194 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1939 



active material found in nature, the atoms of which may be described 

 as minute Fourth-of-July sparklers constantly sending out light of 

 very short wave length and also ejecting with high velocities material 

 fragments resulting from their internal disintegration. These frag- 

 ments have been investigated and found to be of two types. One 

 type is the electron and the other was shown by Rutherford and 

 Royds in 1909 to be the massive portion of the characteristic atoms 

 of the familiar atmospheric gas helium. The electrons are light and 

 easily deflected from their paths, but the helium particles are about 

 10,000 times as massive and go rending through other atoms in their 

 path leaving great havoc on an atomic scale behind them. A study 

 of the way in which these projectiles are scattered by the atoms of 

 other matter that they encounter was shown by Rutherford to imply 

 very interesting consequences. The electrons associated with these 

 scattering atoms are so light as to deflect the helium particles but 

 little, and the observed angular distribution of the scattered particles 

 implies that the massive portions of the atoms responsible for the 

 scattering are concentrated in very minute regions at the center of the 

 electronic structure of an atom. The diameter of this central mass 

 or nucleus is of the order of a hundred-thousandth the diameter of 

 the atom as a whole. The general relative scale and tenuousness of 

 an atomic system is similar to that of our solar system, where the 

 central sun, though much smaller than the outermost planetary orbit, 

 comprises most of the mass. 



These facts and the additional one that radioactive emanations 

 are given off in a statistically predictable way were all that was 

 known of the matter up to 1919. No more was known because no 

 way had been found of affecting the rate or manner of decay of 

 natural radioactive material or of producing any change in ordinary 

 stable atomic nuclei. Without being able to observe directly, or ef- 

 fect any observable alteration, no information could be obtained. 

 The first step in effecting any change in an atomic nucleus was taken 

 by Rutherford in 1919. In this year he observed certain phenomena 

 that could be explained on the hypothesis that he had succeeded in 

 transmuting a nucleus characteristic of nitrogen into one charac- 

 teristic of oxygen. This simple but epoch-making experiment illus- 

 trates one of the techniques early employed in this type of work and 

 also the type of evidence that is obtained and the method of reason- 

 ing involved. It is known that when a rapidly moving, relatively 

 massive particle such as an atomic nucleus impinges on certain 

 crystals such as zinc sulphide, for instance, a tiny star of light 

 appears. This is visible to a dark-adapted eye under suitable mag- 

 nification. These tiny sparks are known as scintillations and indi- 

 cate the stopping of the atomic projectile in the crystal. Their 

 intensity is not a reliable guide to the exact type of atom stopped 



