ATOMIC NUCLEI — HARNWELL 195 



or to its original velocity. A crystal of zinc sulphide scintillates 

 brilliantly in the presence of a bit of radioactive matter owing to 

 its continual bombardment by the helium particles that are emitted 

 by this decaying matter. But if the separation between the radium 

 and the sulphide is increased, the scintillations stop quite abruptly 

 at a critical distance due to the slowing down of the helium parti- 

 cles by collisions with the gas molecules in their path. When the 

 gas in their path was nitrogen, Rutherford found that a few fainter 

 scintillations continued to appear long after the amount of interven- 

 ing gas was sufficient to stop all the original helium particles emitted 

 by the radium. The faintness of the scintillations indicated that 

 they were produced by a lighter projectile than a helium atom, and 

 the only lighter one known is that of hydrogen. Other confirming 

 experiments showed that these were indeed the cause of the faint 

 scintillations, and still further work showed that they came from 

 the nitrogen and not from any hydrogen impurity. Rutherford then 

 suggested the only plausible explanation, which is that a type of 

 nuclear reaction occurs when a helium nucleus impinges with a suf- 

 ficient velocity on one of nitrogen and that one of the resulting 

 products is a nucleus of hydrogen, which is generally called a pro- 

 ton. By the fundamental theorem of the conservation of electric 

 charge, the entity left is most likely an atom of oxygen for this has 

 a charge equal to that of the sum of helium and nitrogen minus the 

 single atomic charge carried away by hydrogen. 



This was a discovery of the greatest importance which has led 

 to the fascinating developments of nuclear physics in the past 10 or 

 15 years. The essential correctness of Rutherford's view was verified 

 by Dr. Blackett, who used a beautiful technique due to C. T. R. 

 Wilson. This is a method for rendering visible and photographable 

 the path of one of these atomic projectiles through a gas. From the 

 length of these threadlike tracks and from the apparent density of 

 their image on a photographic plate, and their curvature in mag- 

 netic field, the mass, charge, and velocity of the atomic projectile can 

 be determined quite unambiguously. A collision with a nitrogen 

 atom resulting in a disintegration is evidenced by the track of the 

 helium particle from the radium suddenly forking; the long, more 

 tenuous tine represents the path of the hydrogen atom that emerges, 

 and the short, heavy tine is that of the resultant oxygen atom that 

 recoils in accordance with the law of conservation of momentum. 

 Photographs of this type are probably the most informative evidence 

 we have as to what actually takes place at an interaction between 

 nuclei. 



Though it was found that many of the lighter atoms could be 

 disintegrated in this way, progress was relatively slow till the advent 



197855 — 40 14 



