168 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



substance that is selectively taken up by tumor tissue. If there 

 were time, I should like to describe work along this line in progress 

 in several laboratories, and especially to speak of the important 

 progress that is being made in the treatment of leukemia, but I must 

 content myself with only mentioning these new developments in 

 medicine, which are so promising for the future. 



ATOMIC ENERGY 



For a long time astronomers have been vexed with a problem, the 

 problem of the source of stellar energy, for there is evidence that the 

 sun has been blazing at its present brilliance for thousands of millions 

 of years, and no ordinary fuel could be responsible for such an eternal 

 fire. 



The discovery of radium posed to the physicist a similar difficulty ; 

 for it was found that radium gives oflf every hour enough energy to 

 heat its own weight of water to boiling, and this it continues to do for 

 more than a thousand years. Such a vast source of energy in the 

 radium atom was as difficult to understand as the evidently limitless 

 store of heat in the sun. The problem was of fundamental interest 

 and all sorts of possibilities were considered even to the abandonment 

 of the principle of the conservation of energy. 



But the first clue to the solution of the problem appeared in 1905 

 when Einstein announced the theory of relativity. One of the revo- 

 lutionary consequences of the theory was that matter is a form of 

 energy and that presumably in nature processes go on in which 

 matter is destroyed and transformed into more familiar forms of 

 energy such as heat, radiation, and mechanical motion. The rela- 

 tivity theory gave as the conversion factor relating mass to equiva- 

 lent energy, the square of the velocity of light — a very large number, 

 even to an astronomer! Thus, the theory indicated that, if a glass 

 of water were completely destroyed, more than a billion kilowatt 

 hours of energy would be released, enough to supply a city with light 

 and power for quite a time ! 



This exciting deduction was immediately accepted by the astron- 

 omers, who said, "Doubtless within the sun conditions are such that 

 matter is being transformed to heat. Thus, slowly through the ages 

 the sun is losing mass ; its very substance is radiating into space." 



Likewise, the physicists, who had other compelling reasons for 

 accepting the Einstein theory, concluded that the source of the energy 

 in the radium atom was a destruction of matter in the atomic 

 explosion giving rise to the penetrating rays. 



Although the fundamental assumptions on which the relativity 

 theory was based were evidently sound, and the explanations of 

 the source of energy of the sun and stars and radioactivity were 



