172 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



many new and interesting types of transformation, in which either 

 a proton or a neutron is released. One of the simplest and most strik- 

 ing of these transformations is produced when deuterium is 

 bombarded by its own ions. Oliphant and others have shown that 

 tw^o distinct types of transformation occur: 



or -^ Jn- + IHe 



In one case, a fast proton and an isotope of hydrogen of mass 3 ap- 

 pear; in the other, a fast neutron and an isotope of helium of mass 

 3. The masses of these hitherto unknown isotopes can be deduced 

 with confidBnce from a consideration of the energy changes. Both 

 of these isotopes are believed to be stable. While ^H and ^H and 

 ^He appear in several other transformations, no certain evidence has 

 so far been obtained of the isotope of helium ^He. A stable isotope 

 of beryllium of mass 8 is also formed in certain transformations as 

 well as radioactive isotopes. It is worthy of note that apart from 

 the mass 5, all the masses from 1 to 20 on the atomic scale are repre- 

 sented either by stable or by radioactive atoms. 



CONSERVATION OF ENERGY IN TRANSFORMATIONS 



In the transformation of the light elements by bombarding par- 

 ticles, the energy released per atom is of the same order of magni- 

 tude as that observed in the radioactive bodies. In a few cases, 

 particularly when deuterons are employed, the release of energy is 

 considerably greater, and a-particles are expelled with higher speeds 

 than from the radio-elements. In some cases too, penetrating y-rays 

 are emitted ef high quantum energj\ For example, the bombardment 

 of ''Li by protons gives rise to intense y-rays of quantum energy as 

 high as 16 million electron-volts — five times greater than the most 

 penetrating y-rays from radioactive bodies. 



It is in general believed that the principle of the conservation of 

 energy holds in these nuclear reactions when account is taken of the 

 change of mass in the system before and after transformation. The 

 equivalence of mass and energy seems now well established. A 

 decrease of mass dm of a system corresponds to an emission of energy 

 c'^dm^ where c is the velocity of light. This law of equivalence is 

 well illustrated in the transformation of the lithium isotopes by pro- 

 tons and deuterons shown below : 



'Li-^-^D-^'He-i-'He 



The relative masses of the nuclei involved are known from the ac- 

 curate measurements of Aston and Bainbridge by the mass-spectro- 



