NON-MATHEMATICAL SCIENCES 175 



into his own scientific consciousness, and has 

 been called chemical instinct (chemisches Ge- 

 fiihl) ; yet it is extraordinary with what pre- 

 cision he will calculate the properties of a sub- 

 stance he has never seen or the consequences of 

 a reaction he has never tried. It is amazing to 

 find how few of the structures which he has 

 assigned to the various molecules are still in 

 doubt. 



A moment of hesitation came when the work 

 of Pasteur required the chemist to explain the 

 so-called optical isomers, which are alike in 

 number and kind of atoms, and in almost every 

 other physical and chemical property, but 

 which rotate polarized light in opposite direc- 

 tions and by equal amounts. But LeBel and 

 van't Hoff showed that the tetrahedral carbon 

 atom, which had already been invented, not 

 only could account for these substances, but 

 should have predicted them. Figure 27 shows 

 two atomic structures which are thus alike in 

 the sense that a man is like his image in a mir- 

 ror, or that a right- and left-handed glove are 

 alike. 



If four different atoms arrive fortuitously 

 where a carbon atom is and combine wdth it, one 

 arrangement is as likely as the other; never 



