n. RITTENBERG AND D. SHEMIN 



such data, however, Knoop deduced diat the oxidation of fatty acids 

 proceeds by the removal of two carbon atoms at a time; but the possi- 

 bihty still existed that the oxidation of the unnatural acids might 

 proceed by a mechanism entirely diflferent from that of the natural 

 fatty acids. Such techniques were of but restricted use. It was clear 

 that little further advance in the study of the intermediary metabolism 

 could be expected from this direction. 



At about the time that Knoop was using the phenyl group to 

 label fatty acids and amino acids, a more powerful technique was in 

 the process of development as a result of the advances being made on 

 the structure of the atomic nucleus. During the 19th century, it 

 was universally believed that the chemical and physical properties of 

 an element were a function of its atomic weight. Mendelyeev, by 

 classifying atoms according to their masses, had been able to predict 

 the properties of elements as yet unknown. In 1906, however, Bolt- 

 wood (6) discovered a radioactive element, ionium, which was chemi- 

 cally and physically identical with thorium but which had a diflferent 

 atomic weight. Other such examples were found among the radio- 

 active elements and it soon became clear that the properties of an atom 

 were determined, not by its mass, but by its nuclear charge, its atomic 

 number. It was therefore possible to have two or more atoms having 

 the same nuclear charge but differing atomic weights. Such atoms 

 were called isotopes by Soddy (41). In 1910 he stated (40), "Chemical 

 homogeneity is no longer a guarantee that any supposed element is 

 not a mixture of several diflferent atomic weights, or that any atomic 

 weight is not merely a mean number." 



With the development of the mass spectrometer by Dempster 

 (11) and Aston (1), it became possible to investigate the iso topic 

 composition of the stable elements; and it is now known that most 

 elements have at least two isotopes, though a few, such as phosphorus 

 and iodine, seem to exist in but one atomic variety. With the excep- 

 tion of the elements from hydrogen to carbon, every element of an even 

 atomic number has three or more stable isotopes, while every stable 

 element with an odd atomic number has two or less stable isotopes. 



Our understanding of the laws governing the structure of atomic 

 nuclei is as yet in a primitive stage. We think that atomic nuclei are 

 built up from two elementary particles, the proton, a particle having 

 a mass of one atomic unit and a single positive charge, and the neutron, 



262 



