204 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



observed fact that either positive or negative electrons are emitted by 

 a large group of radioactive elements to which I will now refer. 



In the early experiments on transmutation by a-particles, it was 

 supposed that a stable nucleus was always formed after the emission 

 of a fast proton. The investigations of M. and Mme. Curie-Joliot 

 showed that in some cases elements were formed which, while momen- 

 tarily stable, ultimately broke up slowly, exactly like the natural 

 radioactive bodies. Most of these radioactive bodies formed by 

 artificial methods break up with the expulsion of fast negative elec- 

 trons, but in a few cases positive electrons are emitted. Since the 

 presence of these radioactive bodies can be easily detected, and their 

 chemical properties readily determined, this new method of attack on 

 the problem of transmutation has proved of great value. Nearly a 

 hundred of these radioactive bodies are now known, produced in a 

 great variety of ways. Some arise from the bombardment by fast 

 a-particles, others by bombardment with protons or deuterons. As 

 Fermi and his colleagues have shown, neutrons, and particularly slow 

 neutrons, are extraordinarily effective in the formation of such radio- 

 active bodies. On account of its absence of charge, the neutron 

 enters freely into the nuclear structure of even the heaviest element, 

 and in many cases causes its transmutation. For example, a number 

 of these radioactive bodies are produced when the two heaviest ele- 

 ments, uranium and thorium, are bombarded by slow neutrons. In 

 the case of uranium, as Hahn and Meitner have shown, the radioactive 

 bodies so formed break up in a succession of stages like the natural 

 radioactive bodies, and give rise to a number of transuranic elements 

 of higher atomic number than uranium (92). These radioactive ele- 

 ments have the chemical properties to be expected from the higher 

 homologues of rhenium, osmium, and iridium of atomic numbers 93, 

 94, and 95. 



These artificial radioactive bodies in general represent unstable 

 varieties of the isotopes of known elements which have a limited life. 

 No doubt such transient radioactive elements are still produced by 

 transmutation in the furnace of our sun, where the thermal motions 

 of the atoms must be very great. These radioactive elements would 

 rapidly disappear as soon as the earth cooled down after separation 

 from the sun. On this view, uranium and thorium are to be regarded 

 as practically the sole survivors in our earth of a large group of radio- 

 active elements, owing to the fact that their time of transformation 

 is long compared with the age of our planet. 



It is of interest to note what an important part the a-particle, which 

 is itself a product of transformation of the natural radioactive bodies, 

 has played in the growth of our knowledge of artificial transmutation. 

 It is to be remembered, too, that our main source of neutrons for 

 experimental purposes is provided by the bombardment of ^beryllium 



