NEW FRONTIERS IN THE ATOM — LAWRENCE 169 



most attractive, until direct experimental verification was forth- 

 coming, Einstein's great deduction could not be regarded as an 

 established lavs^ of nature. 



The first direct evidence of the truth of this fundamental prin- 

 ciple was obtained in the first atom-smashing experiments a decade 

 ago. It was observed that, when the nucleus of a lithium atom is 

 hit by a proton having a kinetic energy of less than a million electron- 

 volts, the result is the formation of two helium nuclei which fly apart 

 with an energy of more than 17 million electron-volts; thus in the 

 nuclear reaction in which hydrogen and litliium unite to form two 

 helium atoms, there is a great release of kinetic energy. 



Now one of the interesting and important occupations of the ex- 

 perimental physicist has been the measurement of the masses of 

 atoms and the weights of atoms are known with great precision — 

 much greater than any individual knows his own weight. In par- 

 ticular, it was known precisely that a lithium atom and a hydrogen 

 atom have a total weight slightly greater than the weight of two 

 helium atoms, and it was a great triumph for the Einstein theory 

 when measurements showed that the excess kinetic energy with which 

 the helium atoms flew apart in the hydrogen-lithium reaction corre- 

 sponded exactly with the disappearance of mass according to the 

 mass-energy relation. Literally hundreds of similar nuclear reactions 

 have been studied in the intervening years, and in each instance the 

 Einstein relation has been verified. At the present time this great 

 principle has as firm an experimental foundation as any of our laws 

 of nature. 



URANIUM FISSION 



Now that it is an experimental fact that matter can be converted 

 into energy, it becomes of great practical importance to inquire 

 whether the vast store of energy in the atom will be tapped for 

 useful purposes. This question has recently taken on added interest 

 through the discovery of a new type of nuclear reaction involving 

 the heavy element uranium. 



It has been known for some years that the heavy elements, such 

 as lead, gold, and uranium, are relatively heavier than the middle- 

 weight elements, such as copper and iron, or more precisely that the 

 average weight of the neutrons, protons, and electrons in the heavy 

 elements is greater than their average weight in the atoms near the 

 middle of the periodic table. Accordingly it is to be expected that, 

 if heavy atoms were split approximately in two forming correspond- 

 ing middle-weight atoms, there would be a vast release of energy 

 corresponding to the disappearance of matter in the transformation. 

 Indeed, from known values of the masses, it can be calculated on the 



