246 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1935 



number of individual atoms entering into the reaction is smaller by 

 many orders of magnitude than those ordinarily involved in chem- 

 ical reactions. The second is that for each atom that is involved the 

 nuclear reaction is much more energetic, an amount of energy being 

 often liberated which is a million times greater than that liberated by 

 the most violent chemical reaction, as, for example, by the explosion 

 of dynamite. Because the atoms involved in the nuclear reactions are 

 so relatively few in number the most sensitive and elaborate forms 

 of physical apparatus must be employed for their detection; but, 

 fortunately, owing to the extremely high energies given to the par- 

 ticle in such reactions, it is often possible to count and measure, 

 individually, single atoms formed in the disintegration. 



A further consequence of the small number of atoms participating 

 in a nuclear reaction is that the total amount of a new element that 

 can be produced is extremely minute. This is illustrated by the fact 

 that if today's most efficient apparatus, adjusted to produce helium 

 out of lithium and hydrogen, were operated continuously for 100 

 years it could produce at the best only a fraction of a penny's worth 

 of helium, whereas the power cost would approach $1,000,000. The 

 physicist has not as yet learned how to produce new elements in bulk. 



INDUCED RADIOACTIVITY 



In 1934 Curie and Joliot reported the fact that many of the light 

 elements, after bombardment by alpha particles, were found spon- 

 taneously to emit positrons. After the alpha-particle bombardment 

 ceased, the emission of positrons continued. Curie and Joliot had 

 found how to make inert substances radioactive; and, appropriately 

 enough, the daughter of the discoverer of radium becomes the co- 

 discoverer of artificially produced radioactivity. This discovery is 

 of far-reaching scientific importance, and today, less than a year after 

 its announcement, radioactive substances artificially produced have 

 already shown promise of competing favorably with the rare and 

 expensive natural product, radium. 



The technique of producing artificial radioactive substances is quite 

 simple. One has merely, for example, to hold a few crystals of boric 

 acid close to a radioactive substance which emits alpha particles. 

 Some of the alpha particles, in striking nuclei of the atoms of boron 

 in the boric acid crystals, will effect a transmutation ; and a new ele- 

 ment, nitrogen, is born. Nitrogen is an exceedingly common element, 

 comprising as it does four-fifths of the earth's atmosphere; but the 

 nitrogen produced in the boric acid differs from the familiar nitrogen 

 of the atmosphere in one very important respect. It contains in its 

 nucleus one neutron less than does the everyday nitrogen and has 

 therefore an atomic weight of 13 instead of 14. 



