CHEMICAL ACHIEVEMENT — PAULING 229 



selves could be used as the method of achieving the low temperature. 

 Let us now return to the basis of chemistry — the atoms of the chemi- 

 cal elements. Tlie last hundred years have seen the systematization 

 of the elements through the periodic system of Mendeleev, the assign- 

 ment of precise atomic weiglits to most of the known elements, the 

 discovery of the elements predicted by the unfilled sequences in 

 Mendeleev's table, as well as the unanticipated series of noble gases, 

 and, finally, in recent years, the development of modern alchemy, the 

 conversion of one element into another, and the artificial production 

 of new elements. Now that four transuranium elements have been 

 reported — neptunium, plutonium, americium, and curium — we may 

 look forward with confidence to the announcement that still more new 

 elements have been made,^ and that practical methods of manufacture 

 in large quantities of the most rare of the lighter elements have also 

 been developed. We may well expect that in the future world nuclear 

 chemistry will be found of the greatest value in many ways, not only 

 in the production of new elements and in the use of radioactive ele- 

 ments as tracers, but also in causing new chemical reactions through 

 bombardment with high-energy particles. 



INORGANIC CHEMISTRY 



Inorganic chemistry has been making steady progress. The in- 

 organic chemist of today has a great advantage over his fellow of 

 preceding generations, in that he has a thorough understanding of the 

 molecular structure of most of the substances with which he is work- 

 ing, and of the relation between the physical and chemical properties 

 of the substances and their structures. An illustration of the useful- 

 ness of structural knowledge is provided by the recent development of 

 substances that are similar to organic compounds, but with silicon 

 atoms, which form the same tetrahedral bonds as carbon, in place of 

 some or all of the carbon atoms. 



The first substance of this nature was made half a century ago. It 

 had not been found possible to make in large quantities the substance 

 diamond, which is a very useful material because it is the hardest of 

 all known substances. However, it was found possible to make a new 

 substance with the same tetrahedral structure as diamond, but with 

 half of the carbon atoms replaced by silicon atoms — the substance 

 silicon carbide, which has now for many years found extensive use as 

 an abrasive. Then it was found that other compounds of silicon could 

 be made, the silicones, which have, in place of long chains of carbon 

 atoms, chains of silicon atoms (usually with oxygen atoms inter- 

 spersed, in a sort of ether linkage) , with methyl groups or other side 



^ The manufacture of two more transuranium elements, berkellum and californium, was 

 announced early in 1950. 



