pass through air or nitrogen, but not 

 through oxygen or carbon dioxide. From the deflec- 

 tion of these particles in a magnetic field it appeared 

 that they were charged hydrogen atoms, indicating 

 that some of the nitrogen atoms were disintegrated 

 by an intense collision with an a-particle. 



In these preliminary experiments it was difficult to 

 get definite information as to the range of these 

 particles from nitrogen, and so to compare them with 

 the H atoms set in motion by the collisions of 

 o-particles with ordinary hydrogen. Recently, im- 

 provement of the optical conditions has made the 

 counting of such weak scintillations much easier and 

 more certain. We have been able to show definitely 

 that the H atoms from nitrogen have a greater range 

 than the H atoms from hydrogen, the ratio being 

 about 1-4 to I. For example, the H atoms liberated 

 by a-particles of range 7 cm. from hydrogen or any 

 hydrogen compound have a maximum range corre- 

 sponding to 29 cm. of air ; while those from nitrogen 

 have a range of 40 cm. This result shows that these 

 particles cannot possibly arise from any hydrogen 

 contamination. 



This observation has opened the way to a series of 

 experiments on other elements. The material under 

 NO. 2680, VOL. 107] 



examination, in the form either of gas or of a thin 

 film of element or oxide, is exposed to the a-rays of 

 radium C. Observation of the number of scintilla- 

 tions is made through a thickness of mica correspond- 

 ing to a distance of 32 cm. of air, so that the results 

 are quite independent of the presence of hydrogen or 

 any tiydrogen compound in the material. 



In this way we have obtained definite evidence that 

 long-range particles are liberated from boron, fluorine, 

 sodium, aluminium, and phosphorus, in addition to 

 nitrogen. \ 



The numbers observed from boron and sodium are 

 much smaller than those from the other elements 

 mentioned. 



The following elements showed very little, if any, 

 efi^ect at an absorption corresponding to 32 cm. fair, 

 viz. lithium, beryllium, carbon, oxygen, magnesium, 

 silicon, sulphur, chlorine, potassium, calcium, • 

 titanium, manganese, iron, copper, tin, and gold. 



The gases oxygen, carbon dioxide, and sulphur 

 dioxide were examined at absorptions of less than 

 32 cm. fair, and no trace of these particles was 

 observed. We have not yet examined whether any 

 of the other elements give rise to particles of maxi- • 

 mum range less than 32 cm. 



The particles liberated from all the first-mentioned 

 elements have a maximum range of at least 40 cm. 

 in air. In particular, the range of the particles from 

 aluminium is surprisingly great, and certainly not less 

 than 80 cm. 



While we have no experimental evidence of the 

 nature of these particles except in the case of 

 nitrogen, it seems likely that the particles are in 

 reality H atoms liberated at different speeds from 

 the elements. Assuming that the law connecting 

 range and velocity of the particles is the same as for 

 the a-particle, it follows that the energy of the 

 particle from aluminium of the maximum range of 

 80 cm. is about 25 p>er cent, greater than the energy 

 of the incident a-particle. 



It is of interest to note that no effect is observed 

 in "pure" elements the atomic mass of which is 

 given by ^n, where n is a whole number. The effect 

 is, however, marked in many of the elements the mass 

 of which is given by 4n+2 or 4^4-3. Such a result 

 is to be anticipated if atoms of the /\n type are built 

 up of stable helium nuclei and those of the 4n4-a 

 type of helium and hydrogen nuclei. 



It should also be mentioned that no particles have 

 so far been observed for any element of mass greater 

 than 31. If this proves to be general, even for 

 a-particles of greater velocity than those of radium C, 

 it may be an indication that the structure of the 

 atomic nucleus undergoes some marked change at this 

 point ; for example, in the lighter atoms the hydrogen 

 nuclei may be satellites of the main body of the 

 nucleus, while in the heavier elements the hydrogen 

 nuclei may form part of the interior structure. 



Until accurate data are available as to the effect 

 of velocitv of the a-particles on the number, range, 

 and distribution of the liberated particles, it does not 

 seem profitable at this stage to discuss the possible 

 mechanism of these atomic collisions which lead to 

 the disintegration of the nucleus. 



E. Rutherford. 

 J. Chadwick. 



Cavendish Laboratory, February 26. 



The Atomic Volume of Isotopes. 



At the discussion on isotopes at the Royal Society 

 on March 3 the question was raised as to within 

 what limits of accuracy the conclusion is justified 

 that the atomic volume of the various isotopes of 

 lead is constant, and the following collected results 



