228 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 3 8 



1. The discovery, measurement, and separation of isotopes of the chemical 

 elements (Aston, Bainbridge, Urey, et al.). 



2. The development of machines for accelerating charged particles to very 

 high energies (1 to 8 million electron volts). 



(a) Transformer-condenser methods (Cockcroft and Walton, Lauritsen, et al.). 



(b) Electrostatic generator (Van de Graaff, Tuve). 



(c) The cyclotron (Lawrence, Livingston, and Cooksey). 



(d) Linear accelerators, resonance transformers, etc. 



3. The transmutation of elements: 



(a) By natural alpha-particles (Rutherford, 1919). 



(b) By artificially accelerated protons (Cockcroft and Walton, 1932). 



(c) By deuterons (Lawrence, 1933). 



(d) By neutrons (Feather, 1932). 



4. Discovery of the positron (Anderson, 1932). 



5. Discovery of the neutron (Chadwick, 1932). 



6. Discovery of induced radioactivity (Curie-Joliot, 1934). 



We shall pass over the field of isotopes without discussion. The 

 heavy isotopes of hydrogen, carbon, oxygen, etc., are of great im- 

 portance to nuclear physics and to biology but their use in the latter 

 field is largely a problem of chemistry — and I shall leave it to the 

 chemists to discuss. 



Similarly, it will not be profitable in this discussion to describe in 

 detail the experimental techniques which have been developed for 

 producing high energy particles. It is the development of these 

 techniques, of course, which has made possible all the other ad- 

 vances: without them it would not be possible even to discuss in a 

 practical way the possible biological applications of nuclear physics. 

 In fact, there would be very little nuclear physics. The time may not 

 be far distant when a cyclotron or a Van de Graaff generator will be 

 standard equipment in large biological and medical research centers. 

 But our chief interest in this symposium is what these machines will 

 do rather than how they work. The principal thing they will do is to 

 produce transmutations of elements, and the products of these trans- 

 mutations are the things which biologists may find useful. I should 

 also mention, however, that resonance transformers and Van de 

 Graaff machines are also suitable, and are even now in use in several 

 medical centers, for the production of million-volt X-rays. Still 

 higher voltages can be produced as soon as they are required, but it 

 will be best to understand something of the effects of 1 -million volt 

 X-rays before 2-million volt machines are built. These machines 

 bring X-ray energies right into the realm of gamma-rays, with avail- 

 able intensities which are thousands of times greater than those 

 attainable from natural radioactive sources. 



The great importance of extending radiological investigations into 

 this region is sufficiently obvious. There is but one interesting point 

 worth mentioning. One advantage usually ascribed to high energy 

 X-rays is their greater penetrating power. However, this increase of 



