A HISTORY OF SCIENCE 



course increasingly difficult, yet, with the affinities for 

 a guide, it is always possible. Of course no one sup- 

 poses that such a formula, written in a single plane, can 

 possibly represent the true architecture of the molecule : 

 it is at best suggestive or diagrammatic rather than 

 pictorial. Nevertheless, it affords hints as to the 

 structure of the molecule such as the fathers of chem- 

 istry would not have thought it possible ever to attain. 



PERIODICITY OF ATOMIC WEIGHTS 



These utterly novel studies of molecular architecture 

 may seem at first sight to take from the atom much of 

 its former prestige as the all-important personage of the 

 chemical world. Since so much depends upon the 

 mere position of the atoms, it may appear that com- 

 paratively little depends upon the nature of the atoms 

 themselves. But such a view is incorrect, for on closer 

 consideration it will appear that at no time has the 

 atom been seen to renounce its peculiar personality. 

 Within certain limits the character of a molecule may 

 be altered by changing the positions of its atoms (just as 

 different buildings may be constructed of the same 

 bricks), but these limits are sharply defined, and it 

 would be as impossible to exceed them as it would be 

 to build a stone building with bricks. From first to 

 last the brick remains a brick, whatever the style of 

 architecture it helps to construct; it never becomes a 

 stone. And just as closely does each atom retain its 

 own peculiar properties, regardless of its surround- 

 ings. 



Thus, for example, the carbon atom may take part in 

 the formation at one time of a diamond, again of a piece 



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