CONTEMPORARY ADVANCES IN PHYSICS 397 



electrons, should then possess the chemical properties of nitrogen in 

 the former case, phosphorus in the latter. 



The important experiment of the Joliots consisted in showing that 

 when a sample of boron (or aluminium) is first exposed to alpha- 

 particle bombardment and then to such chemical processes as would 

 remove nitrogen (or phosphorus) commingled with the boron (or 

 aluminium), the induced radioactivity is itself removed and carried 

 away. I quote verbatim: "We have irradiated the compound BN. 

 By heating boron nitride with caustic soda, gaseous ammonia is 

 produced. The activity separates from the boron and is carried away 

 with the ammonia. This agrees very well with the hypothesis that 

 the radioactive nucleus is in this case an isotope of nitrogen. When 

 irradiated Al is dissolved in HCl, the activity is carried away with the 

 hydrogen in the gaseous state, and can be collected in a tube. The 

 chemical reaction must be the formation of PH3 or SiH4. The 

 precipitation of the activity with zirconium phosphate in acid solution 

 seems to indicate that the radio-element is an isotope of phosphorus." 



The assumed equations are thus substantiated in a very striking 

 way. These experiments are in a sense the first chemical identifica- 

 tions of any product of transmutation; I say "in a sense," because 

 while this nitrogen and this phosphorus are identified by virtue of 

 chemical properties, they are detected only by virtue of their radio- 

 activity.^ 



Some striking photographs, taken at Pasadena with an expansion- 

 chamber containing a block of boron oxide previously bombarded by 

 alpha-particles, show many tracks of positive electrons springing from 

 points in the air of the chamber (Fig. 4). It is inferred that the 

 unstable nuclei formed from the boron (not from the oxygen, since 

 bombardment of Si02 has no effect) are carbon nuclei which unite 

 with electrons to form carbon atoms and then with oxygen atoms to 

 form molecules of CO or CO2 having a natural tendency to diffuse out 

 of the solid mass. The radioactivity may be driven completely out 

 of the solid block in short order by heating to 200° C. The radioactive 

 particles are unable to pass through a liquid-air trap. 



" Inserting mass-numbers into the equations, one finds that since Al has but the 

 one known isotope 27, the value 30 is indicated for the mass-number of "radio- 

 jihosphorus," as Joliot calls it; while since boron has two isotojies 10 and 11, the two 

 values 13 and 14 are indicated for radio-nitrogen, with no certain evidence to dictate 

 a choice between them. Ordinary stable phosphorus has no known isoto]^e 30, and 

 ordinary stable nitrogen has no known isotope 13, but the vast majority of its atoms 

 are of mass-number 14. It seems natural that a very unstable isotope should have 

 a different mass-number from any of the known and stable ones, and this may be a 

 valid argument for inferring that it is B'" rather than B'' wh'ich is concerned in the 

 induced radioactivity of boron; but there is nothing to prohiliit us from su])posing 

 that there may be an unstable isotope of nitrogen agreeing in mass-number with 

 the one which is durable. 



