214 CHEMICAL DISCOVEKY AND INVENTION 



cases. By valency must be understood the habit of some 

 elements, of which hydrogen is the most important, to enter 

 into combination with no more than one other element at the 

 same time, while others may combine with two or more. 



Thus one atom of oxygen may be combined with two other 

 atoms, an atom of nitrogen with three or five others, an atom of 

 carbon at the most with four other atoms. Why atoms are thus 

 limited in their power of combination is one of the fundamental 

 problems of chemistry. 



" Constitutional ? ' formulae based on notions of valency began 

 to be used soon after 1860, but these formulse had no pretension 

 to representing the relative positions of atoms in space. They 

 served merely to show in what order the atoms were supposed 

 to be linked one to another in a molecule, and thus served to 

 some extent to epitomise the chief chemical changes to which 

 the compound would be liable. Thus if acetic acid was repre- 

 sented as CH,.CO.OH or as 



H 



I 

 H C C H 



II I 

 H 



the latter was not designed to serve as a picture of a molecule, 

 though such formulse have been and are very valuable for dis- 

 tinguishing the more prominent cases of isomerism, that is of 

 compounds which, while possessing the same composition, have 

 different chemical properties, and hence, presumably, different 

 atomic structure. 



An acid called lactic acid is produced in sour milk, and another 

 acid having the same composition occurs in flesh, and hence is 

 found in Liebig's meat extract. These acids have the formula 

 CH 3 .CHOH.COOH. They are very much alike, but the latter 

 of these acids is optically active that is it causes the rotation 

 of a plane polarised ray while the other is inactive. The study 

 of these acids by Wislicenus in 1872-3 led to the idea that the 

 differences observed could only be accounted for by supposing 

 different relative positions to be assumed by their constituent 

 atoms in space of three dimensions. But it was not till 1875 

 hat a complete theory was conceived by the late Dutch pro- 

 fessor Van 't Hoff, and set forth in his treatise La Chimie dans 

 VEspace. Almost simultaneously the connection between 



