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SCIENCE 



[N. S. Vol. LIV. No. 1396 



turn, the significance of which I hope we shall 

 thoroughly discuss early next week. The vari- 

 ous consequences of the electronic structure 

 of matter are still unfolding themselves to us, 

 and are increasing our insight into the most 

 varied phenomena at a rate which must have 

 appeared incredible only a few decades ago. 



The enormous and far-reaching importance 

 of the discoveries being made at Cambridge 

 by Sir Ernest Eutherford can not be over- 

 emphasized. These epoch-making discoveries 

 relate to the structure and properties of the 

 nuclei of atoms. At the present time we have, 

 1 think, to accept it as a fact that the atoms 

 ■consist of a positively charged nucleus of mi- 

 nute size, surrounded at a fairly respectful 

 ■distance by the number of electrons requisite 

 ■to maintain the structure electrically neutral. 

 The nucleus contains all but about one two- 

 tliousandth part of the mass of the atom, and 

 its electric charge is numerically equal to that 

 of the negative electron multiplied by what 

 is called the atomic number of the atom, the 

 atomic number being the number which is 

 obtained when the chemical elements are 

 enumerated in the order of the atomic weights ; 

 thus, hydrogen ^^1, helium = 2, lithium = 3, 

 and so on. Consequently the number of ex- 

 ternal electrons in the atom is also equal to 

 the atomic number. The evidence, derived 

 from many distinct and dissimilar lines of 

 inquiry, which makes it necessary to accept 

 the foregoing statements as facts, will be 

 familiar to members of this Section of the 

 British Association, which has continually been 

 in the forefront of contemporary advances in 

 physical science. But I would remind you 

 in passing that one of the important pieces 

 of evidence was supplied by Professor 

 Barkla's researches on the scattering of X-rays 

 by light atoms. 



The diameters of the nuclei of the atoms 

 are comparable with one millionth of one 

 millionth part of a centimeter, and the prob- 

 lem of finding what lies within the interior 

 of such a structure seems at first sight almost 

 liopeless. It is to this problem that Ruther- 

 ford has addressed himself by the direct 

 method of bombarding the nuclei of the dif- 



ferent atoms with the equally minute high- 

 velocity helium nuclei (alpha-particles) given 

 off by radioactive substances, and examining 

 the tracks of any other particles which may 

 be generated as a result of the impact. A 

 careful and critical examination of the results 

 shows that hydrogen nuclei are thus expelled 

 from the nuclei of a number of atoms such 

 as nitrogen and phosphorus. On the other 

 hand, oxygen and carbon do not eject hydro- 

 gen under these circumstances, although there 

 is evidence in the case of oxygen and nitrogen 

 of the expulsion of other sub-nuclei whose 

 precise structure is a matter for further in- 

 quiry. 



The artificial transmutation of the chemical 

 elements is thus an established fact. The 

 natural transmutation has, of course, been 

 familiar for some years to students of radio- 

 activity. The philosopher's stone, one of the 

 alleged chimeras of the mediasval alchemists, 

 is thus within our reach. But this is only 

 part of the story. It appears that in some 

 cases the kinetic energy of the ejected frag- 

 ments is greater than that of the bombard- 

 ing particles. This means that these bom- 

 bardments are able to release the energy which 

 is stored in the nuclei of atoms. Now, we 

 know from the amount of heat liberated in 

 radioactive disintegration that the amount of 

 energy stored in the nuclei is of a higher 

 order of magnitude altogether, some millions 

 of times greater, in fact, than that generated 

 by any chemical reaction such as the combus- 

 tion of coal. In this comparison, of course, 

 it is the amount of energy per unit mass of 

 reacting or disintegrating matter which is 

 under consideration. The amounts of energy 

 which have thus far been released by artificial 

 disintegration of the nuclei are in themselves 

 small, but they are enormous in comparison 

 with the minute amounts of matter ajBFected. 

 If these effects can be sufficiently intensified 

 there appear to be two possibilities. Either 

 they will prove uncontrollable, which would 

 presumably spell the end of all things,^ or they 



- To reassure the nervous I would, however, 

 interpolate the comforting thought that this planet 

 has held considerable quantities of radioactive 



